Ask a physicist anything. (7)

Tuddrussell said:

Why doesn't the chair trick work?

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Because jumping off the chair will won't decelerate you enough to negate the impact. You'd need to jump off the chair with enough force to completely counteract all your downward velocity, and, mechanically, such a jump would have the same effect as slamming into the ground: you would, at the least, break your legs.

^Better your legs than your head.

Tuddrussell said:

^Better your legs than your head.

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Your legs would catapult into your thoracic cavity. Kinda like being a human Transformer, only fatal

Wiccan_Child said:

Because jumping off the chair will won't decelerate you enough to negate the impact. You'd need to jump off the chair with enough force to completely counteract all your downward velocity, and, mechanically, such a jump would have the same effect as slamming into the ground: you would, at the least, break your legs.

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First of all you will have to calculate the distance of free fall to find the velocity with the formula: 10metres per second squared. Once you have your impact speed then you have to exert an opposite force that is equal to or slightly greater than the impact velocity. Basically you will end up experiencing an instant acceleration g force equal to or greater than the impact force and this will end up causing the same damage as if you impacted on the ground. It is all about deceleration! In order to absorb the energy you will have to decelerate quickly in order to dissipate the energy without it causing you any damage!

Don't try this at home!!!!!!!

Wiccan_Child said:

Because jumping off the chair will won't decelerate you enough to negate the impact. You'd need to jump off the chair with enough force to completely counteract all your downward velocity, and, mechanically, such a jump would have the same effect as slamming into the ground: you would, at the least, break your legs.

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There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. Pick a nice day, [The Hitchhiker's Guide to the Galaxy] suggests, and try it.

How to Fly, by Douglas Adams

The important part:
That is, it's going to hurt if you fail to miss the ground. Most people fail to miss the ground, and if they are really trying properly, the likelihood is that they will fail to miss it fairly hard.

Tuddrussell said:

Why doesn't the chair trick work?

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Well, it could actually help somewhat. But I see a number of problems:
1. The chair would have to be a significant fraction of your weight for pushing off of it to be of any use at all. For maximal use you'd need to jump off of an object much heavier than you, which isn't really feasible to take out a window with you.
2. It does increase the amount of time you're spending slowing down, so it might help to reduce injury. However, in order to prevent injury completely you'd have to push off from a much heavier object than a chair really, really hard. As in, hard enough that were you standing on the ground, a push of that magnitude could send you into the window you jumped from. So yeah, that isn't going to happen.
3. The chair itself is something else you can hit as you strike the ground, so it may actually end up increasing injury in the end.

My guess is that if you managed to avoid hitting the chair when you struck the ground, and you used a particularly heavy chair (or other object), you could slightly increase the amount you could fall before being injured. But probably not by much at all.

mzungu said:

So supercooled helium does not act the way they showed it on the video If the video I posted is a scam then please let me know (but explain it not just dismiss it)! I have no problem if superfluids do not exist because this will mean I have learnt something new and that is what science is all about!

Menander: Κάλλιστόν ἐστι κτῆμα παιδεία βροτοῖς.
'Education is the most valuable treasure for mortals.'

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No, the supercooled helium video was completely real

mzungu said:

It is all about deceleration! In order to absorb the energy you will have to decelerate quickly in order to dissipate the energy without it causing you any damage!

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Actually, it's the other way around: you need to spread out the deceleration as long as possible. A good way to think of this is in terms of impulse: in order to change momentum, you have to apply a force over time (called an impulse). So if you increase the time, you decrease the force, increasing survivability. This is why, for instance, modern cars are built to crumple dramatically in the event of a crash: the crumpling gives the passengers a longer time to slow down, which reduces their chance of injury.

The problem with the chair situation is that if you have two objects in free fall pushing off of one another, the lighter one absorbs most of the energy. So if the chair is lighter than you, you can push off of it, but your velocity will hardly change at all. You really need something heavier for it to help much.

Also another way is to design a chair that will fail on impact in such a way as to totally dissipate the energy and stop it from being absorbed by your body. It could start to splinter the legs after a certain amount of energy received and continue to break upwards until the point where the energy received by your body is harmless!

If you do take the Douglas Adam's course then DO NOT FORGET TO TAKE A TOWEL WITH YOU!

mzungu said:

Also another way is to design a chair that will fail on impact in such a way as to totally dissipate the energy and stop it from being absorbed by your body. It could start to splinter the legs after a certain amount of energy received and continue to break upwards until the point where the energy received by your body is harmless!

If you do take the Douglas Adam's course then DO NOT FORGET TO TAKE A TOWEL WITH YOU!

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Well, the problem there is that it's entirely about time. A chair is just too small an object to take much time to crumple, so it can't absorb much of the energy, no matter how intelligently-designed.

Chalnoth said:

Actually, it's the other way around: you need to spread out the deceleration as long as possible. A good way to think of this is in terms of impulse: in order to change momentum, you have to apply a force over time (called an impulse). So if you increase the time, you decrease the force, increasing survivability. This is why, for instance, modern cars are built to crumple dramatically in the event of a crash: the crumpling gives the passengers a longer time to slow down, which reduces their chance of injury.

