Question

The work done by kinetic friction on an object is always negative.

• A. True
• B. False

Answer 1

The correct option is B

False

Consider the situation where a block is in a train and a force is applied such that it is sliding against friction with constant velocity. We know its velocity and displacement will be in the +ve x direction.

Now let us say the train starts moving in the -ve x direction with a velocity greater than the velocity of the block.

Now from the ground frame the velocity of the block will be in the -ve x direction and kinetic friction will be doing positive work.