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Basic Forces Question

  1. Oct 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Pete serves a tennis ball of mass 54 grams at a speed of 53.6 meters per second. The ball is in contact with Pete's racket through a distance of 49.7 centimeters. What force does Pete apply to the ball?

    54 grams = 0.054 kg
    Speed = 53.6 m/s
    49.7 cm = 0.0497 m

    2. Relevant equations
    Force = mass x acceleration
    Vfinal = at + vi

    3. The attempt at a solution
    I am VERY lost, considering the fact that our teacher told us to try and figure it any way we could.

    I tried this. 0.0497 m = 0.5(53.6 m/s) * t2
    t = 0.0420183500 s

    Then I thought of the statement
    It takes 0.0420183500 seconds to travel 0.0497 m when traveling at 53.6 m/s

    Then I tried Vfinal = a*t + Vinitial
    56.3 m/s = a * 0.0420183500 s

    a = 1339.890784 m/s2

    Force = 0.054 kg * 1339.890784 m/s/s
    Force = 72.4 N

    Can anyone help me out with this please?
  2. jcsd
  3. Oct 28, 2009 #2
    Since you are given the distance over which the force was applied, you should probably use an equation that has distance.

    The way I read this question, you are given the following:
    [tex]\Delta[/tex]x = .497m
    vi = 0 (<-- not given, but I'm assuming the ball wasn't moving until he hit it)
    vf = 53.6 m/s

    You are trying to find acceleration.

    Find an equation that has [tex]\Delta[/tex]x, vi, vf, and a in it, and solve for a.

    Once you find acceleration, you can use the F = ma equation to find the force.

    Does this make sense?
  4. Oct 28, 2009 #3
    Yes, what you are suggesting seems to make sense. I will try it and let you know if I figure it out. Thanks
  5. Oct 28, 2009 #4
    Thanks! I got it. One major and obvious problem that I discovered was the fact that I converted it to 0.0497 m rather than 0.497. I ended up using Vf^2 = Vi^2 + 2a * delta x
    I solved for 156 Newtons. Thanks
  6. Oct 28, 2009 #5
    That's the same answer I got too. Good job! Glad to help!
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