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Homework Help: Momentum of child running problem

  1. Sep 20, 2007 #1
    Momentum...please help!!

    1. The problem statement, all variables and given/known data

    A child of mass 31 kg sits on a wooden horse on a carousel. The wooden horse is 4.2 m from the center of the carousel, which rotates at a constant rate and completes one revolution every 6.7 seconds.

    What are the magnitude and direction of the perpendicular component of dp/dt for the child

    2. Relevant equations

    p = mv

    3. The attempt at a solution

    speed = d/t = 3.9 m/s
    p = mv = 31 kg*3.9 m/s = 12.09 kg*m/s

    But I don't know how to compute the components of dp/dt? I know the parallel component is 0 and has no direction, so what would the perpendicular one be? Thanks for the help!!!
  2. jcsd
  3. Sep 20, 2007 #2


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    Staff Emeritus
    Science Advisor

    Well, dp/dt is a force.

    Think of the centripetal/centrifugal forces.
  4. Sep 20, 2007 #3
    so is it essentially the components of the centripetal force?
  5. Sep 20, 2007 #4


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    Staff Emeritus
    Science Advisor

    Essentially that.

    The acceleration comes from the change of the direction of velocity rather than change of the tangential speed. dp/dt = m dv/dt where m is the constant mass, and the speed of v is constant, but the vector changes direction constantly.

    The horse holds the child, who would otherwise fly off the merrygoround tangentially. The horse and child are in static equilibrium - the child's mass imposes a force on the horse which imposes an equal force back on the child holding him or her in place.

  6. Sep 20, 2007 #5
    so dp/dt = m *dv/t = 31 kg * 3.9 m/s / 6.7 s = 18.03 kg*m/s/s. how do I get the components out of this?
  7. Sep 20, 2007 #6
    nevermind...i finally got it :-) thanks!!!
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