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Momentum of ball bouncing off crossbar

  1. Feb 25, 2013 #1
    1. The problem statement, all variables and given/known data

    A soccer ball with a mass of 416 g bounces off the crossbar of a goal and is deflected upward at an angle of 56.7° with respect to horizontal. Immediately after the deflection, the kinetic energy of the ball is 46.9 J. What are the vertical and horizontal components of the ball's momentum immediately after striking the crossbar? (Let the horizontal direction be the x-direction and the vertical direction be the y-direction.)

    2. Relevant equations

    P = mv
    KE = 1/2mv2

    3. The attempt at a solution

    KEf = 0.5 * m * v2
    46.9 = 0.5 * 0.416 * v2
    v = 15 m/s

    P = mv
    Px = 0.416 kg * 15 m/s * cos 56.7 degrees = 3.43 kg m/s
    Py = 0.416 kg * 15 m/s * sin 56.7 degrees = 5.22 kg m/s

    Any help is appreciated. Thanks.
    Last edited: Feb 25, 2013
  2. jcsd
  3. Feb 25, 2013 #2


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    You should keep units everywhere, but I don't see where you need help.
  4. Feb 25, 2013 #3
    Ah sorry... I got those two values for the momentum, they were one of the options but was marked as wrong.
    Last edited: Feb 25, 2013
  5. Feb 25, 2013 #4


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    I get the same answers as you - assuming the ball's not spinning.
  6. Feb 26, 2013 #5
    Oh, thanks. We haven't covered angular momentum yet so shouldn't be... i think. I'll have to ask the instructor next class.
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