# Circular motion

1. Feb 13, 2012

### ScienceGeek24

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

v=2Pi*R/T
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. Feb 13, 2012

### rock.freak667

Your formulas are correct, can you post your exact calculations for it?

3. Feb 13, 2012

### ScienceGeek24

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

0.5kg(12.5^2m/s/0.8m-9.8m/s^2)=92.7N

Last edited: Feb 13, 2012