# Torque Concepts

by 123yt
Tags: concepts, torque
 P: 32 Pretend there are two accelerating masses connected to a massless string with a frictionless pulley between them. How can the frictionless pulley (Rotational inertia and radius given) affect acceleration in any sort of way? Also, why is the net force equal to Acceleration * (Mass of two blocks + I/r^2)? I understand the part with the two blocks, but not with the I/r^2.
Mentor
P: 40,262
 Quote by 123yt Pretend there are two accelerating masses connected to a massless string with a frictionless pulley between them. How can the frictionless pulley (Rotational inertia and radius given) affect acceleration in any sort of way?
The pulley has rotational inertia and thus requires a torque to accelerate it.
 Also, why is the net force equal to Acceleration * (Mass of two blocks + I/r^2)? I understand the part with the two blocks, but not with the I/r^2.
You can think of I/r^2 as the effective mass of the pulley. But that equation is a bit of a short cut. Rather than use it directly, derive your own version by applying Newton's 2nd law to each mass and the pulley itself.
P: 32
 Quote by Doc Al The pulley has rotational inertia and thus requires a torque to accelerate it.
But torque is just a measure of how much a force causes an object to rotate. It doesn't "use up" any force to rotate it, right?

 You can think of I/r^2 as the effective mass of the pulley. But that equation is a bit of a short cut. Rather than use it directly, derive your own version by applying Newton's 2nd law to each mass and the pulley itself.
The rotational inertia of the pulley is I = MR^2 / 2, so shouldn't the mass be M = 2 * I / R^2?

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P: 40,262

## Torque Concepts

 Quote by 123yt But torque is just a measure of how much a force causes an object to rotate. It doesn't "use up" any force to rotate it, right?
It "uses up" force in a manner similar to how pushing a mass "uses up" force.
 The rotational inertia of the pulley is I = MR^2 / 2, so shouldn't the mass be M = 2 * I / R^2?
No. If you derive the equation, you'll see where that I/R^2 term comes from. (No reason to treat the pulley as a uniform disk.)
 P: 32 Alright, thanks for the help. I think I understand torque and rotation a little better now.

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