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I Confusion over 2nd law and conservation of momentum

  1. May 10, 2017 #1
    Hi everybody, I apologize if something like this has been asked before but I have been unable to find an answer through searching.

    If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum. How is it that the cart has accelerated without a horizontal force?

    This issue originally came up while discussing rotational motion. If a figure skater is spinning and brings his hands in he will speed up. How can he accelerate without any torque?

    There must be something very basic that I am missing here, so thank you to anybody who can help with an answer.
  2. jcsd
  3. May 10, 2017 #2


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    2017 Award

    Staff: Mentor

    It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.
    There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..
  4. May 10, 2017 #3
    I knew there must be a simple answer. Thanks for that. Now, what if that weight is taken off by carefully lifting it straight up?
  5. May 11, 2017 #4
    What do you mean by "carefully"? The answer may depend on this.
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