Forces & EnergyOxford AQA IGCSE Physics: Revision notes

Work done is a measure of the energy transferred when a force moves an object:

W = F x d

where W is work done (joules, J), F is force (N), and d is the distance moved in the direction of the force (m).

Energy is transferred whenever work is done. If work is done against friction, the energy transfer is by heating — the object and its surroundings warm up.

When a force stretches or compresses an elastic object (like a spring), energy is stored as elastic potential energy:

E_e = 1/2 x k x e^2

where E_e is elastic potential energy (J), k is the spring constant (N/m), and e is the extension (m). This equation only applies within the limit of proportionality — where extension is still directly proportional to force.

Gravitational potential energy (GPE) is the energy an object has because of its position/height above a reference point:

E_p = m x g x h

where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).

Kinetic energy is the energy an object has because it is moving:

E_k = 1/2 x m x v^2

where m is mass (kg) and v is speed (m/s). Because speed is squared, doubling the speed of a vehicle quadruples its kinetic energy — this is why higher speeds are so much more dangerous on the road: a small increase in speed produces a much larger increase in stopping energy.

Power is the rate of transfer of energy, or the rate of doing work:

P = E / t or P = W / t

where P is power (watts, W), E is energy transferred (J), W is work done (J), and t is time (s). A more powerful device transfers the same amount of energy in less time, or transfers more energy in the same time.