A body of mass $4$ kg falls freely from a height of $45$ m. Find its kinetic energy when it is $20$ m above the ground. (Take $g = 10$ m s$^{-2}$.)

The kinetic energy gained equals the potential energy lost in falling $45 - 20 = 25$ m: $KE = mg\Delta h = 4 \times 10 \times 25 = 1000$ J. Using the full 45 m gives 1800 J, the energy at ground level; using 20 m gives 800 J, the potential energy remaining.