If the power goes out, so that the wheel's motor no longer provides any torque, how long will it take the wheel to come to a stop? you can assume that the wheel rotates on frictionless bearings and that the potter keeps her hands on the pot as it slows.

You have to solve for moment of inertia of a disk spinning around a frictionless axis, which is 1/2*M*Rsquared, where M is the mass of the object, and R is the radius.
 The pot is negligible, so this is the variable I in the equation alpha = net torque/moment of inertia (I)
 So now you need one more variable in order to solve for alpha (angular velocity). You need to calculate net torque. When the power goes out, the only torque being applied to the wheel is the potters hand (the wheel is in free spin, so the existing velocity has no torque). Calculate net torque using the radius of the pot, with the force being perpendicular.
 Now you can use the equation delta t = delta omega / alpha to solve for the time. (change in time = change in angular velocity divided by angular acceleration).

therefore, the efficiency is: