A level Physics · Edexcel 9PH0 · Topic 2D

Work, energy and power

Key points

Work done = force × distance moved in the direction of the force: $\Delta W = F\Delta s$ (or $F s\cos\theta$ when the force is at an angle). Unit: joule.
Kinetic energy $E_k = \frac{1}{2}mv^2$ ; gravitational potential energy change $\Delta E_{grav} = mg\Delta h$ .
Conservation of energy: energy cannot be created or destroyed, only transferred; e.g. falling object: $mg\Delta h = \frac{1}{2}mv^2$ if air resistance is negligible.
Power is the rate of doing work or transferring energy: $P = \frac{E}{t} = \frac{W}{t}$ , measured in watts (J s $^{-1}$ ).
Efficiency = useful energy output ÷ total energy input (or useful power out ÷ total power in), often given as a percentage.
Work done against friction or drag is usually transferred to the surroundings as thermal energy.

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Energy is the quantity that is always conserved when objects interact, so tracking where it goes is one of the most powerful ways to solve mechanics problems. This sub-topic covers work, kinetic and gravitational potential energy, power and…

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