If we try to slide a body over a surface the motion is resisted by a bonding between the body and the surface. This resistance is represented by a single force and is called friction. The force of friction is parallel to the surface and opposite to the direction of intended motions.

If a body is at rest and it is not being pulled, force of friction is zero.

If a pulling force is applied and the body does not move, friction still acts and is called static frication. The maximum value of static friction is called limiting friction that is, static friction is self adjusting with an upper limit called limiting frication.

Limiting frication Fη = µ_s N where N is normal reaction µ_s = tan θ where θ is angle of limiting friction.

µ_s > µ_k > µ_R

Where µ_s stands for coefficient of static frication µ_k stands for coefficient of kinetic friction and µ_R stands for coefficient of rolling friction.

Friction is independent of the surface area in contact but however depends upon the nature of material of the surfaces in contact (that is, force of adhesion or cohesion), their roughness or smoothness, inclination, normally, with increase in smoothness, friction decreases. However, if the surfaces are made too smooth by polishing, the bonding force of cohesion or adhesion increases resulting in cold welding.

Friction may not oppose the motion always in certain cases it causes motion. in cycling, the rear wheel moves by the force communicated to it by pedaling while form wheel moves by itself. Thus while pedaling the force of friction acts on it in forward direction. Front well moves by itself experiencing friction in backward direction. If pedaling is stopped both the wheels more by themselves and so experience a force in backward direction.

In practice O < µ<1, but µ > 1 is observed. Coefficient of friction between Cu – Cu surfaces is 1.6, between glass and glass is 1.

Friction is a non conservative force.

If force is applied and still the body is at rest then the force of friction is equal to force applied?

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