Question

if you push a heavy box which is in rest you must exert some force to start its motion .
however once the box is sliding you can apply a lesser force to maintain that motion. why ?

Answer 1

This is beacuse when the object is at rest the irregularities of the 2 surfaces get interlocked and thus we have to apply more force to unlock those interlockings and bring it into motion.

but when the body is already in motion it gets very less time to interlock the irregularities of the 2 surfaces and thus as the irregularities are not properly interlocked we need to apply less force to maintain it's motion.

hope it helps.....