Work, energy and power · revision map · A-Level 9702, Topic 5
TOPIC 5 · WORK, ENERGY AND POWER
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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ENERGY &
POWER5
POWER5
Work & Power
A force doing work transfers energy from one store to another.
W = F s cosθ
θ = angle between
force and displacement
unit joule (J)
force and displacement
unit joule (J)
- Only the component F cosθ along the motion does work.
- Power is the rate of doing work, in watts (1 W = 1 J s−¹).
Conservation & Efficiency
Energy is never created or destroyed, only transferred between stores.
- The total energy of an isolated system stays constant.
- It changes form, but the grand total is unchanged.
- Useful output is always less than total input; the rest is dissipated, mostly as thermal energy.
A Sankey diagram shows the split: useful and wasted branches add to the input.
Kinetic Energy
Energy stored in motion.
Ek = ½ m v²derived from W = F s with F = ma and v² = u² + 2as
- Work to accelerate a body from rest = the KE gained.
- Scales with v²: double the speed, quadruple the energy.
- A scalar quantity, measured in joules.
Gravitational Potential Energy
Energy stored by raising a mass in a gravitational field.
ΔEp = m g Δhwork done against gravity, valid near Earth where g is uniform
- In free fall, lost GPE converts to gained KE:
only changes in height Δh matter, so the zero of GPE can be chosen freely.
BUILDS ON · Dynamics · Forces · IGCSE Energy
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ENERGY
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LEADS TO · Deformation · Gravitational fields