Does a roadway bridge experience more load when vehicles are parked or when they are moving?

Bridges are designed for the loads that come from the vehicles that are expected to cross them. This includes the weight the vehicle and any dynamic loads that may be introduced from movement of the vehicle. Dynamic loads may be from "bouncing" or from hitting a joint or pothole. Initially it would seem obvious that more load is applied to a bridge while the vehicles are in motion (weight of vehicle plus dynamic load). The dynamic loads are proportional to the travelling speed of the vehicles, but as vehicles go faster, they typically are spaced farther apart. When vehicles are stopped, they typically are much more closely spaced than when they are moving. Can there be a situation where more load is on a bridge because of closely spaced parked vehicles than from moving vehicles that are farther apart? Are these two situations covered in bridge design?

This is to make sure they know what the foundation is made of. For all they knew there may have been an old tunnel underneath that would have collapsed when the new building is put on top. London is built on top of an old marsh, this type of soil is very prone to sinking and uneven settling, digging down and reinforcing the foundation alleviates that. It also ensures the foundation is uniform under the building to avoid a new tower of Pisa. Given the age of the city it may have been to scour the land for potential archaeological...

Parked vehicles vs moving vehicles Closely spaced parked (or slow moving) vehicles are definitely more onerous, as stated on page 89, Appendix 2.A, Clause 2.A.1 of the South African bridge design code TMH7 : It is generally accepted and can readily be shown that except in the very small span range, the worst loading condition occurs under congested (bumper to bumper) conditions caused by a traffic blockage and that the dispersion of traffic at speed caused by increased vehicular inter-spacing, more than off-sets the effects of impact. However, this is only true by inspection for an unlimited number of vehicles...


What material is used to hold molten iron in a furnace?

When iron is melted, I guess it has to be transported and contained. I think the container in which it is has to be able to withstand higher temperatures than what you want to melt. According to this webiste , "Iron, Wrought" has a melting temperature of 1482 - 1593 °C. There are a couple of other metals which have higher melting points (e.g. Wolfram (tungsten) with over 3400 °C), but all I can think of are much more expensive. So what material is the oven / "bottle" / "basin" (or however you call it) made of? (Side question: Iron has been melted for quite a while now. I guess this has changed over the years. Of which materials was it before?)

Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...

Summary Crucibles are lined with refractory materials. Steel processing makes use of graphite or a combination of chromite and magnesite for direct contact with the melt. Cast iron processing often uses engineered clays, also known as alumina-magnesia-silica mixtures. Graphite is harder to form than clay-type refractories. To be suitable as a refractory, a material must meet a number of property requirements to be both economical and safe. Refractory Materials As you noted, iron has a high-end melting point of about 1,540 °C on the far left side of the $\textrm{Fe-C}$ phase diagram below, in the form of pure iron. There...

This will at least depend on the: Rate of Cooling Magnetic field strength Exact composition The magnetic field will alter the microstructure as you can read in, for example, Yudong Zhang, Nathalie Gey, Changshu He, Xiang Zhao, Liang Zuo, Claude Esling, High temperature tempering behaviors in a structural steel under high magnetic field, Acta Materialia, Volume 52, Issue 12, 12 July 2004, Pages 3467-3474, ISSN 1359-6454, http://dx.doi.org/10.1016/j.actamat.2004.03.044 . G.M. Ludtka, R.A. Jaramillo, R.A. Kisner, D.M. Nicholson, J.B. Wilgen, G. Mackiewicz-Ludtka, P.N. Kalu, In situ evidence of enhanced transformation kinetics in a medium carbon steel due to a high magnetic field,...


Why does it take so long to restart a nuclear power plant?

I have heard a couple of times that an operating nuclear power plant which was shut down (non-emergency; e.g. for a regular check) needs over 24 hours (up to 72 hours?) to get up running again. Why does it take that long?

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...

As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%. the middle ,...

When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...


Windmills in empty fields. Why no trees?

I have noticed that windmills are generally built in empty fields with no trees around, and I've been wondering why... A windmill is generally way taller than trees and I can imagine that trees don't actually affect the flow (see picture). But is this the reason why there is nothing around or is there something else that makes them waste so much space around? And is the velocity profile from the first figure a realistic one? If I search for the theory of external convection, the velocity profile looks like this for flow over a plateau: In the second case, wouldn't the velocity be higher if there is an obstacle? Or is it because the obstacle is a porous medium that damps the flow? And lastly, is the velocity profile at earth level looking like that? Is there really an origin to it, or is it fully turbulent all the way?

mechanical-engineering fluid-mechanics thermodynamics heat-transfer

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book " Wind Energy - The Facts ". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...

After the 2014 ICCT report revealed that these light-duty passenger diesel vehicles were emitting too much NOx and US regulators confronted VW about the results, VW did some testing and proposed a voluntary software recall to recalibrate the various emissions control devices on the affected vehicles. From the California Air Resources Board's (CARB's) in-use compliance letter to Volkwagen AG, 2015-09-18 , following that recall: To have a more controlled evaluation of the high NOx observed over the road, CARB developed a special dynamometer cycle which consisted of driving the Phase 2 portion of the FTP repeatedly. This special cycle revealed...

For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...

Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .


What are the engineering principles for a train to get electricity from the railway

How many general methods are there for transferring electricity from the railway to a train? I could see that some trains are connected by a pantograph and some have a third rail. Are there any other methods? What is the general engineering principle behind each? What are the basic differences (pros and cons) of each method? I was looking for these questions but could not find a good review. If anyone has a reference or could answer them kindly do. Also, is it possible to build a small model of such a system and scale it up? I am not talking about a commercial model train. I am aiming at self-designing the same engineering principals and demonstrating them on a scalable small model. If so, I would be happy to have a nice tutorial reference. I am focusing on Electric multiple units (EMU) .

