We already know that static frictional force exists between two surfaces in contact before there is relative motion between two surfaces
When applied force becomes greater then limiting friction force,then motion of the body starts and kinetic friction comes into existence
Thus when bodies in contact moves relative to each other then friction developed between them is called kinetic or dynamic friction
Consider the figure below in which a block of mass m is kept over the surface S
Initially the block is at rest,now we push the block on the surface such that it begins to move or start sliding on the surface
When the block is sliding over the surface each body exerts a frictional force on the other side parallel to the surface in contact
The Force of friction acting on the block B due to surface S is along the direction opposite to the motion of B with respect to S.Thus force of kinetic
friction opposes the relative motion between the bodies in motion
The frictional force acting on the block is along the direction towards the left and an equal force acts on the surface S directed towards the rights
Thus we can say that sliding friction on a body B against surface S is opposite to the velocity of the body B with respect to S
Force of kinetic friction is denoted by fk and its magnitude is always less then the magnitude of limiting friction i.e maximum static friction
i.e fk < fms
kinetic friction force is proportional to the normal contact force between the surfaces i.e
fk is proportional to N
=> fk =μkN
Where N =Normal contact force
μk is the proportionality constant known as coefficient of kinetic friction and its value depends on the nature of the two surfaces in
contact
fk < fms, this inequality shows that force required to start the motion is greater then the force required to maintain the
further motion of the body
