Question about movement in isolated system

  • Thread starter Kenan
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  • #26
HallsofIvy
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Everyone has been telling you the same thing, again and again. The system, rocket and ejecta, does not move in the sense that the center of mass stays in the same place. Of course, you can't "see" the center of mass. What you see is the exhaust, or ejecta, moving very fast in one direction, the rocket itself moving more slowly in the opposite direction. Of course, if you are talking about "inside the rocket" then you are not talking about a "closed system" because the exhaust, that starts inside the rocket, is shot outside.
 
  • #27
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Everyone has been telling you the same thing, again and again. The system, rocket and ejecta, does not move in the sense that the center of mass stays in the same place. Of course, you can't "see" the center of mass. What you see is the exhaust, or ejecta, moving very fast in one direction, the rocket itself moving more slowly in the opposite direction. Of course, if you are talking about "inside the rocket" then you are not talking about a "closed system" because the exhaust, that starts inside the rocket, is shot outside.
I understand that center of mass does not move anywhere.
I'm curious about the motion of objects inside of the isolated system,i don't understand why some objects move and some objects don't in certain isolated systems.

Like Münhausen's body+his hair - momentum conserves and the objects are NOT moving anywhere,gun+bullet - momentum conserves and the objects ARE moving,how so?Because in first example forces are acting on the same object (on the body) and in the second example forces are not acting on the same object?

I'm really SORRY for bothering you guys,it's just I'm really curious about this,I know I sound really dumb,sorry guys.
 
  • #28
A.T.
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Like Münhausen's body+his hair - momentum conserves and the objects are NOT moving anywhere
If he pulls hard enough, his hair will definitely move too.
 
  • #29
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If he pulls hard enough, his hair will definitely move too.
But the fact is a fact right?
If forces will be acting on the same object then there will be no motion,but in gun or rocket case forces are not acting on the same object,no?!
 
  • #30
Doc Al
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I understand that center of mass does not move anywhere.
OK.

I'm curious about the motion of objects inside of the isolated system,i don't understand why some objects move and some objects don't in certain isolated systems.
An object will accelerate (begin to move) if there is an unbalanced force acting on it. It the case of the bullet fired from a gun, there is an unbalanced force acting on the bullet as it begins moving.

Like Münhausen's body+his hair - momentum conserves and the objects are NOT moving anywhere
His hair won't move since it is attached to his head! (If he really pulls it, it could tear off though.) And his body won't move because whatever force he pulls up with (on his hair) is balanced by the equal and opposite force that the hair pulls down with.

Always ask yourself: Is there a net force on an object? If so, it will accelerate. If not, it won't.
 
  • #31
HallsofIvy
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The "parts" inside an isolated system are not isolated!
 
  • #32
A.T.
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If forces will be acting on the same object then there will be no motion,but in gun or rocket case forces are not acting on the same object,no?
Depends on how you define the objects. You can consider Münchausen as arm & body to have two objects as well. Then analyze the forces between them to see what is actually different compared to gun & bullet.
 
  • #33
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OK.


An object will accelerate (begin to move) if there is an unbalanced force acting on it. It the case of the bullet fired from a gun, there is an unbalanced force acting on the bullet as it begins moving.


His hair won't move since it is attached to his head! (If he really pulls it, it could tear off though.) And his body won't move because whatever force he pulls up with (on his hair) is balanced by the equal and opposite force that the hair pulls down with.

Always ask yourself: Is there a net force on an object? If so, it will accelerate. If not, it won't.
Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
 
  • #34
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Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
I hope I'm right now,correct me if I'm wrong.
Thanks again.
 
  • #35
Doc Al
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Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
Close enough.

Have you read this page from the website you linked?: What's[/PLAIN] [Broken] an object and what isn't?
 
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  • #36
Drakkith
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Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
Yes. The net force on each 'piece' of the system is zero in the first example and non-zero in the rocket/gun example.
 

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