Explain why the sliding friction is less than the static friction.

Static friction is little more than sliding friction for any given pair of surfaces. Once the motion starts, the friction becomes slightly less than the maximum static friction called the limiting friction. Once this motion starts, the irregularities act just as an obstruction against their relative motion, they are not interlocked now. Therefore, it is easier to move the object when it is already in motion than to start it from rest.