NCERT Class 8th Chapter -12: Friction

Updated Thu, 02 Jun 2022 12:58 AM IST


If you are searching for notes on friction , then this article is for you. In this article you find everything relating to friction. So read the complete article carefully.

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When a body is moving on another body or an attempt is made to move, then the force present between their contact planes which opposes their motion relative to each other is called friction or ‘frictional force’. The pages of the body are not completely smooth. When these are viewed with a high-power microscope, they have friability and irregularities in many places.

What is friction: Definition
What is dry fiction?
 Types of friction
Friction in liquid
Static friction force
Kinetic friction
Rolling friction
Mathod to reduce friction
The advantage of friction
Disadvantage of friction

 When one surface is placed on top of another, groups of molecules press against each other and irregularities get trapped in each other. Due to which a kind of hot welding is done. This situation arises due to interatomic forces.

Source: Amar ujala

When it slides on a surface, the force required to break the bonds between the molecules is called frictional force.We see many examples of friction everyday in our daily life.
Like lighting a match, riding a motorcycle, rubbing hands warms hands etc.

Definition of the force of friction:-

 “This force is that component of the contact force which is parallel to the contact.”

What is dry friction?

 When two dissimilar solid surfaces in contact slide or try to slide over each other, this force is between them. It is also called Coulomb friction.

Types of dry friction:-

(1) Static friction
(2)Kinetic friction

Friction in liquid:-

 When adjacent layers of a liquid move with different velocities, the friction between them is called viscosity. When a solid object moves in a liquid, the friction produced by the liquid is called viscous force.
Internal frictional force?
 “When a solid object undergoes shear, its various parts slide on each other inside and form a system of frictional forces, which we call internal friction. There is always a loss of energy in this.”

Static friction force:-

 The force of friction which acts before the relative motion between the contact surfaces is called static frictional force.Its nature is to adjust itself.
 The numerical value of static friction is equal to the external force that tries to provide motion relative to the object. The maximum value of static friction is called marginal friction.

 kinetic friction:-

• Kinetic friction means “If the object is moving with relative motion, then the friction opposing its relative motion is called kinetic friction.
• The value of kinetic friction is always slightly less than marginal friction.
• Its value also depends on N.
• Its value also does not depend on the type of motion of the object as it is fixed.
• Its numerical value is fk = uk N. Here uk is the coefficient of dynamic friction.

  rolling friction

 The frictional force that arises due to the rotation of a body or object on a plane is called cylindrical friction. When a body rolls down a path, the contact area is very small, due to which the pressure becomes very high.

Methods to Reduce Friction


Using lubricant:-

 In watches, sewing machines, etc. Thick oil or grease is used in heavy and fast-moving machinery in watches, sewing machines, etc. Thick oil or grease is used in heavy and fast-moving machines.

 Using ball bearing:-

 Movable wheels or small metal bullets placed between the accounts of distances are called ball bearings.
Ball bearings are used to convert object motion into rolling speed. In this case, the effective value of the friction force is very small.

Using polish:-

Friction is reduced by applying polish between two similar surfaces.
The value of friction force can be reduced by polishing the contact surfaces. The surfaces contacted by polish become flat by filling up bulges and troughs. Due to this the intermolecular attractive force makes the bond relatively less and the value of frictional force decreases.


The advantages of friction are as follows:-

I. The force of friction helps us to walk on the surface of the earth. In the absence of friction, we cannot even think of walking on a surface.
II. The grating force between the surface of the paper and the top of the pen helps us to write. It is difficult to write on an oily or smooth surface because there is a lack of friction.
III. The force of friction between the wheels of the vehicle and the road helps to stop the vehicle when brakes are applied. By applying brakes on the vehicle in the absence of Gharshan, the vehicle will slip on the road.
IV. The belt moving around the circumference of the wheel remains on the wheel due to friction.
V. The frictional force between a wheel and a blackboard is helpful in writing on the blackboard.

Disadvantages of friction:-

I. A considerable amount of energy of a moving object due to friction is wasted in the form of heat energy.
II. It does not allow the velocity of vehicles, rockets, trains, etc. to exceed a certain limit.
III. The efficiency of machines decreases due to friction.
IV. The moving parts of the machines wear out due to the force of friction.

Q1-What is the kind of friction?

Answer- Friction is a force, the resistance of motion when one object rubs against another. Whenever two objects rub against each other, they cause friction. Friction works against the motion and acts in the opposite direction.

Q2,-What is the dry fiction?

Answer- Dry friction describes the reaction between two solid bodies in contact when they are in motion (kinetic friction) and when they are not (static friction).

Q3-What factor effect friction?

The frictional force between two bodies depends mainly on three factors: (I) the adhesion between body surfaces (ii) roughness of the surface (iii) deformation of bodies.

Q4-How we can reduce friction?

Answer- Methods for Reduction friction:
Make the surfaces little more smoother. ... 
Lubrication is another way to make a surface smoother. ... 
Make the object more streamlined. ... 
Reduce the Normal force acting between the surfaces in contact. ... 
Reduce the contact between the surfaces, so that less number of bonds will be formed.

Q5-What is the friction in everyday life?

Answer-Without friction between the shoes and the ground, you cannot walk properly. If there is no friction, all of us would be sliding around without any control over ourselves. While walking, you're pushing your foot back as you try to step forward. Friction holds your shoe to the ground so you can walk around.

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