# Motion Problems (tension)

1. Feb 19, 2009

### codz30

1. The problem statement, all variables and given/known data
A ball of mass m on the end of a string is released from rest when the string is held horizontal
eg:
0-----

Prove that as the ball moves through the lowest point in its swing the string has a magnitude equal to three times the weight of the ball.

2. Relevant equations
Fc = mv^2/r
Weight = mg

3. The attempt at a solution
Because its motion in a vertical circular plane, I thought that mv^2/r would be needed, obviously the equation would end up something like
xxxxxx = 3mg
But I'm not sure how or why that would happen?
Any thoughts?

2. Feb 19, 2009

### LowlyPion

Welcome to PF.

Consider what the kinetic energy is of the ball at the lowest point.

Using your knowledge of centripetal acceleration and the weight then what is the force on the string at that point?

3. Feb 19, 2009

### Dr.D

What does it mean to say, "...the string has a magnitude equal to three times the weight of the ball"? How does the string have a magnitude?

4. Feb 19, 2009

### LowlyPion

I'm sure they intend the "tension in the string" ...

5. Feb 19, 2009

### Dr.D

The point is, it would really help if people asking questions would learn to ask those questions.