The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. "Too hot" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...

Pantograph and third rail are pretty much it. Engineering principles: both have a conducting surface on the train (moving) in contact with the stationary rail/wire, in both cases you need a material that's resilient and conductive. The contact strip is a wear material . Differences: overhead catenary wires are flexible and will move around when a train drives underneath them. At high speeds, the pantograph can set up waves in the catenary wire, if these waves aren't damped sufficiently, the pantograph will start bouncing. third rail is limited to low voltages to reduce the risk of arcing between the rails....

If the receiver does not detect the sub-carrier for the "colour burst" signal which is transmitted during the horizontal blanking period the receiver switches on the "colour-killer" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the "back porch" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...


Can we change steel properties by application of magnetic field while quenching?

Austenite is non magnetic while $\alpha$-ferrite and pearlite are magnetic . ( Magnetic properties of pearlite vary as a function of carbon content ) If a strong magnetic field is applied in a particular direction while the steel is being quenched (rather, austenite is being quenched!), would the grain structure change? Is it possible to get superior grain structure and hence tougher steel by application of cyclic magnetic field? My speculation is that, at the eutectoid point while quenching the steel, as low carbon zone of pearlite has more permeability, that zone should align itself to the strong magnetic field by pushing the carbon in orthogonal direction, so grain boundaries should take a different shape. Will it actually happen?

steel process-engineering materials metallurgy magnets

This will at least depend on the: Rate of Cooling Magnetic field strength Exact composition The magnetic field will alter the microstructure as you can read in, for example, Yudong Zhang, Nathalie Gey, Changshu He, Xiang Zhao, Liang Zuo, Claude Esling, High temperature tempering behaviors in a structural steel under high magnetic field, Acta Materialia, Volume 52, Issue 12, 12 July 2004, Pages 3467-3474, ISSN 1359-6454, http://dx.doi.org/10.1016/j.actamat.2004.03.044 . G.M. Ludtka, R.A. Jaramillo, R.A. Kisner, D.M. Nicholson, J.B. Wilgen, G. Mackiewicz-Ludtka, P.N. Kalu, In situ evidence of enhanced transformation kinetics in a medium carbon steel due to a high magnetic field,...

I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...

This is an admittedly North American response. MGT In the US, how much traffic goes over a given track in a year is measured in Million Gross Tons (MGT) e.g. 1 MGT = 2 000 000 000 lbs [spaces instead of commas to be world-friendly]. This is a measure of the total weight of cargo and vehicles but not necessarily the number of individual trains. Rail Life The life of a typical railroad rail is between 1300 MGT and 380 MGT depending on if the rail is on a straight (tangent) track or in a curve. There is more friction...

Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...


How does a traffic light sense the proximity of vehicles?

Some traffic lights don't operate periodically but instead detect when a car is close by and then turns green. I have heard that they use a magnetic sensor embedded in the road to sense cars as they come near. Is this correct? Do they use other means as well?

I can understand your concern. This is going to be difficult for two reasons: Finding out the agency that is responsible for the maintenance may be next to impossible for a layperson. Note that I said "maintenance", because this might be a different entity than the owner due to agreements. Due to the number of complaints that the public agencies receive, it is very likely that your comment will get ignored. This often has nothing to do with whether or not your concern has merit. Public vs. Private Public agencies will be the easiest to contact. In a city, calling...

If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...

As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives....

The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. "Too hot" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...


How many train passes can railway tracks endure?

I know that the rubber on car and truck tires wear, and the road concrete wears out. I wondered: While steel is hard and elastic, it still causes friction (interaction between molecules) and therefore abrasion. Let's say we have on average 20-30 wagons with four axles with a full load of 50-60 tons on each wagon and a traction vehicle. How many train passes can a railway rail endure before it must be replaced? What is the equivalent time frame in which this number of passes is achieved on average?

civil-engineering materials rail transportation

This is an admittedly North American response. MGT In the US, how much traffic goes over a given track in a year is measured in Million Gross Tons (MGT) e.g. 1 MGT = 2 000 000 000 lbs [spaces instead of commas to be world-friendly]. This is a measure of the total weight of cargo and vehicles but not necessarily the number of individual trains. Rail Life The life of a typical railroad rail is between 1300 MGT and 380 MGT depending on if the rail is on a straight (tangent) track or in a curve. There is more friction...

Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...

It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.


How should I position two shelf supports for the best distribution of load?

I'd like to mount a shelf on the wall. I have two shelf supports to do so, like this: Relative to the shelf, where do I place the (blue) supports to achieve the best distribution of load?

When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...

Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...

As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following structure: A rectangular cross-section (such as a plank) will behave equally under positive or negative bending moment, so your objective should be to balance both. To do so, you want your main span to be $2\sqrt2 \approx 2.83$ times the cantilevers. This is found by calculating the cantilever required to offset half of the bending moment due to a uniform load along a simply supported beam:...

The torsion constant $J_T$ relates the angle of twist to applied torque via the equation: $$ \phi = \frac{TL}{J_T G} $$ where $T$ is the applied torque, $L$ is the length of the member, $G$ is modulus of elasticity in shear, and $J_T$ is the torsional constant. The polar moment of inertia on the other hand, is a measure of the resistance of a cross section to torsion with invariant cross section and no significant warping . The case of a circular rod under torsion is special because of circular symmetry, which means that it does not warp and it's...


Just by taking a train ride across my home city I can see truss bridges like the one in above picture everywhere. There are numerous variations, but the most common design seems to be this. But why are they built specifically this way? I can intuitively kind-of see why such a design probably is strong, but is there any kind of in-depth reason? I would be interested to know the answer as much from the physics side of things as possible. Googling didn't help much; I could find information on the different variations and many examples, but none really covered what is it about this design specifically that makes it so popular.

Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .

That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...

Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...


What is the purpose of building foundations?

According to many sources, one of the purposes of building's foundations is "To distribute the weight of the structure over large area so as to avoid over-loading of the soil beneath." (Wikipedia) On the Wikipedia page on foundations, there is the following picture: So the foundation seems to consist of some type of "legs", with a larger surface area on the bottom. But if the point is to distribute the weight of the building over an area, why not put the bottom of the building in contact with ground? Isn't the bottom of the building itself a much larger contact area than the area the foundation can provide? In the picture, the bottom of the building does seem to rest at ground, so why do we require the foundation to have these large surface area rectangles at the bottom if the whole weight of the building is already resting on ground? Here is another picture from the same page: This seems to be some sort of lodge or a cabin, with stones used as foundation. Here it is evident that the building itself is raised above ground with the stones used as a contact to ground. But now the small stones suffer all the weight of the structure. Wouldn't they be under quite a heavy load over small contact area and possibly sink into the ground? Why not simply have the building's floor, a much larger contact area, rest on the ground?

That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...

Down, Not Out Building foundations don't always have to spread the load out to a larger area. Sometimes the load only needs to be transferred to a stronger (harder) layer. This layer may be deep in the earth and have a softer layer on top of it. Layers Say you want to build a building in an area that has a lot of soft clay at the surface. This clay will not support much of anything (especially when it is wet). Underneath this layer is hard bedrock. You now have two choices: Add a basement to your building so that...

Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...


How are passive houses made in very hot regions (like Saudi Arabia)?

I think, here is the main problem the difference between the internal and the external temperature. For example, in Saudi Arabia, in 50 C, a passive house needed probably much sophisticated planning as in Paris. Compared to the traditional cooling systems, in the second case is enough only to get a cooling system with bigger power. I think, they are much more scalable. Is it anyways possible?

If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...

If the receiver does not detect the sub-carrier for the "colour burst" signal which is transmitted during the horizontal blanking period the receiver switches on the "colour-killer" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the "back porch" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...

A yahkchal is an example of a type of passively cooled building in Iran They utilise a combination of passive evaporative cooling and thick thermally insulating walls in order to keep the interior temperatures low enough. First, wind is directed into underground aquifers known as qanat . They are then cooled due to the low humidity desert air causing water to evaporate. The cooled air then flows through the interior of the yakhchal, cooling the interior. The thick insulating walls (filled with earth and various insulating materials such as straw and feathers) help to insulate the cool interior from the...


How does measurement uncertainty combine with tolerances?

Given a tolerance within which your workpiece should be manufactured, say some length should be $10\pm1$mm. If you determine that your uncertainty in measuring this length is $0.2$mm (at 95%). How should a measurement of $9.1$mm be treated? Clearly there is a significant probability that this value will actually be outside of tolerance. Do you need to decrease you tolerance range based on the uncertainty in your measurement?

You need to ensure that even in the worst case scenario, you still meet your measurement spec of $10 \pm 1\text{mm}$. If your tolerance is $0.2\text{mm}$ of your measurement, then a measurement of $11\text{mm}$, while may look like it meets spec, it doesn't because it could be $11.1\text{mm}$. So the worst case that still meets your spec is a measurement of $10.9\text{mm}$, because then with a max tolerance of $0.2\text{mm}$, you still meet $11\text{mm}$. With a $0.2\text{mm}$ tolerance, your $10 \pm 1\text{mm}$ spec becomes $10 \pm 0.9\text{mm}$. How should a measurement of $9.9\text{mm}$ be treated? So revised spec is between...

Answering the question: What are possible types of low cost sensors I can use? There are several types of sensors that can provide millimeter level accuracy. "Low cost" is a very relative term, so you'll need to do some shopping around based on your specific budget. Optical sensors- Included here are those of the type you listed, though it's a very cheap sensor meant more to provide a "yeah something is in front of me, about yay-far-away". There are also laser sensor systems which can provide millimeter level accuracy. Ultrasonic Sensors- Most ultrasonic sensors have relatively low accuracy; in the...

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book " Wind Energy - The Facts ". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...

It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...


Can wifi signal reception be improved by opening a door?

A wifi user is in a different room than the router. The computer is having a hard time connecting and receiving the wifi signal. Can the wifi signal from the router to the computer be improved by opening a door to the room where the computer is?

The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...

Can the wifi signal from the router to the computer be improved by opening a door to the room where the computer is? Maybe, but probably not to a noticeable degree. All structures, including doors, impede the wireless signal from the router by some amount. Generally, the amount of impedance added by the door is a negligible amount and would not be sufficient to noticeably improve the quality of the signal. That said, differing types of door construction have differing impacts on the signal. A hollow core, wooden door won't impede the signal all that much at 4 dB. A...


While browsing a local news site, I stumbled upon this picture of a broken bus door. The glass was fractured due to a rock impact. As far as i know there are no heating elements on the window section in the picture. Now, what interests me is the apparent periodicity and structure of the fracture pattern. Are periodic fractures something commonly encountered in tempered glass? Is the mechanism that generates such fractures understood? Is it a consequence of the manufacturing process? Perhaps the way the glass is affixed to the door? Thanks in advance

It's an artifact of production - in particular, the construction of the conveyor belt of the machine performing the quenching process. The pattern can be observed through a polarizing filter in the undamaged glass: source and it's a result of the structure of the conveyor belt through which the surface of the glass is cooled to generate the stress that gives it the special properties: source The contact area of the glass with the conveyor is small enough that it doesn't negatively impact the process - all throughout the surface the stress is introduced in sufficient amount, but the amount...

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...


