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Calculating Power of a rotating shaft

  1. Feb 21, 2008 #1
    Hi, it's been a number of years since I've even thought about physics, so this is a very simple question. I have a rig set up where I have a power source that is rotating a shaft. Attached to this shaft is a wheel with a string, I have this string going through a pulley in the ceiling, and pulling a hanging weight. What I need to do is calculate the power that I'm getting out of this device, I remember how to calculate work and power, but can't for the life of me figure out where the radius of the wheel comes into play, and the torque. Any help would be appreciated

  2. jcsd
  3. Feb 21, 2008 #2


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    Science Advisor

    If you have the power, you can calculate the torque (neglecting friction) from:

    [tex]P = T \dot{\theta}[/tex]

    [tex]P[/tex] = Power
    [tex]T[/tex] = Torque
    [tex]\dot{\theta}[/tex] = Angular speed

    Just make sure your units are consistent!
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