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Homework Help: My teacher marked me wrong but it think this is right

  1. Jun 17, 2010 #1
    1. The problem statement, all variables and given/known data
    A batter hits a fastball that is travelling at -38 m/s,. The ball has a mass of 0.145 kg. The change in momentum in the ball is 13.1 kg.m/s. what is the balls final momentum?

    B.) what is the final velocity of the baseball?

    2. Relevant equations

    deltaP = Pf - Pi

    P = mv

    3. The attempt at a solution

    p initial = 0.145( -38.0) = -5.51 kg.m/s

    so... 13.1 = Pf - (-5.51)

    Pf = 13.1 - 5.51
    = 7.59 kg.m/s

    b.) Pf = mvf

    7.59= 0.145vf
    vf = 52.3 m/s

    I thought this problem was pretty basic, but i got 0 out of 4 for it, what do you guys get for a solution?
  2. jcsd
  3. Jun 17, 2010 #2
    This looks right to me...
  4. Jun 17, 2010 #3
    The question is a little vague, but I'd answer it as you did.

    What is confusing is that the wording does not make it clear whether the change in momentum is a vector component change, or vector magnitude change. A magnitude change would imply an impossibly fast speed, so I would answer as you did. However, a vector component change is still vague because we don't know what direction the ball was hit in. There may be x, y and z components of the momentum. So, your method is still not really correct unless the ball was hit directly into the pitcher's face.

    Personally, I give your teacher 0 points on this one, unless you've misrepresented the original wording of the question.
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