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Homework Help: Conservation question really quick

  1. Jan 22, 2010 #1
    Two identical carts are held at rest. Between the two carts is a compressed spring of negligible mass. When the carts are released, the spring expands and the carts move in opposite directions. Which of the following statements is not true?

    I. Total momentum of the system is conserved.
    II. Kinetic energy of the system is conserved.
    III. Total mechanical energy of the system is conserved.

    _II and III only _II only _I only _III only _I, II, and III

    What is the answer?

    I thought it was only III since the velocity changes. However choosing only III was marked wrong.
  2. jcsd
  3. Jan 22, 2010 #2
    Presumably, your system is both blocks and the spring. If that is the case, are there any external forces on the system? The sum of the external forces gives you the change in momentum with respect to time.

    Also note that if the spring is not part of the system your answer to the question will be different.
  4. Jan 22, 2010 #3
    so would the answer be I,II, and III
  5. Jan 22, 2010 #4
    No, at the moment when the blocks are released, they have no velocity and no kinetic energy, but at a later time they are moving. Some of the energy at any time may be in the form of potential energy in the spring, but kinetic + potential (both forms of mechanical energy) is constant
  6. Jan 23, 2010 #5
    I know the answer is not only III since i got it wrong, so is it II and III?
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