Explain kinetic friction. Write the laws of kinetic friction. Define the coefficient of kinetic friction.

(N/A) When the magnitude of the force acting on an object lying on a surface exceeds the maximum static friction,the object will start moving in the direction of the external force. Hence,the value of the frictional force reduces to a value less than the maximum static frictional force.
The 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 does not depend on the area of contact between the surfaces.
$(2)$ Kinetic friction force does not depend on the relative velocity of the object in motion.
$(3)$ Kinetic friction force is proportional to the normal force.
$\therefore f_{k} \propto N$
$\therefore f_{k} = \mu_{k} N$
where $\mu_{k} = \text{coefficient of kinetic friction}$,$\mu_{k} = \frac{f_{k}}{N}$.
Coefficient of kinetic friction: The ratio of the kinetic friction force to the normal force is called the coefficient of kinetic friction.
Since $f_{s} > f_{k}$,it follows that $\mu_{s} > \mu_{k}$. Once relative motion starts,by Newton's second law of motion,the acceleration of the object is $a = \frac{F - f_{k}}{m}$.
If the object moves with constant velocity,then $F = f_{k}$.
If the external force applied is reduced to zero,then the acceleration of the object will be $-\frac{f_{k}}{m}$. Hence,it will stop after covering some distance.