You remember the relationship between the change in Kinetic Energy of an object and the Work done by forces on it, right?

For example,

A chunk of ice of massm = 2 kgsits motionless on a frictionless frozen pond. Then a breeze starts to push the ice cube with a constant forceF = 3 N. After the chunk has slid50 m, how fast is it going?

Well, there are very similar relationships between the angular analogues of these quantities: the change in rotational KE of an object and the Work done on it by torques.

For example,

A giant grindstone with moment of inertiaI = 700 kg*m^2is spinning at10 rpm. Joe slows it down by pressing a wooden brake against the rim of the wheel. How much torque must he exert in order to stop it within5 revolutions?

Notice in the example above the fact that torque is involved in some vector arithmetic:

That means that torque must have some direction ... but what direction should it be? How does one figure out the direction of a torque?

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.