# Homework Help: HELP:moment of inertia/angular momentum

1. Nov 12, 2004

### buffgilville

A solid ball of radius 0.4 meters, made from a material of density 1.7 kG/cubic meter, rolls along a rough horizontal plane with a speed of 11.1 meters/sec. What is its angular momentum (in Joule.secs)?

density = mass/volume and volume of a sphere = (4/3)(pi)(r)^3
so the mass is 0.4557kg

I=(2/5)MR^2 ---> (2/5)*0.4557*(0.4)^2 = 0.02917

w=v/r ---> 11.1/0.4 = 27.75
L=Iw ---> 0.02917*27.75 = 0.809

Student response 0.809

2. Nov 12, 2004

### CartoonKid

You have to change the axis of rotation to the contact point of the ball with the plane. It's 2/5mr^2 + mr^2. This will give you the correct answer.

3. Nov 12, 2004

### arildno

"A solid ball of radius 0.4 meters, made from a material of density 1.7 kG/cubic meter, rolls along a rough horizontal plane with a speed of 11.1 meters/sec. What is its angular momentum (in Joule.secs)?"

Cartoonkid has given you the approah they're probably after.
However, as it stands, this question is completely meaningless.
Angular momentum is computed RESPECTIVE to a point. If you haven't been given that, then the exercise is flawed.