Rotation problem: which would stop first?

1. Nov 19, 2008

Misako

Hey! I'm new here in PF and is wanting to solve this teeny bitsy problem about rotation. I'm at 9th grade and is a newbie at physics. :p

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

Two razor fans, one larger and one smaller, were turned on at the same time and turned off at the same time.

Question: which fan would stop first? the larger one or the smaller one?

2. Relevant equations

I'm thinking of: angular velocity = delta s/ r x delta t

3. The attempt at a solution

I need to prove the answer by using an equation. So the equation must include probably a radius(for the smaller and larger blade) and the time. Problem is, i cant seem to find that and is stumped. But for me, I really think the smaller fan would stop first.

Halp? =(

2. Nov 20, 2008

Redbelly98

Staff Emeritus
Welcome to PF.

Has torque been discussed in your class?

3. Nov 20, 2008

tiny-tim

Welcome to PF!

Hi Misako! Welcome to PF!

hmm … that seems rather difficult for 9th grade

let's assume they're turning at the same angular rate …

what is the initial energy proportional to, r r2 or r3?

and what is the rate of energy loss proportional to?