Work, energy and power

Learning objectives

Calculate the work done by a force from the component of the force along the displacement (W = Fs cos θ), and explain why a force at right angles to the motion does no work.

Apply the principle of conservation of energy to transfers between kinetic, gravitational potential, elastic and thermal stores.

Calculate the efficiency of a machine or engine as the fraction of the input energy (or power) that is transferred usefully, and explain where the remainder goes.

Define power as the rate of doing work, and derive P = Fv from P = W/t to solve problems on vehicles moving against resistive forces.

Derive ΔEp = mgΔh from W = Fs and Ek = ½mv² from the equations of motion, and use both in energy calculations in a uniform gravitational field.