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Momentum, impulse, etc.

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data

    My question is:
    Now assume that the pitcher in Part D throws a 0.145 kg baseball parallel to the ground with a speed of 32 m/s in the +x direction. The batter then hits the ball so it goes directly back to the pitcher along the same straight line. What is the ball's velocity just after leaving the bat if the bat applies an impulse of -8.4 N*s to the baseball?


    2. Relevant equations

    The first part of the question is this:
    Assume that a pitcher throws a baseball so that it travels in a straight line parallel to the ground. The batter then hits the ball so it goes directly back to the pitcher along the same straight line. Define the direction the pitcher originally throws the ball as the +x direction.
    The right answer (that I got correct) is this:
    The impulse on the ball caused by the bat will be in the negative x direction.

    3. The attempt at a solution
    Well I know that impulse is just the change in momentum...
    So I did
    mv=mv
    8.4 N*s=(0.145)(v)
    and I got 57.9 m/s... which was wrong

    AND ONE MORE QUESTION PLEASE...
    1. The problem statement, all variables and given/known data

    My question is:
    Olaf is standing on a sheet of ice that covers the football stadium parking lot in Buffalo, New York; there is negligible friction between his feet and the ice. A friend throws Olaf a ball of mass 0.400 kg that is traveling horizontally at 10.9 m/s. Olaf's mass is 73.5 kg.
    If the ball hits Olaf and bounces off his chest horizontally at 7.30 m/s in the opposite direction, what is his speed vf after the collision?


    2. Relevant equations

    The first part of the question is:
    If Olaf catches the ball, with what speed v_f do Olaf and the ball move afterward?
    I got 5.90 cm/s, which is right.

    3. The attempt at a solution
    I found the initial momentum which is (-7.3 m/s * 100cm/1m)(0.400 kg) = -292 kgcm/s
    And this was right

    And then I tried momentum final = mv
    -292 = (73.5kg)(v)
    And it was wrong...
     
  2. jcsd
  3. Nov 18, 2008 #2

    jamesrc

    User Avatar
    Science Advisor
    Gold Member

    For the first problem: Impulse is equal to the change in momentum, not the momentum...

    For the second problem: I don't see you trying to set up the problem correctly. Initially, the momentum of the system is all in the ball (that momentum would be equal to the mass of the ball times the velocity of the ball). After the collision, both the ball and Olaf are moving, so the total momentum would be the sum of the ball's momentum (its mass times velocity) and Olaf's (his mass times velocity; his velocity is what you are solving for).
     
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