About all those oil tankers off the coast of California … | Grist The giant ships burn fuel to keep lights on, power equipment, and heat the large volumes of crude oil resting in their tanks. I'm assuming that crude oil can't be heated in steel frac tanks , places other than salt caverns , and salt caverns . While underground caverns may not seem like the best place to store an emergency oil supply, they're actually very secure. For one thing, since they're 2,000 to 4,000 feet (610 to 1,219 meters) underground, the extreme pressure prevents cracks from forming and leading to leaks [source: DOE ]. Also, the natural temperature difference between the top and bottom of each cavern encourages the oil to circulate, which maintains its quality.

As Solar Mike's answer says, crude oil is viscous - too viscous to easily pump. Crude oil has a "pour point:" the lowest temperature where it will flow under gravity. Heating the crude oil keeps it above the pour point, so it can be pumped. With the large volume of oil in a tanker, it makes more sense to keep it fluid, rather than letting it cool down and then heat it (very slowly) back up). I found one article (PDF) that recommends tankers keep the oil at least 10°C above the pour point to promote circulation within the tank,...

Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book " Wind Energy - The Facts ". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...


I was having a discussion today, which led to the question why do heavy trucks still use air brakes? To my knowledge, it has been used for at least 40 years (I remember that as a kid), and apparently (I was told today but I haven't gotten around to verify it) they are still widely used. From what I remember, one of the things I was cautioned was that if you repeatedly pressed on the air brakes, then after a while the air buffer would empty and it would take time to fill up (thus losing braking capacity). Anyway, I wanted to know what are the benefits and disadvantages of air brakes compared to other technologies, e.g. hydraulic lines, or electrical system, (even KERS systems for more modern electrical vehicles). UPDATE : From the answers I understand that the main issue is reliability and "technical debt". I want to push a bit further and understand, what's stopping air brakes from being used in other vehicles. E.g. is it cost, performance, inability to accompany AC/DC drum rhythm?

The turbulence model can make a big difference in your simulation . There are many turbulence models around. It becomes a tough job to select one out of them. There is no perfect turbulence model. It all depends on several parameters like Reynold's number, whether the flow is separated, pressure gradients, boundary layer thikness and so on. In this answer, brief information about a few popular models is given along with pros and cons and potential applications. However, interested users can see this excellent NASA website and references therein to know more about turbulence modeling. A) ONE EQUATION MODEL: 1....

Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...

Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...

If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...


How did mechanical engineers work before SolidWorks?

I am studying a bunch of stuff related to mechanical engineering and am considering an eventual switch to the field after taking the necessary classes. I'm older, and I took drafting classes in high school and part of college. I'm familiar with the standard drafting procedure with paper, as well as AutoCAD , and these days SolidWorks . It's a shame they don't teach paper drafting much anymore! But I am left with a question I don't have the answer to, nor the resources to answer it. These days it seems most mechanical engineers (at least the ones I know) are really performing the role of both the drafter and the engineer. Prior to 1995 when SolidWorks came about, what was the duty of the mechanical engineer? If we go even further back before CAD was invented, what duties did a mechanical engineer perform?

cad drafting software engineering-history

Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....

Meet Vladimir Shukhov , a Russian architect who first developed hyperboloid structures. He was born in 1853, died in 1939, and created over 200 hyperboloid structures in the intervening years. He was the reason hyperboloids gained the popularity that they did. His first design, the first hyperbolic structure ever, was the Shukhov Tower in Polibino , pictured here : Another tower also bears the name of Shukhov, and it achieved great fame, too. Shukhov also built the Adziogol Lighthouse . In total, Shukhov designed and built 200 hyperboloid structures. He died in 1939, which could one reason for the decline...


Why is it more reliable to use the land/pit transition in a CD-ROM?

I was reading about how CD-ROMs work and I came to this: Although it might seem simplest to use a pit to record a 0 and a land to record a 1, it is more reliable to use a pit/land or land/pit transition for a 1 and its absence as a 0,so this scheme is used. Now, I'd like to know why?

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...

Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .

I'm going to set aside the potential legal liability aspects of your question. In part because you didn't ask about them, but also because liability will vary based upon jurisdiction. Obviously, consult an attorney familiar with the relevant law. Ethically, I think you've done everything that you're obligated to do. You have contacted the customer and notified them of an unsafe situation or configuration of the product. And you have (strongly) advised them that they need to discontinue using the older product in the unsafe configuration. If the customer remains insistent about using the product in an unsafe manner, you...

To elaborate on ratchet freak's answer; CD-ROMs work by sensing the intensity of the light reflected from the CD as it is spinning. More light being reflected stands for a 1 and less light being reflected stands for a 0 (or vice versa). One way to encode the information would be to have highly reflective surfaces for the 1's and dark patches for the 0's. Printing dark patches at the size of the laser spot size is actually used in writable CDs , but the technique suffers from degradation over time. Instead commercial CDs rely on the property of interference...


If aircraft are pressurised, why do our ears pop during liftoff and landing?

Something hit me during my last flight: our ears feel that the pressure around us changes quickly during liftoff and landing, they hurt more and more until we make them pop. However, the cabin must maintain a level of oxygen higher than outside the aircraft because it isn't dense enough at ~10km to breathe normally. Does that mean that the aircraft merely takes air from the outside, heats it up (it's around -40°C outside at 10km) and adds oxygen to it before blowing it inside? How else?

The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...

Because the cabin isn't pressurized to sea level pressure instead it to about 8k ft equivalent. (while the plane is 4.5 times higher) This means there is less differential pressure than if the cabin was pressurized to sea level pressure. But it's still within the limit of what passengers feel comfortable with. This answer on aviation.SE contains a plot of cabin pressure over time during a flight: Source The composition of air doesn't change with altitude just the pressure. This means that you only need to compress the air before blowing it in.


Does an electric vehicle going downhill recover energy?

