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Homework Help: Force experienced by a tennis ball

  1. Apr 30, 2012 #1
    A .065-kg tennis ball moving to the right with a speed of 15 m/s is stuck by a tennis racket, causing it to move to the left with a speed of 15 m/s. If the ball remains in contact with the racket for 0.020s, what is the magnitude of the average force experienced by the ball?

    F=mv/t I think that's the equation to use.

    So if I plug in 0.065 for mass 15 m/s for velocity and .020s for time I would get 48.75 N but that's not an answer choice.
    Last edited: Apr 30, 2012
  2. jcsd
  3. May 1, 2012 #2


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    Homework Helper

    The change of the momentum is equal to the impulse: Δ(mv)=FΔt.

    The velocity of the ball turns to opposite. How much does its momentum change if the velocity changes from 15 m/s to -15 m/s?

  4. May 1, 2012 #3
    It goes from .975 kgm/s to -.975 kgm/s so it changes by 1.95 kgm/s.
  5. May 1, 2012 #4


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    Staff: Mentor

    So does that lead to a match with one of the options offered, now?
  6. May 1, 2012 #5
    So then if thats the case then 1.95 kgm/s=Ft and t equals .020 I would get 97.5 for force but that isn't an answer choice. What am I doing wrong?
  7. May 1, 2012 #6


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    97.5 N should be correct.

  8. May 1, 2012 #7
    I feel dumb now, our teacher likes to round and one of the answer choices is 98. Thanks guys.
  9. May 1, 2012 #8


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    Staff Emeritus
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    That's not just rounding, that's the teacher correctly using significant figures
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