The contact force between a body and the surface on which it is placed can be resolved into two components, one along the normal to the surface in contact and the other parallel to the surface. The component perpendicular to the surface is called normal reaction and the force parallel to the surface is called friction.
An empirical relationship between the normal reaction (N) and limiting frictional force (f) by considering a body of mass m placed on a horizontal table. The static friction fs opposes impending relative motion and the kinetic friction fk opposes the actual motion.
Laws of Friction
1. fs is independent of area of contact.
2. fs ∝ N
fs (max/limiting) = μs N , Where μs = Co-efficient of static friction.
3. fk ∝ N
fk = μk N , Where μk = Co-efficient of kinectic friction.
Limiting friction increases as the mass of the block increases and is independent of the contact area between the two surfaces. The kinetic frictional force is always slightly less than the static limiting frictional force.
When cylindrical or spherical-shaped bodies roll without slipping on a cylindrical or flat surface, the frictional force is minimal. To reduce the friction further, grease or lubricating oil is introduced between the surfaces. Air is used as a lubricant in high-precession and high-speed equipment.
Friction is desirable or undesirable depending on the situation in it occurs. Friction between the moving parts of a machine is undesirable. Friction is desirable when it occurs between the tyres and the road’s surface. Because of this friction, the automobile is able to move forward or stop when brakes are applied. We are able to walk only because of the friction between our feet and the ground. We would fall if we tried to walk on a slippery surface.