Friction is the force that develops at the surfaces of contact of two bodies and impedes (opposes) their relative motion. e.g. air on an aircraft or water in a pipe.
It is not a fundamental force, as it is derived from electromagnetic forces between atoms and electrons, and so cannot be calculated from first principles, but instead must be found empirically.
When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or heat.
The force of friction between two bodies depends on the magnitude of the perpendicular forces of the surfaces in contact. This perpendicular force is known as the normal force (R) and the ratio of the frictional force to the normal force is known as the coefficient of friction.
The deformed surface pushes on the body with a normal force R that is perpendicular to the surface. This is called normal reaction. It balances mg i.e.
R = mg
Suppose a small force P is applied on the block to the right as shown. The force of friction F opposes the motion.
So long as the block does not move,
F = P
This means as we increase P, the friction F also increase, remaining equal to P always.
Types of friction
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Internal friction
It arises on account of relative motion between every two layers of a liquid. Internal friction is also referred to as viscosity of the liquid.
2. External friction
It arises when two bodies in contact with each other try to move or there is an actual relative motion between the two. The external friction is also called contact friction.
External friction is of three types:
1. Static friction
The opposing force that comes in to play when our body tends to move over the surface of another, but the actual motion has yet not started.As seen in the above figure, the magnitude of the static friction is not constant. It always adjusts itself so as to be equal to the applied force.
2. Limiting friction
As we increase the applied force, a stage comes when the body is just at the verge of the moving over the other. The static friction at this stage is obviously maximum. This maximum value of static friction is called Limiting force.
3. Kinetic friction
When we increase the applied force slightly beyond limiting friction, the actual motion starts. This does not mean that friction has disappeared. It only means that the applied force is now greater than the force of limiting force. The force of friction is stage is called kinetic friction or dynamic friction.
Kinetic friction is always slightly less than the limiting friction. This is because once, the motion starts actually; inertia of rest has been overcome. Also, when motion has actually started, irregularities of one surface have little time to get locked again into the irregularities of other surface.
Further kinetic or dynamic friction may be of types:
i. Sliding friction – The opposing force that comes into play when one body is actually sliding over the surface of the other body is called sliding friction. For example, when a flat block is moved over that flat surface of a table , the opposing force is called sliding friction.
ii. Rolling friction – The opposing force that comes into play when one body is actually rolling over the surface of the other body is called rolling friction. For example, when a wheel , a circular disc or a ring or a sphere or a cylinder rolls over a surface , the force opposing it is the rolling friction.
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