What Happens When Air is Drained from a Hot Air Balloon with a Solid Shell?

[Zetan]

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."

 

[sophiecentaur]

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)

 

[russ_watters]

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.

 

[Zetan]

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 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?

 

[Zetan]

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

 

[A.T.]

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.

 

[Zetan]

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!

 

[Zetan]

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 traveling at 30mph and then collides with the stationary rock, would both rocks end up traveling at 15mph because of the equal opposite force from the stationary rock slowing the 30mph rock to 15mph?

 

[jbriggs444]

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 traveling at 30mph and then collides with the stationary rock, would both rocks end up traveling 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]

 

[Zetan]

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?

 

[jbriggs444]

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.

 

[Zetan]

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.