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Homework Help: Momentum problems - really

  1. Nov 8, 2007 #1
    momentum problems - really urgent:)

    Two problems I'm stuck on for a test tomorrow morning...

    1) A .6 kg tennis ball initially moving at 12m/s is struck by a tennis racket causing it to rebound in the opposite direction at 18m/s. What is the average change in momentum?
    I know p=mv but to get the change in velocity, do you just subtract the 12 from the 18 making the answer p=(.6)(6)?

    2) A 15g bullet traveling at 213m/s in a vertical direction buries itself in a 2.4kg block of wood at rest directly above it.
    What is the velocity of the bullet/block combination?
    What is the maximum height the block reaches?
    Is KE conserved? If not, how much is lost?
    Would you just solve this by using m1v1 = (m1 + m2)v' and then using v' = sqrt (2gh) to get the max height? I have no idea about the kinetic energy part of the question..

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

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 8, 2007 #2
    1. no, you're on the right track , but look, the 18 m/s is the OPPOSITE direction. thats the key

    2. lol does it say how far above the block is (it matters). also lol, its funny that the problem assumes that there isn't any gravity until the bullet hits the block (thats what I understand, I'm not the most brilliant student in English). I can tell you that the kinetic energy isn't conserved. Go to wikipedia and look up "inelastic collision" it has great stuff. so tell my a bit more information so i can help

    BTW, "Two problems I'm stuck on for a test tomorrow morning... " why do u know the problems that will be on your test tomorrow?
  4. Nov 8, 2007 #3
    1. hmm.. I don't really get the 18m/s being in the opposite direction. Does it mean the vi - vf is actually 6+18?

    2. I'm pretty sure the problem didn't say how far above the block is but it'll be too late too check. Thanks for your help though :)

    I know those problems are on the test tomorrow because it's a 2 day test and we got to begin working on it today. Our physics tests are always crazy long and we have to take some home, and those two I was really stuck on.
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