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Homework Help: Impulse Average Force

  1. Oct 14, 2013 #1
    1. The problem statement, all variables and given/known data

    A football player whose mass is 85 kg throws a football with a mass of 0.8 kg against a post. The football hits the post traveling horizontally with a speed of 25 m/s, and then it bounces back with about the same speed. The football is in contact with the post for 0.05 s. What average force is exerted on the post by the ball?

    m = 0.8 kg
    vi = 25 m/s
    vf = -25 m/s
    t= 0.05s

    2. Relevant equations


    F = [itex]\frac{m(vf-vi)}{Δt}[/itex]

    3. The attempt at a solution

    F = [itex]\frac{0.8(-25-25)}{.05-0}[/itex] = -800N by the post.

    Is the mass of the football player irrelevant?
  2. jcsd
  3. Oct 14, 2013 #2
    Why would the mass of the football player be relevant? All he does is imparts the initial kinetic energy to the ball. The same thing could be done with a 3,000kg piece of equipment. So yes, the football player's mass is irrelevant.
  4. Oct 14, 2013 #3
    So my calculations are correct without the football player's mass?
  5. Oct 14, 2013 #4
    Looks good to me.
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