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what is friction? merits and demerits of friction ? how to reduce friction ?

Whenever a body moves or tends to move over the surface of another body,a force comes into play which acts parallel to the surface of contact and opposes the relative motion. This opposing force is called friction.

Advantages of friction

(i) It is due to friction between the ground and the feet that we are able to walk.
(ii) The brakes of a vehicle cannot work without friction.
(iii) Various parts of a machine are able to rotate because of friction between belt and pulley.
(iv) The tyres of vehicle are made rough to increase friction.
(v) Nails and screws join various wooden parts due to friction.
(vi) In the absence of friction,it will not be possible to write on a paper with a pen or pencil.

Disadvantages of friction

(i) Wear and tear of the machinery is due to friction.

(ii) A large amount of power is wasted in over coming friction and the efficiency of the machines decreases considerably.

(iii) Excessive friction between rotating parts of a machine produces enough heat and causes damage to the machinery.

As friction has both advantages and disadvantages,we can say that friction is a necessary evil.

Friction between two surfaces can be reduced by any of the following methods:

(i) By polishing.By making the surfaces in contact highly polished and smooth,the bumps and depressions get minimized and so friction is reduced considerably.

(ii) Lubrication.A lubricant is a substance (solid,liquid or gas) which forms a thin layer between two surfaces in contact.It fills depressions present in the surfaces of contact and hence reduces friction.

(iii) Streamlining.Friction due to air is considerably reduced by streamlining the shape of the body moving through air.For example,jets,aeroplanes,fast moving cars,etc have streamline shape.