The kinetic friction force acting on a sliding body moving in a straight line is constant, but it is non-conservative. Explain.

Explanation

Kinetic friction depends on the velocity as it opposes the relative sliding. When the block slide towards right, let it experience, a kinetic friction f towards left at a point P. When it retraces its path, at the same point P, it experience as kinetic friction of same magnitude f but in opposite direction. Hence, work done by friction in to and fro journey will not cancel each other. As friction opposes the displacement, always the total work performed by it is non-zero. It means, the total frictional work done in a closed path depends upon the length of the path. Hence it is non-conservative force. The total work done by kinetic friction acting on two contacting surface is negative. However, the total work done by kinetic friction between two surfaces in contact does not depend on the reference frame.