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Old Nov 8th 2015, 04:09 PM   #21
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OK so, before you answer, here is something to think about.

There are two ways in which energy is transferred from one thing to another.

1) Heat energy can is transferred when there is a temperature difference.

2) A force can do work.

Now there is very little temperature difference ebtween the air and the shuttle as it approaches rentry so energy loss by heat transfer will be small.

So we are left with work.

Work is done when a force moves its point of application. Work = force times distance.

Well the shuttle is travelling very fast and it moves a long way so even a small force will result in a large transfer of energy.

The forces of friction on the shuttle and the atmosphere are equal and opposite so the work done will be the same on each.
Now the atmosphere, although thin, is enormous compared to the shuttle.
So the heating effect of the friction will be spread out over a huge mass of air, but concentrated in the much much smaller mass of the shuttle.
So the temperature of the shuttle rises more than that of the atmosphere.

(This is the same as saying that equal quantities of heat make a kettle hotter than a swimming pool.)

Now consider the impact effect of the shutle on the air. The work done is P x change in volume.
But the change in volume of the enormous atmousphere is negligable so the compression work done is small.

OK so the same amount of work is done on both the atmosphere and the shuttle.

But where does the energy come from to effect this work?

Is there any significant change in the atmosphere during and after passage of the shuttle?
No.
Then there is not significant change in the energy content of the atmosphere ie it does not come from there.

But the shuttle initially has large amoun.ts of both kinetic energy and (gravitational) potential energy.

As the shuttle ploughs into the atmosphere is looses height and thus potential energy is converted into kinetic energy. This at least partially replaces the kinetic energy used to perform the frictional work.

So it is an ongoing dynamic process converting potential energy to kinetic energy to frictional heat.

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Old Nov 8th 2015, 04:52 PM   #22
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Sorry for putting a slight side track on this thread, but I want to comment on something Studiot mentioned a few posts back:
The upper atmosphere is so thin that it is almost exactly a perfect or ideal gas - This means that it does not heat up under compression.
I had not appreciated this subtlety.
Looking at the ideal gas equation:
PV=nRT
I take it that, for an ideal gas, as the pressure changes the volume change is exactly reciprocal, so the temperature does not need to change to keep this equation balanced.
Would I be correct in saying that the fact that the temperature does change in everyday experience (eg the bicycle pump) is that it is not an ideal gas, so the volume and pressure do not change in perfect synchronicity?

Coming back to the original thread, this means that friction is the key mechanism of energy transfer.

As a minor clarification, studiot indicates that there is no change to the atmosphere,
there is of course quite a major temporary local impact on the air, but this soon dissipates into the larger atmosphere as a whole.
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Old Nov 9th 2015, 02:33 AM   #23
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There are also secondary effects to consider.

I noted that there is little temperature difference to drive heat transfer, but after the shuttle heats up it radiates a substantial amount of energy.
Stephan's Law says that the heat radiated is proportional to the fourth power of the absolute temperature of a body so heating parts of it up 500 degrees plus creates a substantial radiator.

Then the passage of the shuttle is noisy. A not large, but non negligable amount of energy is transferred in generating sound.

MBW, the tyre analogy is poor because

1) The air in the tyre is confined. The atmosphere is esentially unconfined.

2) You are adding air mass when you pump a tyre. No mass is added to the atmosphere.

Last edited by studiot; Nov 9th 2015 at 10:35 AM.
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Old Nov 9th 2015, 05:16 AM   #24
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Red face Doh.

Thank you studiot, it's so obvious now you point it out.
I should perhaps have spent more time thinking before posting.
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Old Nov 9th 2015, 12:54 PM   #25
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Originally Posted by studiot View Post
Have you come across work or momentum?

Energy is transferred from the shuttle to the air body by means of momentum transfer.
The work done by the dissipative forces of friction heats both the air and the shuttle.
Hello, I'm going to reveal now physics as i understand it and make a fool of myself in the process. But at least you won't see me blush.

1. Momentum = mv

Momentum = mass of shuttle x velocity of shuttle

The mass of the shuttle will not be changing (because I don't think it's burning fuel but just gliding into the atmosphere).

2. Force = rate of change of momentum

F = (mv-mu)/t

The value for the FORCE will be negative because the shuttle initial velocity will be larger than the final velocity. Don't know what a negative force is !!!!


3.
Originally Posted by studiot View Post

So we are left with work.

Work is done when a force moves its point of application. Work = force times distance.

