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Homework Help: Circular motion

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data

    Part A : A 0.5 Kg ball is attached to a cord 80 cm long. The ball is whirled in a vertical circle at a constant speed. If the ball takes 0.4 seconds to go around once, what is the tension in the cord at the top and bottom of the circle?

    Part B How long does it take for the ball to go around once if the tension in the cord at the top of the circle is zero?

    2. Relevant equations

    T=m(g+v^2/r) (bottom of the circle)
    T=m(g-v^2/r) (top of the circle)

    3. The attempt at a solution

    For part A i used both of the circular motion formulas to fin the tension and i got totally different answers from what it suppose to be for top and bottom. Top it suppose to be 93.8N and bottom it suppose to be 104 N i got 92.7N top and 102.7 N bottom. ( I do not know if the answer is a print error).

    For part B I tried to use v=2Pi*R/T to get the time and i keep getting the same time in part A which is 0.4 secs. But the answer is 1.8 seconds.

    Can someone explain to be what I'm doing wrong??? thanks!
    Last edited: Feb 13, 2012
  2. jcsd
  3. Feb 13, 2012 #2


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    Homework Helper

    Your formulas are correct, can you post your exact calculations for it?
  4. Feb 13, 2012 #3
    m=0.5 kg
    r=0.8 m
    T=0.4 seconds
    v=12.5 m/s

    Bottom part of the circle:

    0.5kg(12.5^2m/s/0.8m+9.8m/s^2)= 102.7N

    Top part of the circle

    Last edited: Feb 13, 2012
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