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 Jun 20th 2008, 09:52 PM #1 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 Newton's third law For application of newton's third law, we mostly applied it in concrete objects. But can it be applied when it is (1) Gas? (2) Liquid? (3) Fluid which fills up the whole container?
Jun 21st 2008, 05:38 AM   #2

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 Originally Posted by werehk For application of newton's third law, we mostly applied it in concrete objects. But can it be applied when it is (1) Gas? (2) Liquid? (3) Fluid which fills up the whole container?
I'm not sure what this question is going for? You can apply Newton's 3rd in all of these cases to talk about different things. Is there a more specific part to this question or does it want examples of how Newton's 3rd might be used for each possibility?

-Dan
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 Jun 21st 2008, 08:17 PM #3 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 Yes, I would like to know some example of application of newton's third law for fluids. Meanwhile, when newton's third law cannot be applied. And sometimes for fluids, it would be compressed and it may flow around. Compressibility of fluids won'r affect the application of newton's third law,right? If I were in vacuum waving my hands, would there be an reaction force that I would experience like that in air? If not, does it mean that newton's third law requires existence of "something" but there is no such "thing" in vacuum? What about antimatter, would it be considered as a the "thing" for Newton's third law to be applied? Last edited by werehk; Jun 21st 2008 at 08:27 PM.
Jun 22nd 2008, 07:47 PM   #4

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 Originally Posted by werehk Yes, I would like to know some example of application of newton's third law for fluids. Meanwhile, when newton's third law cannot be applied. And sometimes for fluids, it would be compressed and it may flow around. Compressibility of fluids won'r affect the application of newton's third law,right? If I were in vacuum waving my hands, would there be an reaction force that I would experience like that in air? If not, does it mean that newton's third law requires existence of "something" but there is no such "thing" in vacuum? What about antimatter, would it be considered as a the "thing" for Newton's third law to be applied?
If I am understanding you correctly I think what we have is a question about "contact forces."

Newton's 3rd sets up a correspondence between two forces, one provided by object A and another provided by object B. For example, take two boxes A and B resting on a horizontal surface. If we use a force to push A into B, why does B move? When we push on object A we are exerting a normal force from it onto object B. This normal force is what gives B a shove. But Newton's 3rd says that there is an equal and oppositely directed force to the force by A on B. So there is a normal force by B on A as well.

This concept applies also to liquids and gases. When a molecule (or atom) strikes the wall of the container its in it applies a force to the wall which we can measure in terms of pressure. But the wall also exerts a force on the molecule, making it bounce away. So when you wave your arms around in the air you will feel a force on them. (Not so much if you are standing still, but stick your arm (carefully!) out the open window of your car while on the highway...)

Why does this not apply to outer space? Because there are no gas molecules to bounce off your arms to create a third law reaction force.

So far as I know Newton's 3rd has been proven in every situation it has been measured in. I know of no case where the law does not apply.

-Dan
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