Consider , let an object of mass, m be made to fall freely from a height h. at the start, the potential energy is mgh and kinetic energy is 0. it is 0 because its velocity is0. the total energy of the object is thus mgh. as it falls, its potential energy will change into kinetic energy. if v is the velocity of the object at a given instant, the kinetic energy would be 1/2 mv2 . as the fall of the object continues , the potential energy would decrease while the kinetic energy would increase. when the object is about to reach the ground, h=0 and v will be the highest. therefore, the kinetic energy of the object would be the same at all points. that is,
potential energy + kinetic energy = constant
mgh + 1/2 mv2 = constant.