Define work and give its SI unit. A labourer lifts a 25 kg cement bag through 2 m. Find the work done against gravity. (g = 9.8 m s⁻²)

1. 1 · Define work. Work is done when a force moves a body in the direction of the force: W = F × s.
2. 2 · Give the unit. The joule (J): 1 J = 1 N × 1 m.
3. 3 · Find the force. To lift the bag, F = its weight = mg = 25 × 9.8 = 245 N.
4. 4 · Find the work. W = F × s = 245 N × 2 m = 490 J.
5. 5 · Real-life link. Carrying the same bag along a level floor does no work against gravity, since the motion is at 90° to gravity.
6. 6 · Close. So the labourer does 490 J of work against gravity.

Scoring tip: Find the force first (weight = mg), then multiply by the height. Always end with the unit J.