# Angular Momentum of a rotating ball

1. Nov 17, 2013

### sammythesp3rmy

1. The problem statement, all variables and given/known data
What is the angular momentum of a 0.210-kg ball rotating on the end of a thin string in a circle of radius 1.35m at an angular speed of 10.4 rad/s?

2. Relevant equations
I am using L = Iω

3. The attempt at a solution
I put I = (2/5)(0.210 kg)(1.35m)^2
ω = 10.4 rad / sec
When I try multiplying these out, I'm not getting the right answer. I come up with 1.592 kgm^2/sec and the answer is 3.98
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 17, 2013

### lucasem_

You're using the moment of inertia of a sphere, But it isn't the sphere that's spinning. The ball is essentially a point mass at the end of the string. The moment of inertia for a point mass is $I=mr^2$. Try this and you will get the answer.