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Homework Help: Finding velocity in an elastic problem

  1. Oct 20, 2012 #1
    1. The problem statement, all variables and given/known data herman(m=135kg) is sitting motionless on a tire swing. eddie hits a baseball (m=2kg) and v=169 m/s. L=3.2m. (A)what is the velocity of the ball after the collision and (B) what is the velocity of herman after the collision. (C) what is herman's change in KE. (D) eddie hits another ball and this time herman catches it. as a result herman moves a height h, how fast was the ball going this time?

    2. Relevant equations p(0)=p(f), KE=1/2mΔv^2, p=J=mgh=mv

    3. The attempt at a solution. (A and B) i started out with my p(0)=p(f), but i have two unknown velocities so iam not sure where to go from there?
    i think for C, i can just use the found velocities to find the change in KE. For the last one i know that h=l-lcos(theta) so i plugged that into mgh=mv.
    thanks for help!
  2. jcsd
  3. Oct 20, 2012 #2


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    hi omc1! :smile:

    according to your title, this is an elastic collision (for parts A-C),

    so you can use conservation of energy as well as the usual conservation of (angular) momentum :wink:
  4. Oct 20, 2012 #3
    so when I do p0+k0=pf+kf, I get that velocity initial equals velocity final, but I am not sure what the height is? I thought that it would be 0 for initial and finial??
  5. Oct 20, 2012 #4


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    how can that be?? :confused:

    we don't seem to be talking about the same question :redface:
  6. Oct 20, 2012 #5
    this is a dumb question but how to I find theta for this problem to solve for the h that herman goes. because I only know the length L.
  7. Oct 20, 2012 #6
    so when I try to find the velocity of the ball, i did 1/2mv^2+mh=1/2mv^2+mgh
    but i didnth this that the ball had a height?
    for the velocity of herman I know he had a final height....L-Lcostheata. but I dont know how to get theta because i only have the length of the one side...
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