Confused about torque vs. energy (Joules)

  • #1
johnschmidt
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Homework Statement



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?

Homework Equations



1 Joule = 1Nm

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...
 
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  • #2
Do you remember the equation...

work = force (N) * distance (m)

Well the equivalent for rotation is...

Work = torque (in Nm) * angular displacement (in Radians)
 
  • #3
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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