13. Types of frictional force and their definition
There are three types of frictional force
13.1 Static friction
13.2 Dynamic friction & Sliding friction
13.3 Rolling friction
13.1 Static friction :
(a) The frictional force which is effective before motion starts between two planes in contact with each other, is known as static friction.
(b) If a body is at rest and no pulling force is acting on it, force of friction on it is zero (and not μR)
(c) Now if a force is applied to pull the body and it does not move, the friction acts which is equal in magnitude and opposite in direction to the applied force [again not μR], i.e. friction is self acting force.
(d) If the applied force is increased the force of static friction also increases.
(e) If the applied force exceeds a certain (Minimum) value, the body starts moving.
(f) The maximum force of static friction upto which body does not move is called limiting friction (f_L)
(g) Static friction is a self adjusting force with an upper limit, called limiting friction.
(h) f_L ∝ R ⇒ f_L= μ_sR
where f_L= limiting friction
R = normal reaction
μ_S = coefficient of static friction
(i) μ_S depends on the nature of surface in contact and force of adhesion between them.
13.2 Dynamic friction (μ_K)
(a) If the applied force is increased further and sets the body in motion, the friction opposing the motion is called dynamic or kinetic friction.
(b) Kinetic friction is lesser than limiting friction.
(c) f_K=μ_KR, where μ_K is called coefficient of kinetic friction.
(d) f_L > f_K ⇒ μ_S > μ_K
i.e. we require more force to start a motion than to maintain it against friction.
(e) The frictional force, which is effective on sliding of two surfaces in contact with each other is known as the sliding friction.
13.3 Rolling friction (μ_R) :
(a) When a body (say wheel) rolls on a surface the resistance offered by the surface is called rolling friction.
(b) In rolling the surfaces at contact do not rub each other.
(c) The velocity of point of contact with respect to the surface remains zero all the time although the centre of the wheel moves forward
(d) μ_R < μ_K < μ_S
(e) Coefficient of friction has no unit and is dimensionless.