Solving Part A & B of a Circular Motion Homework Problem

[ScienceGeek24]

Homework Statement

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?

Homework 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)

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!

 

[rock.freak667]

Homework Statement

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?

Homework 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)

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 A 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!

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

 

[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