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Homework Help: Easy Question about power!

  1. May 16, 2010 #1


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

    A rotating disc is driven by a motor that generates constant power P , and expe-
    riences a frictional drag torque
    γ ω, where ω is the angular velocity of the disc and γ
    is a constant.

    If the moment of inertia of the disc is I , show that the time to accelerate
    the disc from rest to angular velocity ω is

    t= -I/2γ ln (1-w^2/wo^2)

    where ω0 is the value of ω in the steady state.

    2. Relevant equations

    3. The attempt at a solution

    Ok so im trying to solve this by writing out a differential equation..Torque = rate of change of angular momentum..

    Now I know J = Iw so i can write an expression for rate of change of angular momentum..Just having trouble working out what the net torque is..

    it is of the form F - γω..but how can i write an expression for F given the power is constant P?

    I know the integral of Power dt = the integral of F dw..

  2. jcsd
  3. May 16, 2010 #2


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    Sorry - see it now..ts just P/w right?
  4. May 16, 2010 #3


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    i.e F = P/w
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