Friction

When a body is in motion over another surface or when an object moves through a viscous medium like air or water or when a body rolls over another, there is a resistance to the motion because of the interaction of the object with its surroundings. Such a resistance force is called force of friction.

Friction is a result of molecular interaction. According to modern view, the cause of friction is largely due to atomic and molecular forces between the two surfaces at the point of contact.

Types of Frictional Force

Static Friction: The frictional force, which is effective before motion starts between two planes in contact with each other, is known as static friction.

Static frictional force is a self adjusting one. The maximum frictional force when the body is ready to start is called limiting frictional force.

Dynamic Friction: The frictional force, which is effective when two surfaces in contact with each other are in relative motion with respect to each other, is known as dynamic friction.

Rolling Friction: The frictional force, which is effective when a body rolls or rotates on a surface, is known as rolling friction.

Limiting friction (F_s) is independent of the area of contact of the surfaces. Rolling friction depends on the area of contact. Limiting friction is directly proportional to the normal reaction between the surfaces in contact.

F_s ∝ R

F_s = μ_sR where μ_s is called the coefficient of static friction. It depends upon the nature of the surfaces in contact and their state of roughness. The angle between R and the resultant of R and F is called the angle of friction λ.

Tan λ = μ

Characteristics of Static Friction

1. μ_s between two given surfaces is independent of the normal force between the two surfaces.
2. μ_s > 0, it can also be greater than one, but in most of the cases it is less than one.
3. If θ_s is the angle of limiting friction between two surfaces tan θ_s = μ_s.

When one body moves over the other, the force of friction acting between the two surfaces is called kinetic friction. The force of kinetic friction is independent of the area of the surfaces in contact and is proportional to the normal reaction.

F_k ∝ R.

F_k = μ_kR, Where μ_k is coefficient of kinetic friction.

When one body moves over another body, the coefficient of friction is less than limiting coefficient of friction and is called the coefficient of kinetic friction.

F_k is independent of the velocity of sliding provided the velocity is low.

Rolling Friction

When a body rolls over another, the frictional force developed is called rolling frictional force and the corresponding coefficient of friction is called coefficient of rolling friction (μ_r).

1. Rolling friction comes into play when a body such as a wheel rolls on a surface.
2. Rolling friction arises out of the deformation of the two surfaces in contact with each other.
3. Greater the deformation greater is the rolling frictional force.
4. The rolling frictional force is inversely proportional to the radius of the rolling body.
5. The rolling frictional force between two given surfaces is lesser than kinetic and limiting frictional forces.
6. If μ_R is the coefficient of rolling friction, μ_R < μ_k < μ_s for a given pair of surfaces.
7. Ball bearings are used in machinery parts because rolling friction is least.
8. Radial tyres used in cars reduce rolling friction.

When lubricants or viscous liquids are introduced between the surfaces of two solids in contact, they reduce frictional forces because intermolecular forces weaker than those in solids.