I have to model the behavior of an electric car. For this, I use these equations and I can observe by "playing" with parameters that, when going downhill at constant speed, the car has a negative consumption (i.e. recovers energy). I was wondering if this is realistic?

Train Brakes The common brakes on trains are air brakes . As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied. Fail-safe Rail brakes are designed to be fail-safe . That is, when a failure occurs, the safe operation happens. In...

If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a single concentrated load at midspan. The most significant load on a bridge will almost always be its own self-weight; load-trains are heavy but, well, so are bridges. Secondly, I assume you're thinking of the bridge like this: Indeed, we can see here that the bending moment is greater at midspan. However, that's not the bridge we're looking at, it's missing the cantilevers! So...

It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.

You have the right concept, but slipped a decimal point. 5 cm = 0.05 m. The gravitational force on your 450 g mass is 4.4 N as you say, so the torque just to keep up with gravity is (4.4 N)(0.05 m) = 0.22 Nm. However, that is the absolute minimum torque just to keep the system in steady state. It leaves nothing for actually accellerating the mass and for overcoming the inevitable friction. To get the real torque required, you have to specify how fast you want to be able to accellerate this mass upwards. For example, let's say...


From an engineering perspective, what limits the maximum speed you can reach with a regular car? I understand that some of the faster cars are for safety reasons limited to not run faster than say 250 km/h, but that's not my question. I can think of several reasons, but not sure which of these is relevant: Is the limit set by some part (which?) breaking if I increase the rpm, as suggested by red marks on rpm meters? Or is it rather that you cannot get in fuel fast enough to keep increasing the rpm? Or is it that friction/drag increases as you speed up and the engine cannot overcome this as it can only generate a maximum amount of force/torque? If yes, what does this amount of torque/force depend on?

It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.

The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. "Too hot" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...

There are a few simple reasons why the speed of a vehicle (road conditions notwithstanding) may be limited: Gearing -- Production vehicles with conventional transmissions have a limited number of gears. For most modern cars, this is usually 5 or 6, whereas older vehicles may have as few as 2 or 3. If the gear ratio of the highest gear is too low ("lower" gears are expressed as larger numerical ratios), it's entirely possible that the engine will redline before air resistance becomes a factor at all. This ties into your first point about the red zone on the tachometer....


Why is this bridge thickest above the support pillars instead of the mid-span where the bending stress is highest?

Why is a bridge designed like this? The depth of the section at pillars is more than the depth at middle. If I model this as a simply supported beam having load at mid span then the bending moment will be maximized at the middle and the area is also less at the middle. So, this will lead to higher bending stress. So, why is it designed like that?

mechanical-engineering structural-engineering civil-engineering beam bridges

A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either because the hull is especially long or the channel it is being launched into is narrow. There is also the consideration that a sideways launch can be done from any quay that will take the weight but a bow first launch requires a specially constructed slipway. There is also the consideration that if you go in sideways the...

Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....

When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...


What is the most efficient means of warming a building with a high ceiling?

Consider a large auditorium, a church, or some other very large, essentially one roomed building with a high ceiling. Suppose that the building has many entrances which enable cold air in/hot air out and traffic in and out of the building is unavoidably large. I would imagine that any attempts to control the temperature of such a large building would be very inefficient in terms of energy and cost, particularly because warm air rises. Assuming that it's very cold outside and we're interested primarily in keeping the building warm at the ground level so that it is comfortable for humans to work and interact, what is the best method to keep such a large, essentially one roomed building with a high ceiling warm when outdoor cold air exposure is frequent and unavoidable? When I say "best," I'm interested in balancing energy, maintenance, and monetary costs over the life of the building.

civil-engineering building-physics energy hvac

There are a few main reasons why suspension bridges aren't used for railroads. The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers. The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Third is that trains don't really have suspensions, especially freight trains....

When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...

Your question sort of has two parts: How to supply heat, and how to keep it in. Large open rooms with a high ceilings are most efficiently warmed with radiant ceiling heat. Warm air rises, which renders forced-air systems inefficient because the pumped heat ends up at the ceiling and the coldest part of the room is near the floor where you actually want the heat. Radiant floor systems are limited to about 87F because they are in contact with occupants, and so their peak output may not be enough to keep the space comfortable. They also lose more heat...


While going downhill, does my car consume less fuel when in neutral or in gear?

I once meditated on the fact that a car, in downhill, would consume more fuel when in neutral to keep the engine on, rather than in gear. To keep the engine on when in gear, in fact, you mostly need the gravity and enough thrust to keep it rotating, and therefore no fuel (just motor oil). This is what I'd expect cars to do but I'm probably missing something. How's actually the story? And if it's not like this, would/could my idea work somehow? (note: patent pending :P)

It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...

Note: This is specific to gasoline based engines. Assuming your car was produced after the ~1990's it is equipped with a Deceleration Fuel Cut-Off (DFCO) system which cuts the fuel flow to the engine when the gas pedal is not depressed and the car is in gear. In this case it is more fuel efficient to stay in gear because you are letting gravity run the engine instead of fuel. In addition, it saves wear on your brakes because the partial-vacuum created in the engine leads to engine-braking . In many states in the US it is actually required that...

Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....


Why are hyperboloid towers not popular anymore?

Hyperboloid towers were very popular in the end of 19th and the first half of 20th centuries - water towers, powerline anchor towers, sometimes tall radio towers were built using this design. The claimed advantage is using less steel compared to other designs for the same strength. They are very rarely used nowadays (to such extent that old hyperboloid towers are treated as cultural heritage objects and protected by the state in some countries). Why did they lose popularity? Is there any inherent defect it the design? Is steel not expensive anymore?

If the receiver does not detect the sub-carrier for the "colour burst" signal which is transmitted during the horizontal blanking period the receiver switches on the "colour-killer" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise. Figure 1. The colorburst signal is transmitted on the "back porch" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately...

Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '

Meet Vladimir Shukhov , a Russian architect who first developed hyperboloid structures. He was born in 1853, died in 1939, and created over 200 hyperboloid structures in the intervening years. He was the reason hyperboloids gained the popularity that they did. His first design, the first hyperbolic structure ever, was the Shukhov Tower in Polibino , pictured here : Another tower also bears the name of Shukhov, and it achieved great fame, too. Shukhov also built the Adziogol Lighthouse . In total, Shukhov designed and built 200 hyperboloid structures. He died in 1939, which could one reason for the decline...


How should the public raise questions about unsafe structures in the United States?

In a program on NPR that I was listening to, there was a bit about a bridge that from the description sounded to a layman as unsound and is still in use. The program described it as an old wooden railway (and I'm aware that has its own set of challenges) bridge with rotting timbers. In the United Sates, if a member of the public sees a bridge (railway, tramway, car/truck, foot, bike, etc...) that is of questionable soundness, what is the process for him or her to determine who us responsible for it, and if they (the responsible parties) should look at it and have it evaluated? Is there a particular agency that is responsible for regulating bridges and ascertaining their safety?

I can understand your concern. This is going to be difficult for two reasons: Finding out the agency that is responsible for the maintenance may be next to impossible for a layperson. Note that I said "maintenance", because this might be a different entity than the owner due to agreements. Due to the number of complaints that the public agencies receive, it is very likely that your comment will get ignored. This often has nothing to do with whether or not your concern has merit. Public vs. Private Public agencies will be the easiest to contact. In a city, calling...

The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...

As others stated before, induction loops are the primary - most reliable method: the coils (usually just several loops of wire) embedded in the road; fed given frequency from a generator, in presence of metal the frequency of the LC circuit changes and the sensor circuitry detects the change of frequency, producing a presence signal. In some cases these may fail to detect bicycles, but they are by far most common as they aren't affected by weather (or more precisely, the detection circuit tunes in to slow changes of frequency caused by weather) and are immune to accidental false positives....


Purpose of spheres and fins on submarine propeller

On a recently launched Russian diesel-electric submarine, the rear propeller has two distinct features. You can see spheres at the base of every propeller blade: Also, the trailing edge of the shaft has four adjoining fins in line with the axis of rotation: Most importantly, what is the purpose and function of the spheres, and what's beneath the veil in the first image? I believe that it may differ from the second image. It's made of a different material, so possibly a sacrificial element. Is it to reduce noise? Is it to smooth flow or prevent cavitation? Notice that in the first image the spheres are at the leading edge and in the second image the spheres are at the trailing edge. There is a related youtube video .

mechanical-engineering fluid-mechanics propulsion marine-engineering naval-engineering

From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...

I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint tightening characteristics (see joint diagram). reduce contact stresses on surfaces. The basic idea is that since contact stress is defined as: $$\sigma = \frac{F}{A}$$ Obviously the larger the area the less the stress. Also, like Nuclear Hoagie pointed out the area changes with a square law of the diameter. UPDATE 2 - Calculation for M6 bolt (thanks to BenC) *The question gathered enough interest for me to carry out a...

In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...


How do I size metal plates to get the correct dimensions after folding?

I am designing a metal plate that will be laser-cut (or machine-cut) and then folded. I want to know how to size the pre-folded plate in order to get the right dimensions after folding. My actual part is not exactly like this (I simplified it for ease of drawing) but it shows what I want to achieve. In this case it's a 2mm aluminium plate, the red arrows show the inner dimensions after folding that I want to specify and achieve. The holes must also line up and the window should be correctly placed. Intuitively I would expect some compression along the inner part of the folds and stretching on the outer parts - ideally along the centre of the plate - but I don't know if this is what will happen. Assuming the red arrows are 100mm each, should the plate be 300mm? I'm guessing not, so how do I calculate the radius of curvature that will be achieved and if I need to add (or remove) material at the folds in order to achieve my required dimensions?

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...

Your assumption is right! A 300mm long plate with two foldings won't do! This is because you need to take into account the bend allowance and the bend compensation! But why is so? Here is a diagram of what's going on: When you bend a material, part of it will extend (the external part of the bend), while another part will retract (the internal part). The line (in the thickness of the plate) where the dimension doesn't change is called the Neutral line. The neutral line is usually located between a third and a half of the material thickness (from...


Can I use a ultrasound sensor to measure water level?

If I use an ultrasonic sensor will it detect the water level? I was thinking about a product to read water level on water boxes (common in Brazil). I researched about instrumentation for this measure, and I think that an ultrasonic sensor is the best option. Will the water correctly reflect ultrasound and not change the normal measurements against a solid obstacle?

electrical-engineering measurements sensors control-engineering

Technology used in depth finder, in marine application is mostly like is the best to measure the water from the top. To measure water level from the bottom of tank one could use a piezo electric ceramic transducer combined with an Analog Front End (AFE) and a micro-controller to measure water level. The diagram below best explains the configuration. You can use a piezo electric ceramic transducer from Steminc, TDC1000 AFE from Texas Instrument and a MSP430 micro-controller also from Texas Instrument. There might be other configurations, but currently I am only aware of this configuration. Piezo Electric Transducer Analog...

From the point of view of the driver of a car, impacting another car is about as bad as crashing against an ideal wall (a wall with zero deformation whatsoever). If there were a plane reflection between the two cars, then vs. Car would be exactly equal to vs. Wall (the contact points between both cars would all be on the same plane, due to reflection, so each car could be considered a wall for the other). But this plane reflection does not exist: What we have instead is a 2-fold rotational reflection . Let's say the left part of...

As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%. the middle ,...


