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Homework Help: Can you confirm the answers?

  1. Apr 11, 2012 #1
    1. The problem statement, all variables and given/known data

    A baseball pitcher throws a fast-ball (mass 50g) to stop a bullet (mass 5g), which has been shot at him at a speed of 400m/s by an angry fan. Can the pitcher save himself by stopping the bullet with a fast-ball of 100mph assuming a perfectly inelastic head-on collision, where the bullet gets stuck in the baseball?

    What is the final momentum and the final speed of the baseball+bullet system?

    2. Relevant equations

    P = mv+mv

    3. The attempt at a solution

    So I did:

    Final momentum:

    P= bullet = (400m/s)(0.005kg) = 2

    p = fast-ball = 100mph = 44.7m/s = (44.7m/s)(0.05kg) = 2.235

    Pf = 2.235 + 2 = 4.235kg.m/s

    Final velocity:

    If the question were find the final velocity of the bullet, I guess I´d have written 0, right (because it´s perfectly inelastic)?

    But since I am being asked the final speed for both baseball +bullet system, I am attempting:

    v = p/m
    v = 4.235kg.m/s divided by 0.055kg (total mass) = 77m/s.

    So, is that correct?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 11, 2012 #2
    You are right all the way up to the point where you get your Pf. That is incorrect, as the momentum is in opposite directions. You should subtract, not add.
  4. Apr 11, 2012 #3

    pf = 0.235kg. m/s

    vf = v/m = 0.235kg. m/s divided by 0.055kg (total mass) = 4.27m/s.

    Is that correct now?
  5. Apr 11, 2012 #4
    Yes. So what can we conclude from your answers? The pitcher can indeed save himself from the bullet. Don't try this at home though, it's a crazy physics question :P.
  6. Apr 11, 2012 #5
    hahaha!! Thank you SO MUCH!! You are great !!! =)
  7. Apr 11, 2012 #6
    Np, glad I could help :)
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