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Homework Help: I am an idiot, please help me. (Find the force of a tennis ball)

  1. May 31, 2012 #1
    This seems like such a pathetically elementary problem, and I feel like a total moron right now. Well, here it is:

    When a 58 gram tennis ball is served, it accelerates from rest to a speed of 45 m/s. The impact of the racket gives the ball a constant acceleration over a distance of 44 cm. What is the magnitude of the net force acting on the ball?

    The answer is 130 N. The only force formula I know is f=ma. I know that the distance (44 cm) is supposed to be used, and maybe gravity (9.8 m/s2) should be considered but I have no clue how. Thanks for any help :(
  2. jcsd
  3. May 31, 2012 #2


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    Why would acceleration of gravity have anything to do with this? It doesn't.

    Do you know how the work done on an object is related to the object's change in kinetic energy?
  4. May 31, 2012 #3


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    SammyS's suggestion that you look at the change in energy, the work done by the racket, is probably the simplest way to do this.

    However, since you say "The only force formula I know is f=ma", you can do it that way. With a constant acceleration, a, and initial speed 0, the speed after the acceleration is v= at and the total distance moved would be (1/2)at^2. You know, then, that at= 45 m/s and (1/2)at^2= .44 m. You can solve the first equation for t in terms of a and then put that into the second equation to get an equation in the single variable a. After you know a, you can use "f= ma".
  5. May 31, 2012 #4
    Before you do any of that, convert your mass to kg and your distance to meters.
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