A car of mass $1000$ kg travelling at $20$ m s$^{-1}$ is brought to rest by its brakes in a distance of $50$ m. Find the average braking force.

The work done by the brakes equals the kinetic energy lost: $Fd = \dfrac{1}{2}mv^2$, so $F = \dfrac{\frac{1}{2} \times 1000 \times 400}{50} = \dfrac{200000}{50} = 4000$ N. Omitting the half gives 8000 N.