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Homework Help: Physics: When a 58-g tennis ball is served?

  1. Sep 22, 2009 #1
    1. The problem statement, all variables and given/known data

    When a 58-g tennis ball is served, it accelerates from rest to a speed of 36 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 36 cm. What is the magnitude of the net force acting on the ball?

    2. Relevant equations

    3. The attempt at a solution

    Fnet= ma
    to find a i need time
    so i used v=d/t to find the time. 36=36/t so t=1sec
    now to find a i used V-V_0=at so 36=a*1 so a=36
    now i plugged it in
    Fnet= 58g*36=2088

    what am i doing wrong?
  2. jcsd
  3. Sep 22, 2009 #2
    You are very sloppy with your units. Go back through your calculations making sure the units are correct. I can see at least two major place where you units are incorrect. See if this fixes the answer.
  4. Sep 22, 2009 #3
    ok so i changed the m=0.058kg, and d=0.36m
    so t=0.01s which makes a=3600m/s2 so fnet= 3600*0.058=208.8. did i do a conversion wrong/
  5. Sep 22, 2009 #4
    The speed of the ball isn't 36 m/s during the entire acceleration, but only at the end of it, so you can't use v = d/t and the time you computed is wrong.
    use x = (1/2) a t^2 to find out when the ball has moved 36 cm.
  6. Sep 22, 2009 #5
    how can i use x= 1/2 at^2 if i dont know the t or a? what do i use for t? i tried using v^2-V_0^2=2a(x-x_0) but that didnt work out?
  7. Sep 22, 2009 #6
    You know x = 0.36m = (1/2)at^2 and v = at = 36 m/s
  8. Sep 22, 2009 #7
    oh ok thanks its 104.4
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