Question

prove that the law of conservation of energy taking a stone vertically downwards.

Answer 1

assume a stone on a point P 40 meter above the ground .it has potential energy of 40 joules and kinetic energy 0 at time it doesn't start moving. It starts moving downward when it reach 30 meter above the ground the kinetic energy is 10 and potential energy is 30 . when it reach 20 meter above ground both kinetic and potential energy become 20 . on 10 meter above the ground kinetic energy is 30 and potential energy is 10 . and just above the ground kinetic energy become 40 and potential energy is 0. when it hit the ground the kinetic energy become 0 and potential energy become  40. 
                              in all cases the total energy is 40 joules