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Confused about torque vs. energy (Joules)

  1. Jan 14, 2014 #1
    1. The problem statement, all variables and given/known data

    Solid disk, 50cm in diameter, with a mass of 5kg. Disk is located in space by a bearing. The friction torque of the bearing is 10Nm. A string is wrapped around the disk and connected to a weight. The weight is allowed to fall and spin the disk. How much energy is lost to friction?

    2. Relevant equations

    1 Joule = 1Nm

    3. The attempt at a solution

    Since the friction torque is 10Nm, the energy lost to friction is 10J (same as 10Nm). Is this right? I'm confused by torque having the same units as Joules...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 14, 2014 #2

    CWatters

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    Do you remember the equation...

    work = force (N) * distance (m)

    Well the equivalent for rotation is...

    Work = torque (in Nm) * angular displacement (in Radians)
     
  4. Jan 14, 2014 #3

    SteamKing

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    Even worse for you, the total energy lost due to friction is not the same as the frictional torque. There is a relation between work and the torque and the total angle turned. It is similar to the work resulting from a force translating a certain distance.
     
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