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Homework Help: Merry-go-round => what is conserved?

  1. Apr 19, 2009 #1
    A boy of mass m = 40 kg running with speed v = 4 m/s jumps onto the outer edge of a merry-go-round of mass M = 160 kg and radius R = 1.8 m, as shown in the picture above. The merry-go-round is initially at rest, and can rotate about a frictionless pivot at its center. You may assume that the inital velocity of the boy is tangent to the edge of the merry-go round.

    Which of the following quantities are conserved throughout this problem for the system consisting of the boy and the merry-go-round?

    A) only kinetic energy
    B) kinetic energy and angular momentum
    C) only linear momentum
    D) linear momentum and angular momentum
    E) only angular momentum

    I believe the answer is D, linear momentum AND angular momentum because both are conserved regardless of an inelastic or elastic collision right?

    Can somebody please check out my answer and help me if it is wrong? Thank you very much in advance.
  2. jcsd
  3. Apr 19, 2009 #2
    Wait, wouldn't only the angular momentum be conserved since the boy's velocity changes after contact with the merry-go-round?

    So only the angular momentum is conserved?
  4. Apr 19, 2009 #3


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    It's an inelastic collision, so there's goes thinking kinetic energy would be conserved.

    For linear momentum, when the boy contacts the rim of the go-round there is a force coming from the hub that resists the linear motion, but not the angular momentum. I think your choice of angular momentum only would be the correct answer.
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