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108

There are two main reasons why glass is still preferred over say PMMA. The first is durability. As long as it isn't broken, the glass in a window can easily last for hundreds of years in good condition. In particular it is a lot more resistant to scratches than comparable plastics and isn't really subject to much in the way of environmental degradation. ...


46

Here is a plastic (most likely PMMA) window, in a boat, after only 37 years. In addition to the obvious scratches, the outer surface has developed a cloudiness : possibly from degradation due to UV light, and (towards the LH end) you can see a cubelike pattern of stress cracks, rather like a toughened glass windscreen after a pebble hit it. You really ...


31

What are the requirements for window glass? Resistance to moisture Resistance to UV radiation Resistance to cleaning agents Very high transmission in the visible spectrum (There are many more, from an engineering point of view) Most transparent plastics are not resistant enough. Plexiglas is one example which fulfills the requirements enough to be used ...


30

Firstly, the incident happened as the train was leaving a siding passing a shunt signal. These provide less authorisation than a normal signal does, even when they are not at danger (the train can permit as far as the line is clear or the next signal, there is no guarantee the track ahead is clear). Now, in the UK, there are four train protection systems: ...


29

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 ...


25

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 ...


21

While I'm sure it caused less damage than a train collision Exactly. This is a last-resort response intended to avoid a full-on collision. A derailed train causes some damage and is a mess to clean up. A collision between two trains will likely cause much more damage, be even harder to clean up, and quite likely get people killed. It can also impact the ...


15

I actually did make windows out of plexiglas for an outbuilding. I could work them to the shape I wanted and they were lightweight. However, I discovered that they indeed scratch easily: just trying to get the protective paper off I both scraped and melted the plastic. We want hard windows for houses and primary buildings. So what you describe would be ...


13

That is a ring-grip quick-release pin, sometimes referred to as clevis pins. McMaster link


11

In the limit of the cars being identical and the wall being immutable, I would argue that the two situations are the same based on symmetry. Consider the collision of the two cars with no wall. Conservation of momentum implies that the end result is both cars at a stand-still. If they hit each other perfectly head on, the vehicles will buckle and absorb ...


11

Why derail? If the train went past a red signal, then all of the other safety systems already failed! The method of last resort is to make absolutely sure that the train will not travel down the track. That is done by derailing it. How did it get to this point? The real question is how did it get to the point of passing a red signal? At that point, ...


10

After some research, we are using glass windows as it was used traditionally. Actually plastic windows exists. Though it may take time for most of us to adapt to it. While the reasons to consider glass windows are: Up to 80% of all recycled glass can be reclaimed. Recycled glass uses 40% less energy than manufacturing new glass. Recycling doesn’t ...


10

Almost every manufactured product can be abused in an unsafe way by a creative enough user. If the customer asked your opinion on doing something which is evidently unsafe, I would suggest (on the grounds of common sense, not on the laws of any particular country) that you put your objection in writing, so there can be no future argument in court about "who ...


10

In general a modern commercial/military explosive device will not explode if dropped. The most common commercial/military explosives such as C4, TNT, ANFO are catagorised either as Secondary or Tertiary explosives, which means they are fairly insensitive to shock/fire and require a severe shock (usually from a detonator) in order to explode. Tertiary ...


8

xkcd has a fantastic description of how lightning works. This video is also quite good (especially as of 0:56, but the rest is worth watching as well) since it shows lightning in super slow motion, so you can see it "searching" for a path down to earth. The important thing here is to learn that lightning searches for a path in steps. Xkcd mentions 60-meter ...


8

This is a pretty huge topic, but I can summarize a couple major factors. More population, more damage, more awareness The effect of a disaster that kills thousands of people and causes property damage in the billions of dollars (current equivalent) is felt much more intensely than that of a disaster of similar magnitude that affects a much smaller ...


8

The most straightforward way would be to mount them between flanges, after a section of pipe has been valved off and flushed. This is correct. Blanking spades, also known as slip plates, slip blinds or spectacle blinds, are usually slipped in between two flanges after a process line has been double blocked and bled. They are usually secured in place by the ...


8

It's a combination of 1, 3, 4, the product development cycle, and the nature of the product. 1 - Damage in materials is a cumulative process, the product of hundreds or thousands of little events (e.g. the tightening and loosening of the belts) or one big event (a crash). So, the product has to be designed to always have a certain damage capacity ...


7

Another issue is that the coefficient of thermal expansion of many plastics is higher than that of window glass. Frames for plastic windows would have to be able to accommodate more expansion / contraction than those for glass windows. http://www.eplastics.com/Plastic/plastics_library/Coefficients-of-Thermal-Expansions-of-Plexiglass http://www.sdplastics....


7

Why would a train automatically derail if a signal is passed at danger? When the cost of derailment is lower than the cost of a train collision, and the possibility of train collision exists, then a train derailment device may be used. A red signal means danger, stop. If a train continues past that point it is likely to encounter another train or similar ...


6

Products can be given a UL Listing Mark or the can be classified as a UL Recognized component. The simplest way to think about the difference between these two is that a UL Listing Mark is meant for products that are "complete products." Things like appliances or equipment that you could, for example, buy in a store. A UL Recognized Component, on the ...


6

I am skeptical as to whether this component needs to be grounded at all; it's not conductive, so my intuition is that it isn't able to cause a shock. My question is basically (1) can a PVC pipe cause a static discharge Don't confuse conductivity with ability to hold charge. If you've ever rubbed a balloon against your hair to make it statically cling to ...


5

A common refrain in these kind of discussions is "there is no such thing as safe, only safer." Nothing we make is perfectly safe in an absolute manner - we don't design buildings to withstand asteroid impacts and we don't design cars to survive falling off a bridge. Instead, when we say safe, we usually mean "safe enough" - the level of risk is acceptable ...


5

For something to be safe there must be no risk of injury, harm, disability or death to anyone when something is being assembled, used or in the future as a result of the something being used. Where anyone is defined as any person - whether they be employee, government or military official or member of the public. Local laws may qualify this by setting ...


5

I'm guessing you haven't seen the Mythbusters episode on this. Then, see this writeup that explains why they are correct, since Mythbuster's explanation leaves something to be desired. In short, it's exactly the same. In the 2 car example, each car has the same amount of energy (as each other, and as the car in the 1 car example) since they are all going ...


5

The answer will depend on the wall, and on the other car. Consider the comparison to an ideal, immovable wall. In the case of a hypothetical collision between a Humvee (2500 KG) and a VW Beetle (850 KG), for the Humvee "vs. car" is better, whereas for the Beetle "vs. wall" is better. Now consider the comparison to a paper thin, soft wall. For both the ...


5

I have tried, but have not found a way to respond to this question that is not opinion based. By their nature, a person's ethics are just that; a person's ethics. They are usually deeply held, personal beliefs. Other words for ethics might be morals, values, conscience, etc., again, these are not necessarily universal in their importance. They are personal. ...


5

Tell him to add a proper (double pole) shutoff switch and educate the techs to use it when replacing motors. (Or make a mechanical interlock so the exposed contacts are never live) This will make it actually safe because when it's disconnected the accessible contacts will not be connected to the power source. In general try to give a safe alternative. ...


5

A derailer is used rather than a device to trip the brakes because there is no guarantee there are any brakes to trip. These things are not intended to stop trains moving under power with their air compressors pounding and their brake reservoirs at pressure. They are intended to stop loose strings of cars which have started rolling and are headed for the ...


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