Notes of chapter: Friction are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 8.
Friction is a force which opposes the relative motion of the two surfaces.
Push a book from the right side on the table. The book will move in opposite direction, ie, left direction and stop.
Now, push the book from the left side of the book, the book will move in the opposite direction, ie, right direction and stop.
It happens because force of friction acts between the book and surface of the table.
(2) The irregularities between the surfaces affect the force of friction between those surfaces. The irregularities between the surfaces lock into the each other. The force of friction is applied to overcome the interlocking.
(i) The force of friction is very high when both surfaces are rough because rough surfaces have many irregularities.
Eg:- Roll a pen wrapped in sand paper on the inclined plane of sand. It will move short distance.
(ii) The force of friction is very small when both surfaces are smooth because smooth surfaces have less or almost no irregularities.
Eg:- Roll a pen on the inclined plane of smooth surface. It will move to a long distance.
(iii) The force of friction is comparatively high when one surface is rough and one surface is smooth because rough surface has irregularities and smooth surface has almost no irregularities.
Eg:- Roll a pen wrapped in sand paper on the inclined plane of smooth surface. It will move to a longer distance than the rough surfaces and shorter distances than the smooth surfaces.
(3) Types of the friction-
The force which is required to move an object from the position of rest is known as static friction.
Eg:- The force which we applied on a box to move from the rest is a static friction.
The force which is required to keep an object moving with the same speed is known as sliding friction.
Eg:- The force which we apply on the box to keep it moving is sliding friction.
Once the box begins to move or slide the contact points of the both surfaces can not get time to lock. Therefore, sliding friction is slightly smaller than the static friction. It is always easy to move the box already in the motion than to get it started.
The force which is required to roll an object over the surface is known as rolling friction. Rolling reduces friction.
Eg:- (i) Rolling luggage over wheels is easy because it reduces friction.
(ii) Ball rolls over the ground and stops due to rolling friction.
(4) Uses of the friction-
(i) Glass tumbler can be hold only because of friction.
(ii) An object can be stop from its moving stage because of friction.
(iii) Fixing a nail and tying a knot can be only possible only by friction.
(iv) Friction produces heat. Our palms feel warm when we rub them together.
(5) Harms of the friction-
(i)Friction wears out the material.
Eg:- (a) Ball bearings of wheels are wear out because of friction. Ball bearings have to move against friction which causes wear in ball bearings.
(b) Screws also wear out because of friction.
(c) Soles of shoes also wear out because of friction. We have to put force of friction to move against interlocking of shoe’s sole and earth which causes wear out of the sole of shoes.
(ii) Friction opposes movement.
Eg:- It stops ball, bicycle or any other objects which is in motion.
(iii) Friction produces sound in the machines which causes noise pollution.
(iv) Due to friction, automobiles consume more fuel.
(6) Increasing and reducing friction-
We can increase the friction for better grip on the floor in many ways. Some of them are discussed below:-
(i) Groves increases force of friction between two surfaces even if one surface is smooth and other is grooved. It happens because of good interlocking between the two surfaces ,ie, one is grooved(not smooth) and other is smooth. When we want to move, slide or do any activity we have to put more force of friction against this interlocking. Therefore, movement becomes slow and safe.
Eg:- (a) Groove in sole of shoes.
(b) Treaded tyres also used to increase friction for safe driving.
(ii) Brake pads increases force of friction between two surfaces, ie, wheel and the earth. When we press the brake lever it works against the motion of vehicle. The brake pads arrest wheel and wheel stops motion.
(iii) Coarse substance increases friction between two surfaces, ie, one surface where coarse substance is used and the other surface. Coarse substance increases the force of friction between the surface where coarse substance applied and the other surface. The interlocking between these two surfaces is good because one surface is rogue (Coarse). Therefore, we have to apply more friction to increase grip.
Eg:- (a) Gymnasts put coarse substance on their hands to increase friction which increases grip on the gymnasting tools.
(b) Kabbadi players rub their hands with soil to increase friction which increases their grip on their opponents.
(7) Reducing friction can be done by many ways which are listed below:-
(i) Use of powder lowers down the friction between the two surfaces because of smoothness of powder a weak interlocking or no interlocking occurs between the two surfaces which causes less friction.
Eg:- We spread powder on the carom board to reduce friction between the board and coins.
(ii) We pour few drops of oil over the hinges of the door which reduces the friction between hinge and door.
The substances which reduce friction are called lubricants.
Eg :- Oil, grease etc.
(8) Friction can not be eliminated completely because if there is no friction, no movement is possible.
(9) Gases and liquids are commonly known fluids in science. Fluids also exert friction which is called drag.
(i) The frictional force of the object depends on the speed of the fluid in which an object is moving.
If speed of the fluid is high the friction is also highs and vice-versa.
(ii)The frictional force also depends on the shape of the object and the nature of the fluid.
The transport mode of fluids designed to reduce friction between them and fluid which causes less energy consumption. Their design must have inspired by the natural things as birds and fishes. They have shapes which would make them lose less energy in overcoming friction.