A stone of mass 1kg is dropped from a height of 10m above the ground and falls freely under gravity. Its kinetic energy 5m above the ground is then equal to
-
A.
its kinetic energy on the ground -
B.
twice its initial potential energy -
C.
its initial potential energy -
D.
half its initial potential energy
Correct Answer: Option D
Explanation
At the height of 10m, its total energy is potential, given by P.E. = mgh = 1 x 10 x 10 = 100J.
But as the stone descends, its potential energy decreases, while its kinetic energy increases.
Half way during the fall, (h = 5m), its P.E. = 50J, and consequently, its K.E. should be 50J, such that from the principle of conservation of energy, the total energy of the stone at any instant should be conserved.