Surprisingly this hasn't been asked before, so I must be missing something simple. We use engineering stress and engineering strain in this eq. Stress = (Young's modulus) × (strain). This eq. is used in analysis of bending beams, twisting shafts and in buckling. So the final equation of bending $(\frac{M}{I} = \frac{\sigma}{y})$ and torsion $(\frac{T}{I} = \frac{\tau}{r})$ will give us value of engineering stress but not the value of stress. Why are we considering engineering stress instead of true stress while we know it will not give correct value of stress? Some things I read are: Difficult to measure. Not that much of a difference and we can just apply a Factor of Safety. "We don't consider materials to change their cross-sectional area after loading, since we design to have no plastic deformation the elastic region is most important, therefore what happens after the proportional limit is not important" Firstly, 1 and 2 are not real reasons for me. Number 3 seems plausible since we always design in the elastic region, but is this it? Does engineering strain even give valid information after the proportional limit?

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book " Wind Energy - The Facts ". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...

We use engineering strain even though it is not the "correct" value because in most cases, specifically in the elastic regime, engineering strain differs negligibly from true strain. For linear elastic, Hookean materials, it is generally the case strain at the elastic limit is very small. Even the strongest steels, for example, have an upper limit when cold worked of about $\sigma_{\textrm{el}}=1\times 10^{9}\ \textrm{Pa}$. The modulus of steel is approximately $E=200\times 10^{9}\ \textrm{Pa}$. Thus $\varepsilon_{\textrm{el}}=0.005=0.5\%$ for the strongest steels. So at the onset of plastic deformation, engineering strain is $0.5\%$. Many useful elastic materials have much lower engineering strain at...


What is this large steel structure used in excavation?

I keep seeing these large rectangular structures on job sites they put in the ground. I'm guessing something for strengthening the surface structures?

Parked vehicles vs moving vehicles Closely spaced parked (or slow moving) vehicles are definitely more onerous, as stated on page 89, Appendix 2.A, Clause 2.A.1 of the South African bridge design code TMH7 : It is generally accepted and can readily be shown that except in the very small span range, the worst loading condition occurs under congested (bumper to bumper) conditions caused by a traffic blockage and that the dispersion of traffic at speed caused by increased vehicular inter-spacing, more than off-sets the effects of impact. However, this is only true by inspection for an unlimited number of vehicles...

The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. "Too hot" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...

It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...


Recently I was weighting stuff with my dad and as I was playing with one of the old 2 kg weights I felt with my fingers number "5". After cleaning and upon further inspection it turned out the writing on the weight says "5Φ". We checked and it indeed weights 2 kg as my dad remembered. It was done with a simple balance scale by comparing with another weight of similar age that is properly marked as "2 kg". There are no other marks on that weight. On the opposite side is a bulge but it's an irregular blob - I think it's remainder of sprue. So, question that arised in my head is - what does that Φ mean? Is it some rare unit of mass? Some sort of preferred series or standard for weights? I was looking for answers and found nothing. I ruled out pounds that was first thing that came to my mind because the difference was to big (5 pounds is approx. 2.26 kg) - though there are some signs of wear and corrosion it's in rather good shape and I don't believe it weighted 0.26 kg more originally. Also, considering I'm from Poland and the weight is really old it probably was made during the soviet era (maybe even in USSR) - so that phi could be actually cyrilic fe - but that also gave me no results. As a last resort I decided to ask here - maybe some member of that community will know the answer. Below is a photo of the weight in question.

As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%. the middle ,...

You are probably right (it being of Russian origin). The weight seems to be 5 funt. 1 funt is the Russian equivalent of a pound. It is written as Фунт, funt, and around 1900 it was the basic unit of weight measurement in Russia (so it survived in the USSR days), but now its obsolete. 1 Φ is about 409.5 grams.

It is more efficient to transmit DC using about the same infrastructure. This is because of several effects: Skin effect experienced with AC. There is no skin effect with DC. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only...


As I heard, the life expectancy of photovoltaic cells is normally some decades, as so. This is very long, regarding their high cost, it has significant (negative) effect on the total costs. Why do they age? I can see the only possibility which could damage their atomic structure, and this is the very high energetic ultraviolet (or even soft röntgen) photon spectrum of the sun (which is only a small part of its power). And, I think, these photons could be maybe easily filtered out by a transparent plastic layer over the silicon. Thus, why do the solar cells age, and what happens in them on the atomic level?

The current limit specification for a wire is limited by the heat that current will produce, and how much heat the wire can dissipate before getting too hot. "Too hot" depends on the circumstances. You will see higher current ratings for the same type of wire in chassis wiring applications than the electrical code allows for home installations, for example. This is mostly due to how hot too hot is. The ultimate limit for extreme applications is that the conductor not melt. Temperatures anywhere near that would be unsafe running along wooden supports inside a wall in a house. As...

The National Renewable Energy Lab (NREL) estimates something like 0.4%-1% degradation per year for solar panels. I don't know what the chemical action is here (the electrical result being an increase in series resistance of the PV cells), but the long-term degradation of the panels is correlated with UV exposure. Although damaging to the PV material, the UV rays do contain energy that we want to collect, so you won't be able to filter it out completely without defeating the purpose of the panel. Other causes for degradation are mostly weather-related, but in the form of mechanical stresses to the...

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...


Are pontoon bridges being considered to extend bridge span?

Pontoon bridges differ from traditional bridges in that they are supported not by structures anchored to the floor of the body being spanned but by floating pontoons that are connected by a more rigid structure that supports a roadway. They're often used by militaries to provide a temporary crossing point, but they're also used for permanent civilian crossings . I would assume that they make it easier to cross larger bodies of water because there is less structure to be secured below the surface. In areas of deep water, support structures can become unfeasibly large. These larger spans could, though, make the pontoon bridges susceptible to damage from strong winds and currents. Are there plans to use pontoon bridges to cross long distances?

