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Homework Help: Finding an expression for torque

  1. Apr 21, 2012 #1
    1. The problem statement, all variables and given/known data

    http://www.physics.ox.ac.uk/users/yassin/mechanics/problems/probs6_2011.pdf [Broken]

    Q 2b.

    2. Relevant equations

    3. The attempt at a solution

    T is torque and J is angular momentum.
    T=dJ/dt=I dω/dt

    I tried solving dω/dt=ωk to get dω/dt=ce^kt, but this may not help.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 21, 2012 #2


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    They represented torque as N.

    You're given N = kω ---equation 1

    You also know that N = -I[itex]\frac{dω}{dt}[/itex] ---equation 2
    (the negative sign is to keep N positive since they're concerned with the numerical value for torque).

    You can equate equations 1 and 2 to get kω = -I[itex]\frac{dω}{dt}[/itex]

    and solve the differential equation. Remember to apply the given bounds using definite integrals on both sides after separating the variables ω and t.

    After that, express k in terms of the other variables, and put this back into equation 1 to get N.
    Last edited by a moderator: May 5, 2017
  4. Apr 22, 2012 #3
    Great, I've got the right answer now.

    Thank you very much! :-)
  5. Apr 22, 2012 #4


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    You're welcome. :smile:
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