Air density, vacuum and gravity

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  • #51
sophiecentaur
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What creates the mutual attraction?
The electric forces between the charges in adjacent molecules produce bonding between adjacent molecules. A molecule may (will) have neutral charge but the arrangement of the charges can mean that a nearby molecule will find itself closer to a net negative charges than to positive net charge. This is the basis of why liquids are liquids and solids are solids.
(This post is nowhere near enough as a proper description of the situation; it is just a hint in the right direction.)
 
  • #52
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It doesn't need to be expelled. Same weight with more volume means less density.

Ah yes, thanks.
That must mean that when the balloon is inflated to the point where neutral buoyancy is achieved, the balloon material must be able to expand further for the volume to increase to achieve lift off then?

You'll be glad to hear that I am not a hot air balloon pilot! :eek:
 
  • #53
sophiecentaur
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You may find this interesting.
 
  • #54
russ_watters
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It doesn't need to be expelled. Same weight with more volume means less density.
Hot air balloons aren't very elastic: when you heat the air inside, some is expelled through the bottom and the volume remains roughly constant.
 
  • #55
A.T.
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Hot air balloons aren't very elastic: when you heat the air inside, some is expelled through the bottom and the volume remains roughly constant.
Yes, but during the initial inflating the volume increases. The point was that in general, density matters, not the total weight.
 
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  • #56
That's what I wanted to know, thanks. It was thinking about why warm air rises that started me off. I suppose as the air inside of a hot air balloon warms up and expands it is expelled through the hole at the base until there is less weight of air inside?

Essentially, that is correct but instead of thinking about "weight of air", think about density, the mass per unit of volume. Warm air has lower density than cold. And, if you've ever seen a hot air balloon being filled, you'll understand that air isn't actually expelled from the bottom. This is because the balloon is laid out on the ground empty and the propane heaters are started in order to fill the balloon
 
  • #57
russ_watters
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I don't think the precise sequence of the inflation (when, exactly, they start heating while filling) is critical to the question and can lead to misleading interpretation of the normal operation of the balloon:

When in normal flight, the volume of the balloon is fixed and the pilot controls the mass of the balloon by adding or removing air mass by heating it (removing mass) or alowing it to cool (adding mass).

This is different from most helium balloons, which have a fixed mass (over the short term) but a volume that changes with altitude. This means that in general the hot air balloon has a max altitude which it will reach and stabilize at, while the helium balloon may just rise until it over inflates and pops.
 
  • #58
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It's probably my fault for not explaining exactly why I wondered whether a vacuum would have a similar lifting effect to warm air, as I didn't know about buoyancy and why less air mass would lift the weight of the entire balloon and basket, etc. It has ended up even more interesting though, learning about all sorts of other branches of the subject as well.
I originally wondered what the warm air was doing exactly and pondered on what would happen if you took "less air mass" to the extreme and removed all of it, eliminating air altogether (in an imaginary situation where it were possible to do that of course).
I have plenty to read about now though, thank you.
 
  • #59
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Another question I have is (this might need a new thread?): If you have two balls exactly the same size, one made of lightweight foam and the other of steel, why do they fall at the same speed in a vacuum? When you hold them in each hand, why does it take less effort to hold one up than the other and if you caught them as they were falling?
 
  • #60
sophiecentaur
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Another question I have is (this might need a new thread?): If you have two balls exactly the same size, one made of lightweight foam and the other of steel, why do they fall at the same speed in a vacuum? When you hold them in each hand, why does it take less effort to hold one up than the other and if you caught them as they were falling?
The effect of gravity is to produce the same acceleration on all masses (assuming the masses are very small compared with the planet. That's because the weight force is mg and the acceleration that the weight force produces on a mass m is g. The m's cancel. If you are holding them, however, you are experiencing the weight forces, which will not be the same.
 
  • #61
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The effect of gravity is to produce the same acceleration on all masses (assuming the masses are very small compared with the planet. That's because the weight force is mg and the acceleration that the weight force produces on a mass m is g. The m's cancel. If you are holding them, however, you are experiencing the weight forces, which will not be the same.

Interesting, thanks. I had to change the image in my mind of objects "dropping", to one of objects being pulled, from a standstill by the same force?

Came across this on Yahoo to:

"The keyword is INERTIA. It's a tendency of an object to conserve its momentum: a stationary object will try to stay stationary; a moving object will try to carry on moving in the same direction. Inertia is proportional to the object's mass.

The reason a hammer will not fall faster than a feather, is because it's got stronger inertia and will require more force to be moved. This difference in inertia of the hammer and the feather balances out their difference in mass."
 
  • #62
sophiecentaur
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Personally, I can see no point in using the word 'inertia' in this sort of discussion because the word 'mass' fits in its place. If one says that it's the 'Inertia' of a floating barge that makes it hard to shift, surely you could say it's Mass is what counts. F=ma describes how an object "tries to stay stationary" (but I am allergic to anthromorphisms, personally)
 
  • #63
russ_watters
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Personally, I can see no point in using the word 'inertia' in this sort of discussion because the word 'mass' fits in its place.
Personal preferences are what they are, but personally I find it adds clarity to mention the sub-properties of mass separately. Mass is an amount of matter, but what does it do? It occupies volume, absorbs heat and in this case, resists acceleration.
 
  • #64
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Personally, I can see no point in using the word 'inertia' in this sort of discussion because the word 'mass' fits in its place. If one says that it's the 'Inertia' of a floating barge that makes it hard to shift, surely you could say it's Mass is what counts. F=ma describes how an object "tries to stay stationary" (but I am allergic to anthromorphisms, personally)

Yes, that's what happens when one looks on Yahoo for information :smile: Probably best avoided. Although some of it helped.
Just to check - is it right to say that it is the 'Inertia' of a moving barge that makes it 'hard' to stop? In other words, a 'stationary' object has no inertia?
 
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  • #65
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Personal preferences are what they are, but personally I find it adds clarity to mention the sub-properties of mass separately. Mass is an amount of matter, but what does it do? It occupies volume, absorbs heat and in this case, resists acceleration.

It definitely helps build a clearer overall picture
 
  • #66
A.T.
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F=ma describes how an object "tries to stay stationary"
In other words, a 'stationary' object has no inertia?
I'm puzzled how you arrived at this interpretation of what sophiecentaur wrote.
 
  • #67
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I'm puzzled how you arrived at this interpretation of what sophiecentaur wrote.

Because this is all new to me and dare I say it, I had no idea what F=ma described. I should have looked that up first or asked. However, I didn't want to test everyones patients to much!

I have just looked up the definition of inertia and F=ma which has made it slightly clearer!
 
  • #68
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One last question to clarify if I have understood this or not!:

If there were 2 rocks in space, exactly the same mass, one of them is travelling at 30mph and then collides with the stationary rock, would both rocks end up travelling at 15mph because of the equal opposite force from the stationary rock slowing the 30mph rock to 15mph?
 
  • #69
jbriggs444
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One last question to clarify if I have understood this or not!:

If there were 2 rocks in space, exactly the same mass, one of them is travelling at 30mph and then collides with the stationary rock, would both rocks end up travelling at 15mph because of the equal opposite force from the stationary rock slowing the 30mph rock to 15mph?
Yes, that's right.

[Assuming they stick together rather than bouncing off]
 
  • #70
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Yet another question:
If all matter is moving through space, is there such a thing as a truly stationary 'object', how would we know it was with nothing stationary to measure it against?

In other words - If an object in space is described as being in a complete state of rest, what is that relative to? Is there anything that is in a complete state of rest?
 
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  • #71
jbriggs444
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Yet another question:
If all matter is moving through space, is there such a thing as a truly stationary 'object', how would we know it was with nothing stationary to measure it against?

In other words - If an object in space is described as being in a complete state of rest, what is that relative to? Is there anything that is in a complete state of rest?
No. There is no such thing as a complete state of rest. The laws of physics do not allow for any experiment to detect a state of absolute rest. They can only detect a state of rest relative to something.

One way of stating this principle is that the laws of physics are "invariant" with respect to choice of rest frame. That is, you can pick any non-accelerated object that you like and consider it to be at rest. The frame of reference in which this object is at rest is as good as any other such frame. The laws of physics work identically in all of them.
 
  • #72
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No. There is no such thing as a complete state of rest. The laws of physics do not allow for any experiment to detect a state of absolute rest. They can only detect a state of rest relative to something.

One way of stating this principle is that the laws of physics are "invariant" with respect to choice of rest frame. That is, you can pick any non-accelerated object that you like and consider it to be at rest. The frame of reference in which this object is at rest is as good as any other such frame. The laws of physics work identically in all of them.

Thank you, that's given me a nice clear picture.
 

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