Friction

 When a body move(or slide or roll), or tries to move over the surface, a tangible force is come into action between their surface of contact, against their relative motion. This opposing force is known as friction.

Friction is depend on the mass of the body and roughness of the surface in contact and magnitude of friction(which increase with the increase of roughness and mass).

static friction: when the body is in rest it is called static friction. It is self adjusting force.

limiting friction: when body move from rest state it is called limiting friction.it is powerful froce and it is always more than static and dynamic force.

dynamic friction:  when body start moving it give rise to dynamic friction.

→?? Usually, smoothness decreases the force of friction. However, when the surfaces in contact are made too smooth by polishing, the binding force of adhesion increases and hence, the frictional force increases. This is called ‘cold welding(bind two metal without heating).

→ Friction is a non-conservative force and hence, the mechanical energy(potential and kinetic energy) is not conserved. In fact, friction converts mechanical energy partly into heat, light(spark), sound, electricity, etc.

→ Generally, friction opposes motion.However, in certain cases friction is essential for motion. Without friction, motion cannot be started, stopped or transferred from one body to the other. Thus, friction is a necessary evil.

Advantages of Friction:

-When a person pushes the ground backward, the rough surface of the ground exerts a forward force due to friction. It makes possible a person to move on the ground. Due to lack of sufficient friction on ice or wet soil, it is difficult to walk

-The working of the brakes of vehicles is possible due to friction only.

-The friction between the tyres of vehicles and road makes the motion of the vehicles possible

-The cleaning action of sandpaper occurs due to friction only

-In absence of friction, we would not be able to hold a pen and if we could, the pen would not write on paper

-Writing on the blackboard with a chalk is possible due to friction

-The transfer of motion from one part of a machine to the other part through belts is possible by friction.

-The working of nuts and bolts for holding parts of machinery together is based on friction.

-The knots in woven clothes are possible due to friction.

Disadvantages of Friction

-Since, the force of friction opposes the relative motion between any two bodies in contact, hence, extra work is done to overcome the force of friction. It involves extra loss of energy. About 20% of the petrol used in an automobile is used up to overcome the force of friction in the engine and in driving

-The force of friction heat the part of machine, that may damage the part of machine.

-Friction causes wear and tear of the parts of the machinery

Order of the magnitude of the  force of friction is:

Rolling friction < sliding friction < limiting friction