(This is the same as saying that equal quantities of heat make a kettle hotter than a swimming pool.)
So what is doing work? is the shuttle doing work because it is a force moving a distance W = Fd ?? Confused


4.

Originally Posted by studiot View Post

As the shuttle ploughs into the atmosphere is looses height and thus potential energy is converted into kinetic energy. This at least partially replaces the kinetic energy used to perform the frictional work.

So it is an ongoing dynamic process converting potential energy to kinetic energy to frictional heat.

Have you just revealed the answer to me in that last comment?

The energy transfer is from POTENTIAL ENERGY (because of the height of the shuttle from the Earths surface) to kinetic energy (as it travels downwards to the Earth) to FRICTION (between the air molecules).

So the shuttle loses energy to friction and slows down. Simple???? A one sentence that will give 10/10 marks ???

6.

Originally Posted by studiot View Post

The upper atmosphere is so thin that it is almost exactly a perfect or ideal gas - This means that it does not heat up under compression.
But the air does get thicker as the shuttle travels further into the atmosphere. The Boyles law will apply and the air will be compressed and heat will be created. So shuttle will lose energy to the air resistance it experiences when it compresses the air (PRESSURE??). The air converts the kinetic energy of the now faster moving air molecules into heat (or faster moving molecules just means heat???).

Pressure is inversely proportional to volume. I know the atmosphere is thin but it does get thicker nearer the earth and it travels so fast that it is compressed. The volume decreases and the pressure increases.

Pressure is proportional to temperature so as the pressure increases the temperature increases.

7.
Originally Posted by studiot View Post

The upper atmosphere is so thin that it is almost exactly a perfect or ideal gas - This means that it does not heat up under compression.
So how does friction occur if the air is so thin? It must be because the air will get thicker nearer the Earth



THERE ENDETH MY ANALYSIS. I know its long .... I'll be grateful for any comments.
The COLOURS and BOLD lettering is to make for easier reading.
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Old Nov 9th 2015, 02:17 PM   #26
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1) correct
2) if you consider the force as a vector, in other words the magnitude of the force and the direction of the force, then a negative force is just acting in the opposite direction to the positive force. Which direction is positive and which is negative is pretty much arbitrary, choose one and the other will follow.
3) The shuttle is doing work moving the air, the air is doing work slowing the shuttle, its part of the action and reaction thing.
4) Bingo
5) where is 5?
6) The air is thin, which means the distance between air molecules is relatively large, which in turn means that the interactions between neighbouring air molecules is small. I guess this is what makes it behave like an ideal gas.
However I don't think you should get too bogged down by this, it just means that heat generated by compression will not be a major factor.
7) The distance between air molecules is (relatively) large, but the velocity of the shuttle is huge, thus the number of air molecules hitting each square centimetre of the shuttle per second is still ample to generate significant friction.

This is a good example of how a seemingly innocuous question can lead you ever deeper down the rabbit hole of physics.
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Old Nov 9th 2015, 03:41 PM   #27
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Originally Posted by MBW View Post
1) correct
2) if you consider the force as a vector, in other words the magnitude of the force and the direction of the force, then a negative force is just acting in the opposite direction to the positive force. Which direction is positive and which is negative is pretty much arbitrary, choose one and the other will follow.
3) The shuttle is doing work moving the air, the air is doing work slowing the shuttle, its part of the action and reaction thing.
4) Bingo

6) The air is thin, which means the distance between air molecules is relatively large, which in turn means that the interactions between neighbouring air molecules is small. I guess this is what makes it behave like an ideal gas.
However I don't think you should get too bogged down by this, it just means that heat generated by compression will not be a major factor.
7) The distance between air molecules is (relatively) large, but the velocity of the shuttle is huge, thus the number of air molecules hitting each square centimetre of the shuttle per second is still ample to generate significant friction.

That summary made a lot of sense. Thank you

As for NUMBER 5 .... lost in a black hole
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Old Nov 9th 2015, 03:49 PM   #28
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Smile

Thanks for all the help. Thanks Studiot for revealing the answer.

Though I did not find the experience of having to work through a series of riddles to get an answer pleasant, I did learn lots of physics principles along the way. That's a good thing.

Have you noticed we're onto TWO pages of posts on this thread. Not many threads that lengthy .... well done team.


One more thing please. What is energy is created by FRICTION when the kinetic energy is transferred into friction? Is it just heat? Kinetic energy is converted to thermal energy due to friction with the air molecules. This causes the immense heat experienced by the shuttle on re-entry?
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