It is a trench shield. It gets placed in a trench after the trench is dug to prevent workers from being hurt or killed in the event of a trench collapse. This picture from GMC trench shield shows a partially collapsed trench with a shield installed that would protect the workers installing the blue brute pipe.

That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...

Theoretically pontoon bridges with rope anchors keeping them to the bottom would work against wind and flow, overcoming the problem jhabbot mentioned in his answer (same as train length limit - stretching force). In practice these come with more problems of their own. They drift on water surface and as result, rise and fall with water waves. The larger the body of water they span, the higher the waves; at certain point in stormy weather the bridge would just launch the vehicles into the air. The anchoring isn't exactly simple if it's to withstand such forces. You could just as...


What happens when you put the wrong type of fuel in an internal combustion engine?

What happens to the working of petrol engine when it is emptied and filled with diesel or to a diesel engine emptied and filled with petrol? Will the engine be able to operate and, if not, why not?

If you were going to turn left 90 degrees, without turning the wheels, then you wind up dragging the wheels sideways while you turn. 16 seconds into this video shows exactly what I'm talking about . So every time you try to back out of your driveway, or a parking spot, or turn into a parking spot, or turn anywhere for any reason, you're going to lay down rubber because the tires are rotating quickly while remaining nearly stationary as you turn. Again, look at the wheel slip in the video I linked. You'd be hard pressed to find "four...

Putting diesel fuel in a gasoline engine is just about impossible. Diesel fuel nozzles are larger than gasoline nozzles, and modern gas caps are too small for diesel nozzles to fit into. However, if you managed to get it in there, the diesel fuel is too heavy and evaporates too slowly for the spark plugs to ignite it effectively. One source says that it won't start at all, another source says that it probably won't start, but there's a chance it will, it'll just run horribly and end up as a smokey disaster. The converse is possible, since the gasoline...

OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while...

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...


If the emergency brake in a train is broken, do the passenger car brakes still work?

Scenario: Suppose the emergency brake button in the cab of a train is not working. Would the emergency brake cords / buttons in the passenger cars still work? This question is inspired by this question on the SciFi StackExchange , asking why the passengers in the train fight scene from the movie SpiderMan 2 did not simply pull the emergency brakes. In that particular scene, a R46 City Subway Car was rendered "unstoppable" by ripping out the speed control lever in the car, which also happened to disable the emergency brake button (see 16 seconds into the video). With movies, we suspend our disbelief in order to enjoy the story that is presented. But the above SciFi question got me to thinking about how emergency brake systems are designed for trains. Given trains significant mass and momentum when moving, it seems like there would be multiple, redundant safety systems to provide braking capability for the train. My Question: Is there a common safety design used for the braking system of trains? Does that design account for portions of the system failing and allowing other portions to compensate for the failed components? (ie. would the passenger car emergency brakes still work?)

For long freight trains and those that will be climbing to stations at higher altitudes, an extra or two locomotives are attached to the front. I've always wondered why. As usual there are multiple issues. Most important is landscape. If more power than a single locomotive can provide is not needed for the whole trip, but only a short stretch, like climbing or crossing a mountain range, then it would be a huge waste of resources to attach them the whole trip. For example, take a 10 hour trip time, where all, except for a 1 hour stretch, can be...

Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the load in groups. The digrams are from SKF ball-bearings. '

Electricity production optimization is a very complex subject. It is also affected by many parameters , which I will try to outline below. TL;DR: Demand is constantly monitored and supply is constantly adjusted TL;DR 2: Lshaver's excellent post is a suggested reading after reading this, because it expands and explains what happens at timescales ranging from the micro-second to minutes timescales. Changes in energy demand during the day First of all, the energy demand during the day, week, month of year can change quite a lot. Below is a typical day in New England. Fluctuation in power energy consumption a...

Train Brakes The common brakes on trains are air brakes . As the name implies, these work off of air pressure. The braking power isn't controlled in the way that you would immediately think of though. They do not work like car brakes where the harder you press on the brake pedal, the harder the pressure goes through the lines to the brake cylinders. They work the opposite. The less pressure in the line, the more braking force is applied. Fail-safe Rail brakes are designed to be fail-safe . That is, when a failure occurs, the safe operation happens. In...


Is there a method for plating steel with stainless steel? If so, is it chemical, electrical, or electrochemical? I did a quick search on the internet but was unable to find a service. I'm interested in applying a food safe finish to something that would otherwise be cost prohibitive to make out of solid stainless steel.

Keeping things simple, steel is an alloy of iron and carbon, whereas stainless steel is essentially an alloy of iron, carbon and chromium or iron, carbon chromium and nickel. All forms of steel, whether they be ordinary iron and carbon alloy or stainless steel are made from a melt in furnaces. Because of this stainless steel cannot be plated to ordinary steel by chemical means either. Stainless steel can be welded to ordinary steel but a TIG welder is required, but this wouldn't suit your purposes. Hot dipping is unlikely to be an option due to the melting temperatures of...


Why are lasers used for concentrated light applications instead of incoherent light sources?

Some laser applications consist simply of concentrating light into a small spot. Two example applications are laser welding and cutting. In these cases a CO 2 laser is often used which needs a regulated power supply, a water cooling system, and a supply of CO 2 gas. Why do these applications use a laser instead of a simpler (i.e. incoherent) light source such as an AC powered arc lamp?

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...

Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...

There are many reasons why highly monochromatic light, such as that emitted by a laser, is useful for delivering a large amount of power to a small spot. First of all, incoherent light sources such as a lamp are extended sources which means that they are emitting light from a piece of material which takes up a finite amount of space. When focusing this light to a point, the focal spot is limited by the size of your source multiplied by the magnification of your imaging system. This may sound like a small effect, but if you want to focus...