Work done
Work is done when a force moves an object in the direction of the force. The work done equals the energy transferred. If a force F moves an object a distance d in the direction of the force, then work W = F × d. The unit is the joule (J); 1 J = 1 N·m.
Key idea
W = F d (work, in J). P = W / t (power, in W). 1 watt = 1 joule per second. Power can also be written P = E/t, where E is the energy transferred.
Power
Power is the rate at which work is done, or the rate at which energy is transferred. A more powerful machine transfers the same energy in less time. The unit of power is the watt (W), equal to one joule per second.
Worked example
A crane lifts a 500 N load through a height of 4.0 m in 8.0 s. Find (a) the work done and (b) the power.
(a) W = F d = 500 × 4.0 = 2000 J.
(b) P = W / t = 2000 / 8.0 = 250 W.
Remember
- No movement means no work done, even if a force is applied.
- Work and energy share the same unit, the joule.
- Power tells you how fast energy is transferred, not how much.