The work done by a force is found using the equation:
work done (J) = force (N) × distance moved in the direction of the force (m)
In symbols, E = F × d, with force in newtons (N) and distance in metres (m). Work done is the energy transferred, so it is also measured in joules. A newton metre (N m) is the same as a joule.
Worked example: a force of 40 N moves a box 3 m. The work done is 40 × 3 = 120 J, so 120 J of energy is transferred to the box. The equation can be rearranged: force = work done ÷ distance, and distance = work done ÷ force. If a force does 150 J of work moving a trolley 5 m, the force is 150 ÷ 5 = 30 N.
Only movement in the direction of the force counts. A person carrying a bag along a level path pushes upwards on it, but the bag moves sideways, so the upward force does no work on the bag.