Question

Select the correct alternative.
(A). Work done by kinetic friction on a body always results in a loss of its kinetic energy.
(B). Work done on a body, in the motion of that body over a closed loop is zero for every force in nature.
(C). Total mechanical energy of a system is always conserved no matter what type of internal and external forces on the body are present.
 (D). When total work done by a conservative force on the system is positive then the potential energy associated with this force decreased.

Answer 1

Hint – Here we will proceed by explaining all the parts one by one, in order to find the correct alternative we have to use the approach of conversion of energy, power and work.

Complete step-by-step solution -

For part (a).
The first statement that work done by kinetic friction on a body always results in a loss of its kinetic energy, is not true because as always as work done by kinetic friction on a body, it may increase its kinetic energy as well. Hence, statement 1 is incorrect.

For part(b).
The second statement is that work done on a body, in the motion of that body over a closed loop is zero for every force in nature. It is not true because the external forces on the body may change the total energy of the body. The work done in the motion of a body over a closed loop is zero, it is indeed true but only for conservative forces. This is because in such collisions, there is always a loss of energy in the form of heat, sound etc. therefore it is not true for every force in nature.

For part ( c ).
The third statement that is Total mechanical energy of a system is always conserved no matter what type of internal and external forces on the body are present. It is incorrect because internal forces will not change the energy of the system but external forces can change the total energy by changing their magnitude or direction. So, this statement is also incorrect.

For part (d).
The fourth statement is that When total work done by a conservative force on the system is positive then the potential energy associated with this force decreases. This statement is true because a conservative force does a potential work on a body when it displaces the body in the direction of force. As a result, the body advances towards the centre of force. It decreases the separation between the two, thereby decreasing potential energy of the body.

Therefore, Option D is the correct answer.

Note – Whenever we come up with this type of question, one must know that here we have used the approach of work, energy and power that is work is done by using energy and power is the rate at which work is done. By using the basic concepts of work, energy and power we can easily solve questions related to it.