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Momentum Question

  1. Jan 10, 2007 #1
    1. The problem statement, all variables and given/known data
    A tennis ball with a mass of 110g is traveling 18.5 m/s east. It is struck by a racquet that applies a force of 950 N west. The ball and the racquet are in contact for 3.2ms. The change in momentum of the tennis ball is..


    2. Relevant equations
    As far as I can tell p=mv and p=w/t are relevant to the equation


    3. The attempt at a solution
    At first I tried to find the sum of the momentum before and subtract it by the sum of the momentum after, but that didn't seem to work. So I'm not really sure. Momentum has been my worst unit all year.

    Any help would be appreciated. Thanks
     
  2. jcsd
  3. Jan 10, 2007 #2

    Doc Al

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    Staff: Mentor

    Last edited: Jan 10, 2007
  4. Jan 10, 2007 #3
    So I figured if impulse is equal to the change in momentum then I could use the force and time in the equation to get the change in momentum.

    so 950(3.2/1000) = 3.04

    Which is the correct answer apparently (3.04 kg m/s, west).

    What I'm still puzzled by is there is so much more info in the question. It appears to me that the velocity is not factored in at all. Should not mv = Ft ? (supposed to be a change in velocity, I cant find a triangle icon) Or is the change in velocity not 37m/s.
     
    Last edited: Jan 10, 2007
  5. Jan 11, 2007 #4
    There's an important thing to remember; the change in momentum of an object is equal to the impulse [force] acting on it.
     
  6. Jan 11, 2007 #5

    Doc Al

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    Staff: Mentor

    Right. Momentum is a vector, so direction is important.

    Since you've found the change in momentum you can easily find the change in velocity (since you know the mass of the tennis ball) using:
    [tex]\vec{F}\Delta t = m \Delta \vec{v}[/tex]
    But how did you get 37 m/s?

    And once you correctly find the change in velocity, you can use the intial velocity to find the final velocity.
     
  7. Jan 11, 2007 #6
    K thanks for clarifying that for me.

    It was more of a brain fart on my part, I realized you can find the change in velocity using the equation, but it was different then what I guessed because the ball was initially going at 18.5 m/s then I figured it would be rebounding back 18.5 m/s so I thought the change in velocity would be 37 m/s. Which is in fact wrong. The velocity coming back is not going to be the same as it was when it hit the object.

    Thanks for your help guys. I had a couple similar questions on a quiz in class today and got them correct.
     
    Last edited: Jan 11, 2007
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