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Momentum and Energy

  1. Apr 30, 2008 #1
    1. The problem statement, all variables and given/known data
    An isolated system is initially at rest. Is it possible for parts of the system to be in motion at some later time? Explain.

    2. Relevant equations

    3. The attempt at a solution

    My guess is no because if the system is at rest, the only way the system could get into motion is if some outside force acts on it. Can anyone tell me if im right??
  2. jcsd
  3. Apr 30, 2008 #2
    What would happen if I took a glass of icewater and put it in an well-insulated box full of warm air? The system would be isolated, but you'd have quite a hard time convincing me that the glass would be exactly the same if I took it out after a few hours.
  4. May 1, 2008 #3
    I'm not entirely sure what you mean by that. Are you saying that parts of the system are in motion at some later time?
  5. May 1, 2008 #4
    Okay, consider the picture below:

    There is a block of some material placed atop a chunk of ice that prevents the block from falling into the cup. The temperature of the air in the box is much higher than the melting point of ice. We assume the box is well-insulated, so there is no heat transfer with the surroundings.

    What happens to the ice as time goes on?
  6. May 1, 2008 #5


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    That's kind of an exotic example. But I see why you suggested it. Just take a stretched spring and put it far away from any forces. Now let it go. Initially it's at rest. Later?
  7. May 2, 2008 #6
    So even though the spring is at rest, when you release it, the spring is no longer at rest, therefore the answer to the question is yes it is possible for parts of the system to be in motion at some later time. Is this right?
  8. May 2, 2008 #7


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    When you release it, the spring is at rest. After the release, parts of it start to move, yes. I suspect this question is concerning conservation of momentum. The center of mass of the whole system can't move. But the parts can move.
  9. May 2, 2008 #8


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    Indeed. Imagine, say, a person floating in space with a bowling ball in his hands. Now he can throw the bowling ball and will begin moving! No external forces. In essence, this question sets the basis for momentum conservation on a type of problem called explosion, where initial momentum is zero.
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