# Homework Help: Rotational inertia

1. Apr 19, 2015

### goonking

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I assume the energy stored is = 1/2 (I) (ω^2)

I (moment of inertia) is MR^2 since it's a hoop? or is it a solid cylinder?

do we need to convert the rpm (revolutions per minute) to radians per sec?

2. Apr 19, 2015

### B0b-A

3. Apr 19, 2015

### MaxwellsCat

You're on the right track, just realize that a disc is a cylinder with vanishing height.

Otherwise yeah, try it out and report back on what you end up with. It'll be important to do a sanity check on the answer that you get - that'll tell you whether you got the right answer or not most of the time.

4. Apr 19, 2015

### AlephNumbers

No, that's a thin disk. A disk is a solid cylinder.

As you defined it, ω is angular velocity, which is typically measured in radians per second.

Last edited: Apr 19, 2015
5. Apr 19, 2015

### MaxwellsCat

Sorry, I should have been explicit - I meant a solid cylinder.

6. Apr 19, 2015

### goonking

after doing all the math, answer came out to be 1.4 x 10^6 Js.

7. Apr 19, 2015

### MaxwellsCat

So right away there's something wrong - the units should be J not J$\cdot$s, unless you meant Joules :P

Does that answer make sense? In the context of the problem, does that seem reasonable?

8. Apr 19, 2015

### goonking

yes, i meant joules.

9. Apr 19, 2015