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Physics impulse momentum problem

  1. May 3, 2014 #1
    1. The problem statement, all variables and given/known data

    A 60.0 gram tennis ball is served at 45.0 m/s and is returned with an average force of 100 N over 50 ms. Find the speed with which the ball is returned. Assume the ball is still moving at 45.0 m/s when it is returned.

    2. Relevant equations
    Impulse = Average Force * time
    3. The attempt at a solution
    Pi= mv= .06*45 = 2.7
    Pf= mv= .06*v
    Impulse = Average Force * time= 100 * .05= 5
    dp= pf-pi
    5= .06v-2.7
    v= 128.3
    The answer should be 38.3. What did I do wrong?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 3, 2014 #2

    SammyS

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    First of all: The statement in the above quote, which I highlighted is a bit puzzling. (Oh! I see what's meant. The ball is still moving at 45.0 m/s when it gets to the returner's racket.)


    What you did wrong was to ignore that the velocity ( and thus the momentum ) of the ball changes direction, thus it changes sign. This assumes that the ball is returned exactly 180° from the direction it was received.
     
  4. May 3, 2014 #3
    Check the sign convention on your velocity.
     
  5. May 3, 2014 #4
    Okay, so abs (change in impulse) = force *times.
    So I change the velocity final to -v.
    abs (-.06v -2.7) = 5
    v= -128.3
    v= 38.3
    I use the positive 38.3 because I already put in the negative sign?
    And yeah "Assume the ball is still moving at 45.0 m/s when it is returned." confused me too... The ball returns at the same speed that it leaves?
     
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