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Homework Help: Collision and impulse

  1. Mar 27, 2008 #1
    [SOLVED] collision and impulse

    1. The problem statement, all variables and given/known data
    A 1.0 kg ball drops vertically onto a floor, hitting with a speed of 20 m/s. It rebounds with an initial speed of 5 m/s.
    (a) What impulse acts on the ball during the contact?

    (b) If the ball is in contact with the floor for 0.020 s, what is the average force exerted on the floor?

    2. Relevant equations

    3. The attempt at a solution

    For (a) i used p=m*v and deltap=J

    so I got (1*-20)-(1*5)=-25

    and for (b) i used that F=(m*deltav)/deltat

    which came out to be... 1(-25)/.020=0.5 N

    both answers were wrong and I'm not sure what is wrong with the formulas I used...

    Any help is appreciated! Thanks!
  2. jcsd
  3. Mar 27, 2008 #2
  4. Mar 27, 2008 #3
    For a) the impulse is P_after - P_before and not P_before - P_after.

    For b) see The reply of Mikelepore. I think you need to add the force of gravity as well.
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