FRICTION

FRICTION

DEFINITION: - When a body moves or tends to move over another body then the resistance offered by the second body against the movement of the first body is called friction.

TYPES: -

1. STATIC FRICTION: - Friction experienced by a body when it is at rest, is called Static friction.
2. DYNAMIC FRICTION: - Friction experienced by a body when it is in motion.

a. SLIDING FRICTION: - Friction experienced by a body when a body is sliding over another body.

b. ROLLING FRICTION: -Friction experienced by a body when it is rolling over another body.

LIMITING FRICTION: -The maximum value of frictional force, which comes into play, when a body just begins to move over the another body, is known as limiting friction.

LAWS OF FRICTION (IMPORTANT POINT): -

1. The force of friction is independent of the area of contact between the two surfaces.
2. Depends upon the roughness i.e. nature of the surfaces.

NORMAL REACTION: -When a body is lying on a horizontal or inclined plane, in equilibrium condition, its weight acts vertically downwards through its C.G. The plane in turn exerts on upward reaction on the body. This reaction, which is taken to act perpendicular to the plane, is called normal reaction.

Fig.1 | Normal Reaction

It is generally denoted by ‘R’.

CO-EFFICIENT OF FRICTION: - It is defined as the ratio of limiting friction to the normal reaction between the two bodies.

Generally it is denoted by µ.

Co-efficient of friction u = F/R_N

ANGLE OF FRICTION: - It is defined as the angle which is the resultant of friction and normal reaction, makes with the normal reaction (R_N).

Fig. 2 | Angle of Friction

tanФ = F/R_N = µR_N / R_N = µ

ANGLE OF REPOSE: - The maximum inclination of the plane to the horizontal on which a body start to slip down.

Fig.3 | Angle of Repose

W sinα = F

= µR_N

= µW cosα

Wsinα = µWcosα 

therefore, tanα = µ = tanФ

α = Ф

i.e. Angle of Repose = Angle of Friction

Fig. 4 | Cone of friction