Question

Select the correct alternative

• A. (1) Work done by kinetic friction on a body always results in a loss of its kinetic energy.
• B. (2) Work done on a body, in the motion of that body over a close loop is zero for every force in nature.
• C. (3) Total mechanical energy of a system is always conserved no matter what type of internal and external forces on the body are present.
• D. (4) When total work done by a conservative force on the system is positive then the potential energy associated with this force decreases.

Answer 1

Answer: (D)

(4) When total work done by a conservative force on the system is positive then the potential energy associated with this force decreases.

Answer 2

Statement (1) is false because kinetic friction can do positive work, e.g., when a block is placed on a moving belt. Statement (2) is false as non-conservative forces do non-zero work over closed loops. Statement (3) is false because mechanical energy is conserved only in the absence of net work by non-conservative forces. Statement (4) is true, as the definition of potential energy for conservative forces is $W_c = -\Delta U$, so positive work done by a conservative force implies a decrease in potential energy.