Work & Energy Transfer
Work & Energy Transfer
- Energy moves from one object to another when a force pushes something through a distance. Physics calls that transfer .
- Work is the starting idea of this whole chapter: energy, power and efficiency are all built on it.
The definition
Symbols
- = work done (J)
- = force (N)
- = distance moved in the direction of the force (m)
- The one-mark definition: work done is the product of the force and the distance moved in the direction of the force (9702/21/O/N/24 Q3(a)).
- The unit is the joule: . One joule is the work done when a force of 1 N moves something 1 m along the force's own direction.
- Work done = energy transferred. The two phrases describe the same number of joules.
Worked example
Smallest case: force along the motion
A 25 N force pushes a box 4.0 m along a horizontal floor. Find the work done by the force.
- Force and movement point the same way, so .
Answer
When work is done, and when it is not
- In everyday language, holding a heavy bag feels like hard work. In physics it is not work: the force does not move.
- No movement along the force = no work = no energy transferred to the object.
| Doing work | Not doing work |
|---|---|
| pushing a car and it moves: its kinetic energy grows | pushing a car that does not move: no energy transferred |
| lifting weights upward: their potential energy grows | holding weights still above your head |
| gravity pulling a falling stone downward | gravity pulling the Moon in its (circular) orbit: the pull is at 90° to the motion |
Work is not about effort or feeling tired. Work is done only when the force moves through a distance in its own direction.