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Homework Help: Board and man sliding on ice problem

  1. Oct 28, 2012 #1
    1. The problem statement, all variables and given/known data
    A 250-kg board, 2.4 m in length slides broadside along the surface of ice with a speed of 21m/s . A 63 kg man at rest grabs one end as it goes past and hangs on as both he and the beam go spinning down the ice. Assume frictionless motion, and assume that the man can be regarded as a point mass. [Editor's note: that does not seem like a realistic speed for anybody to catch and hold on to the board! Work the problem anyway.] (Figure 1)

    Part A. How fast does the center of mass of the system move after the collision?

    Part B. With what angular velocity does the system rotate about its new center of mass? (Note that this is not the original center of mass of the board.)

    2. Relevant equations

    L=Iω, TE = 1/2mv^2 + 1/2Iω^2

    3. The attempt at a solution

    The answer to part A is 16.77 m/s. I found this by using cons of momentum.
    Is kinetic energy conserved in part B? Can someone set it up for me?
  2. jcsd
  3. Oct 28, 2012 #2


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    Hi Zach981! :smile:
    Certainly not … this is a completely inelastic collision.

    Use conservation of angular momentum :wink:

    (which, like ordinary momentum, is always conserved in collisions)​
  4. Oct 28, 2012 #3

    Simon Bridge

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    Where else would the energy go? Note that there is translational and rotational motion.
    Isn't angular momentum supposed to be conserved as well?
    Have you been introduced to center-of-mass coordinates yet?
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