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Homework Help: Force on a bouncing ball?

  1. Mar 25, 2009 #1
    Force on a bouncing ball???

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

    this should be easy but for some reason im making a mistake here is the problem

    A ball whose mass is 0.2 kg hits the floor with a speed of 5 m/s and rebounds upward with a speed of 4 m/s. If the ball was in contact with the floor for 1.5 ms (1.5 E -3 s), what was the average magnitude of the force exerted on the ball by the floor?

    The attempt at a solution

    what i did was use delta(P) = Fnet * delta(t)
    change in momentum equals net force times change in time

    i can find change of momentum of the ball by

    P3 = ((m (+-) M) / (m + M)) * P1
    P1 is inital momentum of ball so just m*v .2 * 5 = 1 but would be - because movind down
    m is just .2 and big M is mass of earth
    solve equation i got P3 to be 1
    so i took final P minus inital P and get 2
    then divided this by 1.5 E -3 to find Fnet and get some answer like 1333.333 N
    that seems too big so was wondering where i messed up (or is it right)
  2. jcsd
  3. Mar 25, 2009 #2
    Re: Force on a bouncing ball???

    I don't understand your methodology for solving for the momentum. Both the initial and final velocities of the ball are given meaning that both momenta can be calculated directly, right?
  4. Mar 25, 2009 #3
    Re: Force on a bouncing ball???

    wow no for some reason i was thinking the given speeds was the acceleration. then i had to use that to figure out velocity and then momentum.

    so i would use p = m*v to find final and inital
    final = .2 * 4 = .8
    inital = .2 * 5 = 1 (which will be -1 because it is moving towards the earth)
    final - inital = 1.8 this is my change in momentum

    then could i just use the change in momentum = fnet times change in time?
    would my change in time just be the time it is in contact with the floor?
  5. Mar 25, 2009 #4
    Re: Force on a bouncing ball???

    Well, the time given is the time that the ball is in contact with the floor, which is when the momentum change is taking place, so yes I would say that is the correct delta T to plug in.
  6. Mar 25, 2009 #5
    Re: Force on a bouncing ball???

    oh ok... i got it now.
    im sleep deprived and my brain
    is not working right now. need to
    get more than 3 hrs of sleep tonight
    thanks for the help!!!!!
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