Which of the following is not a conservative force?
A. Electrostatic force
B. Gravitational force
C. Frictional force
D. Elastic spring force
C
Even if the other forces are unfamiliar, a frictional force does not satisfy the conditions for a conservative force. For an object moving along a closed path, some work is always done to overcome friction. Thus, the net work over a complete closed path is not 0, and friction cannot be a conservative force.