When magnitude of force acting on object lying on surface exceeds maximum static friction the object will start moving in direction of external force. Hence, value of frictional force will reduce less than maximum static frictional force.

Frictional force which opposes relative motion between surfaces in contact is called kinetic friction. It is denoted by $f_{k}$.

Laws of kinetic friction:

$(1)$ Kinetic friction do not depend on area of contact between surface.

$(2)$ Kinetic friction force do not depend on relative velocity of object in motion.

$(3)$ Kinetic friction force is proportional to normal force.

$\therefore f_{k} \propto \mathrm{N}$

$\therefore f_{k}=\mu_{k} \mathrm{~N}$

$\mu_{k}=\text { coefficient of kinetic friction, } \mu_{k}=\frac{f_{k}}{\mathrm{~N}}$

Coefficient of kinetic friction : Ratio of kinetic friction force and normal force is called coefficient of kinetic friction.

$f_{s}>f_{k}$ hence $\mu_{s}>\mu_{k}$ once relative motion start, then by Newton's second law of motion acceleration of object $a=\frac{\mathrm{F}-f_{k}}{\mathrm{~m}}$.

If object move with constant velocity, then $\mathrm{F}=f_{k}$

If external force applied is reduced to zero, then acceleration of object will be $-\frac{f_{k}}{\mathrm{~m}}$. Hence, it will stop after covering some distance.