# Homework Help: Moment of Inertia/angularvelocity question

1. Nov 17, 2005

### donjt81

A cylinder with moment of inertia 14.2kg m(squared) rotates with angular velocity 11 rad/s on a frictionless vertical axle. A second cylinder, with moment of inertia 21.4 kg m(squared), initially not rotating, drops onto the first cylinder and remains in contact. Since the surfaces are rough, the two eventually reach the same angular velocity.

Calculate the final angular velocity.

Any pointers on how I could approach this problem?

2. Nov 17, 2005

### Fermat

Pointer: Conservation of Angular Momentum.

3. Nov 18, 2005

### donjt81

Thanks for the pointer and sorry for posting in 2 places.

so
Ii Wi = If Wf
Wf = (Ii Wi)/If

Ii = 14.2 kg m2
If = 21.4 kg m2

Is this the correct approach?

also do I need to change any units above?

4. Nov 18, 2005

### Staff: Mentor

What's the rotational inertia of the combined system of both cylinders?

5. Nov 18, 2005

### donjt81

moment of inertia of the combined system will be the sum of the two cylinders...

so Moment of inertia of the system = 14.2 + 21.4

is this correct?

6. Nov 18, 2005

### Staff: Mentor

That is correct.

7. Nov 18, 2005