1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Easy Question about power!

  1. May 16, 2010 #1

    bon

    User Avatar

    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..

    Thanks!
     
  2. jcsd
  3. May 16, 2010 #2

    bon

    User Avatar

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

    bon

    User Avatar

    i.e F = P/w
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook