Angular Momentum of a rotating ball

[sammythesp3rmy]

Homework Statement

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?

Homework Equations

I am using L = Iω

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

 

[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.