Work done
In science, work is done when a force moves an object in the direction of the force. It is calculated with W = F × s, where F is the force in newtons (N) and s is the distance moved in the direction of the force in metres (m). Work is measured in joules (J). If an object does not move, no work is done — for example, pushing hard against a wall that stays still does zero work.
Key idea
Work: W = F × s (joules). Power: P = E ÷ t (watts). 1 W = 1 J per second.
Energy
Energy is the ability to do work, and it is also measured in joules. When work is done, energy is transferred from one form to another; the total amount of energy is always conserved. Common forms include kinetic, potential, heat, light and electrical energy.
Example
A boy pushes a box with a force of 20 N and moves it 3 m. Work done = 20 × 3 = 60 J. If this takes 2 s, power = 60 ÷ 2 = 30 W.
Power
Power is the rate of doing work, or how fast energy is transferred. It is found from P = E ÷ t, where E is the energy (or work) in joules and t is the time in seconds. Power is measured in watts (W); one watt is one joule per second. A more powerful machine does the same work in a shorter time.
Remember
- Work and energy are both measured in joules.
- No movement means no work is done.
- Power is how fast work is done, in watts.