Friction

“The force which opposed the motion of the body when it is in continuous contact with the surface of the earth or any other object, is called ‘force of friction’ or ‘frictional force’

OR

“When a body slides on a surface on another body, a force comes into play which opposes the motion of the body. This force is known as ‘Force of Friction’“.

• Friction occurs due to the roughness of the two surfaces in contact with each other.

• Friction is unavoidable because there exist no perfect smooth surface.

• Friction is a force therefore its unit it Newton.

• Friction is represented by ‘$F$’ or ‘$f$’.

• The force of friction always acts parallel to the surface in contact and opposite to the direction of motion.

• Friction occurs due to the interlocking of the crests and troughs of the surface of the two bodies when they are in contact with each other.

DERIVATION:

Consider an object of mass ‘$m$’ lying on a table having a weight ‘$W$’ acting towards the surface. Then according to Newton’s third law of motion, reaction ‘$R$‘ of the surface will be offered against the weight of the body.

Let ‘$F_s$’ be the limiting frictional force, if this body is dragged onto a surface, then it can be experimentally concluded that,

• Frictional force is directly proportional to the normal reaction of the surface.

$F_s \propto R$

$F_s = (constant) R$

$\boxed {F_s = \mu R}$

Where, ‘μ’ is the constant of proportionality known as the Coefficient of friction.

$\boxed{\mu = \frac{F_s}{R}}$

Therefore, it can be concluded that the value of coefficient of friction depends upon the nature of the surfaces of the two bodies in contact.

As we know that,

$W =R$

And,

$W=mg$

Therefore,

$R=mg$

Hence, the above formula for friction can be re-written as

$\boxed{F_s=\mu mg}$

The value of μ is constant for any material.

TYPES OF FRICTION:

Static Friction:

“The resisting force between the two surfaces, before the body comes into motion is called its static friction.”

• Static friction is self-adjusting.

• The value of static friction is proportional to the applied force which is tending to change the state of an object from rest to motion i.e. more the applied force on a resting body, the more will be its static friction.

Limiting Friction:

“The maximum force of friction which just stops the body from sliding or moving is called limiting friction.”

• Maximum value of static friction is called Limiting Friction.

Kinetic Friction/ Sliding Friction:

“The friction offered to a body during motion i.e. sliding onto a surface is called its kinetic friction or sliding friction.”

• The value of sliding friction remains constant if the nature of the two surfaces remains same.

• The value of sliding friction is always slightly lesser than the value of limiting friction.

Rolling Friction:

“When a body moves with the help of wheels or rollers or bearing, in this case the friction offered to that body will be termed as rolling friction.”

• The value of rolling friction is far lesser than the value of limiting friction.