FRICTION

When a body slides over the surface of another body, an opposing force is set up between them to resist    the motion. The force which opposes the motion is called friction OR Force of Friction.

Force of friction tends to decelerate a body and always acts in the opposite direction of motion.

CATEGORIES OF FRICTION    (1) Contact friction    (2) Fluid friction

 LIMITING FRICTION : -

When an external force is applied against the force of friction, the force of friction also increases by the    same amount. Therefore, It adjusts itself in such a way that it is equal and opposite to the external    force. It has a maximum value just before the motion starts. So friction is a self-adjusting force. The    maximum force of friction that stops the body from moving is called LIMITING FRICTION.    It is denoted by Fs.    LIMITING FRICTION is directly proportional to the surface reaction.

Limiting friction Fs is: Fs ∞ R Fs = µR Fs = µW Where R = normal reaction But R = W and W = mg So the above equation becomes Fs = µmg    Where µ = constant known as coefficient of friction. COEFFICIENT OF FRICTION : -  Coefficient of friction is the ratio of LIMITING FRICTION to the NORMAL REACTION. µ = Fs / R    Coefficient of friction is constant for a given pair of surfaces but different for different pairs.    Unit of µ:    Since it is a ratio of two similar quantities, therefore it has no unit as shown. ROLLING FRICTION: - When a body rolls over a surface, the force of friction is called ROLLING FRICTION. When a sphere rolls    over a surface it experiences an opposing force called ROLLING FRICTION.    Rolling friction is much less than the sliding friction because in case of rolling contact area of two surfaces    is very small as compared to sliding.