If the applied force is increased further and sets the body in motion, the friction opposing the motion is called kinetic friction.
(i) Kinetic friction depends upon the normal reaction.
Fk R or Fk = kR
(ii) Value of k depends upon the nature of surface in contact.
(iii) Kinetic friction is always less than limiting friction
Fk < FI ∴ k < s
i.e. coefficient of kinetic friction is always less than coefficient of static friction.