Whenever surface of a body tend to move on or actually moves relative to another surface in contact, an opposing force is generated between the surfaces. This phenomenon is called ‘friction’ and the opposing force is called frictional force.
There are of three types of friction:
- Static Friction.
- Sliding Friction or Kinetic Friction.
- Rolling Friction.
Properties of Static Friction
- Force of static friction is a contact force.
- Force of static frictions is generated tangential to the surfaces in contact and it is in the direction opposite to the direction in which body tend to move.
- Force of static friction is a self adjusting force i.e. it can adjust its value according to the net external force acting on the body up to a maximum limit. This maximum limit up to which the force of static friction can increases its value is known as limiting force of static friction [fsm or fs max]
- The value of fsm doesn’t depend upon the area of contact between the two surfaces.
- Value of fsm is directly proportional to the normal force acting between the two force i.e. fsm ∝ N
Or fsm = μs N ……….………..(1)
Where μs is a constant for the pair of surfaces in contact and it is called ‘coefficient of static friction’.
Static Friction (Video)
Angle of Repose (θc)
Maximum angle of inclination upto which a block kept on the inclined surface can remain at rest is known as angle of repose . Also above this angle the block starts sliding down due to its own weight.
Value of angle of repose depends upon the value of coefficient of static friction
Or, μs = tanθc ………..(2)
Or θc = tan-1μs ………..(3)