The problem with the chair situation is that if you have two objects in free fall pushing off of one another, the lighter one absorbs most of the energy. So if the chair is lighter than you, you can push off of it, but your velocity will hardly change at all. You really need something heavier for it to help much.

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When I said "Quickly" I meant quick enough so that you do not impact the floor with a force capable of damaging you but not "Quick" enough where the the acceleration of jumping off the chair will also damage you. Either way it cannot be done!: http://dsc.discovery.com/videos/mythbusters-elevator-of-death-minimyth.html

mzungu said:

When I said "Quickly" I meant quick enough so that you do not impact the floor with a force capable of damaging you but not "Quick" enough where the the acceleration of jumping off the chair will also damage you. Either way it cannot be done!: MythBusters: Elevator of Death MiniMyth : Video : Discovery Channel

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Love it! I hadn't seen that myth before, but it's a great one

And it illustrates just how little you can help yourself by jumping, because in this case what you're jumping from does have much more weight than you, so you do genuinely gain some of the energy from your push-off. Unfortunately, it isn't going to be much: at most you'll slightly reduce your injury, so maybe it would increase the amount you can fall before dying by a few feet, but no more.

The problem with the elevator, however, is how would you push off? Once the elevator is released, it's going to be falling at near free-fall, and you're basically going to be floating weightless. Your ability to be on the floor of the elevator to push off at all is going to be minimal at best. And then, how would you know when to push off?

mzungu said:

Also another way is to design a chair that will fail on impact in such a way as to totally dissipate the energy and stop it from being absorbed by your body. It could start to splinter the legs after a certain amount of energy received and continue to break upwards until the point where the energy received by your body is harmless!

If you do take the Douglas Adam's course then DO NOT FORGET TO TAKE A TOWEL WITH YOU!

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It might work as a parachute (I can't believe that's not on H2G2!)

mzungu said:

First of all you will have to calculate the distance of free fall to find the velocity with the formula: 10metres per second squared. Once you have your impact speed then you have to exert an opposite force that is equal to or slightly greater than the impact velocity. Basically you will end up experiencing an instant acceleration g force equal to or greater than the impact force and this will end up causing the same damage as if you impacted on the ground. It is all about deceleration! In order to absorb the energy you will have to decelerate quickly in order to dissipate the energy without it causing you any damage!

Don't try this at home!!!!!!!

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Karate...

Very good documentary by the BBC. Enjoy:
Also here: http://topdocumentaryfilms.com/seeing-stars/
Extreme Astronomy - Horizon: Seeing Stars. 720p - YouTube!

Would you please explain blackbody radiation in your own words?

AV1611VET said:

Would you please explain blackbody radiation in your own words?

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Black body radiation is radiation emitted from a black body, so the first question is, what's a black body?

Basically, it's anything that absorbs all light that falls on it. They're abstract objects that don't exist in reality, but there are a number of objects that are close enough, or which act in the same way as black bodies. The empty backdrop of space, the blackness between stars in the sky, is basically a black body, because it absorbs all light - insofar as light shone into the night sky doesn't reflect back, so it's like it's all been absorbed.

The key feature of a black body is that any light emitted by it is purely a product of heat. It absorbs incoming light, but its temperature still makes it shine. The light emitted can be plotted as a graph of its frequency (from microwaves and infra-red, through visible and ultra-violet, to X-ray and gamma ray) against the intensity of each type of light:



So if you have something that is, or operates like, a black body, then its spectrum will look like one of those curves. We can thus work out its temperature very accurately by looking at the outputted light.

You may have heard of the CMBR, or the Cosmic Microwave Background Radiation. As I mentioned, the blackness of space can be treated as a black body, so any light 'emitted' by it must, according to the Big Bang theory, fit a black body curve. More than that, the theory predicts which curve it should fit (namely, it should fit the '2.75K' curve). When we measured its spectrum and plotted on the same graph as the predicted black body spectrum of that temperature, this is what we got:



As the graph states, theory and observation agree. This is one of the single most impressive pieces of evidences in all of science (in my humble opinion).



So, what is black body radiation? It's the idealised spectrum of light emitted by an object by virtue of its heat alone (and not due to, say, the reflection of light being shone at it, whether due to total absorption of light, or the lack of any such light). Some objects are pretty close approximations of a black body (such as the Sun), and some aren't objects but operate like a black body (such as the CMBR), so the concept is very useful in astrophysics.

Thank you, sir!

Another question -- (okay, two):

If I have the windows in my car up, and sunlight hits the dashboard, is it blackbody radiation that heats the interior of my car?

And in those graphs you posted, does "K" stand for "Kelvin"?

AV1611VET said:

Thank you, sir!

Another question -- (okay, two):

If I have the windows in my car up, and sunlight hits the dashboard, is it blackbody radiation that heats the interior of my car?

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Yes and no. The Sun is a black body, so its light is black body radiation, so the heat is ultimately from black body radiation - but reason it's noticably hot when you get in is more due to the still air and a sort of mini greenhouse effect. It is black body radiation, but that's incidental - any IR/visible-heavy spectrum of light would do the trick.

AV1611VET said:

And in those graphs you posted, does "K" stand for "Kelvin"?

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Yes. Kelvin is the standard scientific unit of measurement for temperature, where 0K = -273.15°C.

Thank you --