If a rock weighing 2,200 n is dropped from a height of 15m, what is the kinetic energy just before it hits the ground?

We can solve the problem by using the law of conservation of energy.

Initially, the rock has only gravitational potential energy, which is given by

where mg is the weigth of the rock (2200 N), while h is the height at which the rock has been released (h=15 m). If we calculate it, we get


Just before hitting the ground, the rock height is zero, so its potential energy is now zero. So the total mechanical energy of the rock now is just kinetic energy:

however, the mechanical energy of the rock must be conserved, so


and so we have that the kinetic energy of the rock just before hitting the ground is equal to its initial potential energy: