Work done (spec 4.11, 4.12)
Force × distance in the SAME direction.
Definition. Work done = force applied × distance moved IN THE DIRECTION of the force.
- in joules (J)
- in newtons (N)
- in metres (m)
1 J = 1 N m. One joule is the work done when a 1 N force moves an object 1 m in the direction of the force.
Force must be parallel to motion. If a force acts at an angle, only the COMPONENT of the force along the direction of motion does work. A horizontal force lifting nothing (no vertical motion) does no work against gravity; the weight of a horizontally-moving trolley does no work because weight is perpendicular to the motion.
Spec 4.12: work done = energy transferred. Whenever a force does work on an object, energy is transferred TO or FROM the object. The kind of energy transferred depends on the situation:
- Pushing a box along a floor: work done by the push transfers to KE (if accelerating) and/or to thermal energy of the floor/box via friction.
- Lifting a box vertically at constant speed: work done by the lift force transfers to the GPE store of the box.
- Stretching a spring: work done by the pulling force transfers to elastic PE of the spring.
Sign convention. Work done BY a force in the direction of motion is POSITIVE; work done AGAINST the motion (friction, drag) is NEGATIVE — it removes energy.
- — force in the direction of motion.
- 1 J = 1 N moved 1 m.
- Work done = energy transferred (spec 4.12).
- Perpendicular force components do NO work.