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78 votes

Why are two wires used in railway overhead equipment?

Overhead line worker here. It's about speed. To answer your question, the purpose of the catenary wire is to get the contact wire to lay flat, so it isn't catenary itself. In your second photo, there ...
Harper - Reinstate Monica's user avatar
56 votes
Accepted

Why are railroad tank cars bent in the middle?

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 ...
NMech's user avatar
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44 votes
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Why are engine blocks so robust apart from containing high pressure?

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 ...
NMech's user avatar
  • 24.4k
44 votes

Why are bulldozers so slow?

I wonder why can't we engineer.. We CAN engineer a vehicle which is both fast, and has a lot of torque. (E.g. a tank, as you mentioned yourself) The reason why we don't do this for a bulldozer (I.e. &...
Chris_abc's user avatar
  • 799
42 votes

Why are screw holes in most of the door hinges in zigzag orientation?

More compact configuration that fully utilizes the area of the leaf. Avoid stresses concentrated on a single plane that is likely to cause the base material to progressively fail in shear or split. ...
r13's user avatar
  • 8,227
40 votes
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Why do heavy trucks use air brakes?

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 ...
Solar Mike's user avatar
  • 15.9k
38 votes
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Windmills in empty fields. Why no trees?

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, ...
410 gone's user avatar
  • 3,761
36 votes

How do bike wheels keep turning when the rider stops pedalling?

The soft clicking sound you hear when coasting is the pawl going over the ratchet as in the picture below.
Jordon's user avatar
  • 361
36 votes

Why are screw holes in most of the door hinges in zigzag orientation?

Because the screws go into wood and if the screws are in line then the wood will most often split between the screws in the grain direction and then the screws come loose.
Solar Mike's user avatar
  • 15.9k
35 votes

Why does this fence have a kink?

They're V-Mesh panels. V-Mesh panel from Irish Wire. The offset of the vee gives rigidity in the horizontal direction. The vertical rigidity is addressed by the fence posts. It's available in 2, 3 or ...
Transistor's user avatar
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33 votes
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Why exactly does a washer help distribute the stress around a bolt?

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 ...
NMech's user avatar
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32 votes
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Does an electric vehicle going downhill recover energy?

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. ...
Mark's user avatar
  • 1,012
32 votes
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Why is this bridge thickest above the support pillars instead of the mid-span where the bending stress is highest?

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 ...
Wasabi's user avatar
  • 13.1k
27 votes

Why does moving air feel cold?

There are a couple reasons. First of all, it's important to note that the sensation of warmth or coolness is only indirectly related to temperature. The receptors in your skin that deal with ...
Air's user avatar
  • 3,211
27 votes
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What limits the speed of a car?

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 ...
Arnon's user avatar
  • 402
27 votes

Why exactly does a washer help distribute the stress around a bolt?

It is for spreading out the stress. But it is also for giving the bolt a bearing surface to turn on. The washer always goes on the side (nut or bolt) that is being turned. It prevents it from marring ...
DKNguyen's user avatar
  • 5,641
26 votes

How do bike wheels keep turning when the rider stops pedalling?

it is called a freewheel. Inside the sprocket gear, there is a mechanism that lets the chain engage the rear wheel only when it is moving faster than the wheel is turning; else the wheel will turn ...
kamran's user avatar
  • 22.5k
26 votes

Why does "potential energy" have the word "potential" in it?

Because it has the potential to produce mechanical work.
NMech's user avatar
  • 24.4k
26 votes

Why are bulldozers so slow?

From memory, but pretty much realistic: a tank will use e.g. 1000 liters of diesel fuel just to move 100 km and will probably require a track maintenance afterwards. A truck with the same tank on its ...
fraxinus's user avatar
  • 499
25 votes
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How should I position two shelf supports for the best distribution of load?

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 ...
Wasabi's user avatar
  • 13.1k
25 votes
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How do vending machines prevent people from stealing goods?

The vending machines have different product delivery box implementations. One of the most common ones -I've encountered- is the following: Basically you have a box which has an opening, as shown in ...
NMech's user avatar
  • 24.4k
24 votes
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Why are most jigsaw blades designed to cut on the up stroke and have teeth that point towards the shank?

The photo you have is not for the standard jigsaw blade. Standard blades teeth point upward and cut upward for the obvious reasons: less wobble, more control, center of force near the handle. This is ...
kamran's user avatar
  • 22.5k
23 votes
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Why does a gear have extra holes?

Different gears have different drivers for having holes. One way you can categorize gears is by whether they are used primarily for transferring: motion: transferring angular position and angular ...
NMech's user avatar
  • 24.4k
21 votes
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Benefit of removing the center "engine hub" inside a fan

For a large and/or high speed fan, there are several mechanical disadvantages, for example The outer ring is highly stressed and therefore heavy. It may also need a heavy containment system to avoid ...
alephzero's user avatar
  • 12.5k
21 votes

Why does temperature not increase by putting more logs into the fire

Adding more fuel doesn't permit an arbitrarily high combustion temperature because of an unavoidable intrinsic limitation: The reaction has to heat its products. (In the case of wood, this is mostly ...
Chemomechanics's user avatar
21 votes
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Why are multiple locomotives attached only to the front for larger trains?

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 ...
Raffzahn's user avatar
  • 326
20 votes

Aluminum weight saving

I believe it has less to do with strength and more to do with stiffness. A rod of aluminum of the same length and weight as a a steel rod will be just as strong (force required to break) but have ...
DKNguyen's user avatar
  • 5,641
19 votes
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Why does "potential energy" have the word "potential" in it?

potential (adj.) Capable of being but not yet in existence; latent or undeveloped. The block is stationary and therefore not performing work. If the block never moves, the potential energy remains ...
fred_dot_u's user avatar
  • 7,227
18 votes
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Why do electric trains arc at high speeds?

Several factors influence this: at high speed, there's a higher chance of the pantograph losing contact with the catenary wire: at higher speed, bumps in the wire cause a more violent excursion that ...
Hobbes's user avatar
  • 746

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