Work, power and efficiency
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 5 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Pushing a wall
- Work done
- Work and energy
Read the transcript of the teaching part
Last time
Before we start: last time, in Gravitational, kinetic and elastic potential energy, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Pushing a wall
You push a wall as hard as you can for ten minutes. You are exhausted, but the wall has not moved. In physics, you have done no work on it. Work needs a force and a movement in the direction of the force.
Work done
Work done by a force is the force multiplied by the distance moved in the direction of the force. The unit is the joule: one newton moving an object one metre. Work is a scalar. If the force is at an angle theta to the movement, only the component along the movement counts. A fifty newton force at sixty degrees, moving an object four metres, does a hundred joules of work.
Work and energy
Work done on an object is the energy transferred to it. Doing work of one joule transfers one joule. A force at right angles to the movement does no work. Gravity on a satellite in a circular orbit is an example. A ball of mass one point two kilograms moving at three metres per second is stopped by a cushion. The work done on it equals its lost kinetic energy. Half times one point two times three squared: five point four joules. The stopping distance is not needed.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Efficiency · Power · Work done.
The whole chapter: Work, Energy and Power, Work, Energy and Power.
Chapter 5.1: Work, energy and power
- Work, power and efficiency 6 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.