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Men in a box (very simple question)

  1. Jan 3, 2010 #1
    there is a box with wheels and man inside it.my teacher is said there is no way to man move the box while his inside. i said opposite. i know im right but i cant prove. can you help me?

    sry for bad eng.(not my natural lang.)
  2. jcsd
  3. Jan 3, 2010 #2


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    Staff Emeritus
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    Then how do you know you're right? Why do you think you're right?
  4. Jan 3, 2010 #3
    It depends what you mean by "move".
  5. Jan 3, 2010 #4
    i mean replace or something. box has wheels. i think im right cos i can turn myself in my chair without touching the ground. i think if man jumps forth and back or hit the sides with shoulder box may move.
  6. Jan 3, 2010 #5

    Doc Al

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    Question for you: If the center of mass of the "men+box" system is to accelerate, there must be an external force acting. What provides that force?
  7. Jan 3, 2010 #6
    there is no external force as far as i know.
  8. Jan 3, 2010 #7

    Doc Al

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    Staff: Mentor

    Right. Assuming no friction, there is no external force available to move the center of mass. (Note that you cannot ignore friction in your example of you and your chair.)
  9. Jan 3, 2010 #8
    Neglecting friction, the center of mass of the box/man system will remain in the same place. By moving inside the box the man can change the center of mass and thus the box's location.

    For some fixed values of friction the man can take advantage of the fact that the static friction is typically higher than the moving friction and "store" that extra force over time by moving in a jerking manner.

    If he accelerates slowly to one side the box will not move if the force of his acceleration is less than the static friction force. Then he can accelerate (decelerate) fast enough to slide the box a little.

    The net forces add up to zero over time, but for short periods they could exceed the static friction forces in only one direction.
  10. Jan 3, 2010 #9
    i understand now. thanks.
  11. Jan 3, 2010 #10


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    The distinction you may want to make with your teacher is whether the two of you are talking "real world" or "ideal world". In the ideal world, no, the man in the box cannot make it move. In the real world he can take advantage of friction to get the box to move.
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