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Homework Help: Rotation problem: which would stop first?

  1. Nov 19, 2008 #1
    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. jcsd
  3. Nov 20, 2008 #2


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    Welcome to PF.

    Has torque been discussed in your class?
  4. Nov 20, 2008 #3


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    Welcome to PF!

    Hi Misako! Welcome to PF! :smile:

    hmm … that seems rather difficult for 9th grade :redface:

    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? :smile:
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