Class- 8th Chapter- 12 Friction (घर्षण)
Friction:- When one surface is moving over another surface, a force comes into play that opposes their relative motion. This force is called friction or force of friction.
Or, The force that opposes the relative motion between the two surfaces of objects in contact is called friction.
* Friction always opposes motion because it acts in opposite direction to motion.
* Friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.
Factors Affecting Friction:
* Surfaces roughness/Iregularities of surface: The friction will be greater in case of rough surface than in case of smooth surface because friction
is created due to roughness/irregularities on surfaces. The roughness on surfaces result in interlocking of two surfaces.
The interlockingbetween two surfaces creates friction.
* Pressure Between two Surfaces: The force of friction increases with increase in pressure between two surfaces. Due to this moving a heavy box is difficult compared to moving a light box. Pressure is higher in case of a heavy box.
* The force of friction is proportional to the weight of an object placed on a surface. If the weight doubles, force of friction also doubles.
* The regularity of a surface or its smoothness: If the surfaces of either the objects or are smooth, the force of Friction would be less and the object would move easily over the surface. Even smooth surfaces have a certain irregularity.
Type of Friction (6)
Static Friction स्थैतिज घर्षण Sliding Friction सर्पी घर्षण Rolling Frictions लोटनी घर्षण
Limiting Friction सीमांत घर्षण Kinetic Friction गतिज घर्षण Fluid Friction द्रव घर्षण
Static Friction:- The friction between astationary object and the surface on which it’s resting is called static friction.
* The static friction is equal to minimum force that is required to make an object.
Sliding Friction:– The friction between two surface of sliding objects is called sliding friction.
* Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.
* Static friction is greater than sliding friction. This is because it takes more force to break the interlocking between two surfaces than it does to keep them sliding once they are already moving.
Rolling Friction:- The friction between two surface of rolling objects is called rolling friction.
* Rolling friction is less than sliding friction.
Limiting Friction:- The maximum value of the friction force at which the body resting on the another body start to slide is called limiting friction.
Kinetic Friction:- The friction between two surfaces of moving objects is called kinetic friction.
Fluid Friction: – The friction between solid surface of object travelling through a fluid(liquid, air) is called fluid friction.
| Rolling Friction < Sliding Friction < Static Friction |
Drag:- The frictional force generated when a body moves within a fluid is called drag.
Air resistance is a form of friction that acts to slow down any object moving in the air.
Streamlined Shape:-A shape which is wide in middle and tapered atends is called a streamlined shape. Such a shape reduces drag or fluid friction.
Hence, a streamlined body can easily move through a fluid.
* Birds and fish are naturally endowed with streamlined body.
* Boats and airplanes are also made streamlined so that they can easily move through air or water.
Q. 1. Is friction desirable in some cases? If yes, give an example.
Ans:-Yes, friction desirable in many cases. For example, we cannot walk on the road without friction. Friction provide necessary grip between our shoes and the road so that we can walk comfortably.
Q. 2.Why does bicycle slowdown when brakes are applied?
Ans:- On applying brakes, you provide a pressing force between the brake pad and rim of the wheel. This creates a friction between the brakes pads and the rim of the wheel. Friction makes the bicycle slowdown or stop.
Friction: A Necessary Evil (abhishap) Friction is pervasive (sarvyapi). Even a simple task; like writing on a page; cannot be done without friction. Walking on road won’t be possible because friction. But friction causes wear of machine parts, engines, soles of shoes and wastage of energy. Thus, friction is beneficial as well as harmful. Hence, friction is called a necessary evil.
Benefits of Friction
* We walk on the road due to friction. Walking on ice is almost impossible because of lack of friction.
* Friction is necessary to keep a ladder against a wall. The ladder will not find grip against the wall in the absence of friction.
* Without friction, we would not be able to grip object such as pen, pencil.
* Without friction, we would not be able to hold the vessels like glass, tumbler, kulhar (earthen pot)
* Cars and buses are able to run on roads because of friction between the tyres and the road.
* Friction of the brakes and tyres helps to drive car and to stop.
Harms of Friction
* It results in wear and tear of shoes and tyres, machine parts, clothes, etc. It causes monetary loss.
* Heat is generated by friction, which results in wastage of energy. Moreover, a significant amount of energy is wasted in overcoming friction.
* The force of friction slows down or stops the movement of objects.
Ways to Increase Friction
* Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving better grip on these objects.
* A surface can be made rough with the help of sand paper; in order to increase friction.
* Soles of shoes are made rough to increase friction. This helps in providing better grip on surface while walking.
* Treads of tyres are deliberately made so that a good road grip can be achieved with the help of friction.
* Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.
* A carpet is used to cover the tiled floor.
Ways to Reduce Friction
* Lubricating oil and grease are used in machines to reduce friction between moving parts.
* Some machines use air cushion for reducing friction because use of oil is not ideal in such machines.
* Graphite powder is used in some machines to reduce friction.
* Ball bearing converts sliding friction into rolling friction. You have read that rolling friction is less than sliding friction. Thus, ball bearing helps inreducing friction.