What term describes the rate at which work is done?

• A. Momentum
• B. Force
• C. Power
• D. Energy

Answer: (C)

Power is the rate at which work is done. When you do a certain amount of work W (in joules) in a time t (in seconds), the rate at which that energy is transferred is W/t, called power. If the force you apply moves an object a distance d in time t, then work is W = Fd and power becomes P = W/t = Fd/t. Since velocity v = d/t, power can also be written as P = F·v when the force acts in the same direction as the motion. The unit for power is the watt, equal to one joule per second.

This is what the term expresses: how quickly energy is being transferred or transformed through doing work. Other terms describe different ideas—momentum is a measure of motion, force is a push or pull that can change motion, and energy is the capacity to do work—so they don’t capture the idea of rate like power does.