Question

When a stone falls freely towards the earth, its total energy?
(A) First decreases and then becomes zero
(B) Remains constant.
(C) Increases
(D) Decreases

Answer 1

When a body falls freely then it moves under the influence of gravity only. We know that from Newton’s First law of motion if a body is at rest it will remain at rest, if the body is in motion it will remain in motion until and unless it is acted upon by some external force. So, for the motion to continue a force must be exerted and here that force is the gravitational force.

Complete answer:
When the body is at some height and it is at rest, then the energy possessed by the body is the potential energy and when the body falls freely then its potential energy starts decreasing but its kinetic energy keeps on increasing. Assuming there is no loss of energy, at any point of time of its total energy remains constant. Also, when it strikes the ground, it has completely kinetic energy. Thus, the energy of the body remains constant.

So, the correct option is B.

Note: Mechanical energy can be summed up as the sum of potential and kinetic energy stored or possessed by the body which is either at rest, either in motion. Mechanical energy is used to do work. Also, there is a very important theorem called work energy theorem. Work-energy theorem states that the net work done by the forces on an object equals the change in its kinetic energy.