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Friction Chapter 12 Class 8 Notes




Welcome to Class 8 Science chapter 12 notes. Here we go into detail about the various types of friction that we encounter in our daily lives, advantages and disadvantages of friction and much more.
Friction is essentially the resistance that any object encounters when travelling across or through a surface or medium. In the absence of frictional force, an object's movement would either not begin or, if in motion, would not come to a halt.

Friction class 8 science chapter 12 Notes

Meaning of friction

  • When two objects with their surfaces in contact rub against each other, they cause friction.
  • Friction is the force opposite to the direction of motion.
    friction class 8 notes
  • Friction is the force that acts when one object tries to move or slide over the surface of another object.
  • It can be defined as the resistance of motion when one objects rub against other.
  • Friction works against the motion and acts in the opposite direction. However, friction does not always oppose the motion, it also helps in motion.
  • The S.I unit in which frictional force is measured is, Newton.
  • Examples of friction
    1. Friction between the tyres of vehicles and the road.
    2. To make the box move, we have to apply a force to overcome the force of friction.
    3. Friction between pen and book.

What is Friction?
The force, which opposes the relative motion between two surfaces in contact is known as friction. It occurs due to surface irregularities of the two objects in contact.
What is Surface Irregularities?
This unevenness of the surface is called surface irregularities or roughness.

surface irregularities
What are the examples of friction?
1. Writing on paper using a pencil: Friction between the graphite in a pencil and a sheet of paper leaves a mark on the paper.
2. Warming hands in winter: As friction produces heat you can warm your hands in winter by rubbing them against each other. So heat from friction can be useful.
3. Gymnasts apply some coarse substances on their hands to increase friction for better grip.

Cause of friction

  • The root cause of friction is the irregularities or rough edges that is ridges and grooves present on the two surfaces which are in contact. Due to this they get interlocked with each other and cause friction.
  • Even smooth surfaces like tiles in houses, office, school have a large number of irregularities when seen under a microscope.

Why is it difficult to walk on a smooth or wet floor?
Due to less friction between our foot and floor. As the interlocked irregularities possess a layer of water which decreases friction.
What Causes Friction?
Friction exists between two surfaces due to abnormalities on the surfaces of the objects in contact, as well as the interlocking of the two surfaces' micro-level imperfections. So, this force is caused by molecular adhesion, surface roughness, ploughing effect and deformations.

Factors affecting friction

The force of friction depends on the following factors

  • It acts in a direction opposite to the direction of motion.
  • It depends on the mass of the body
  • It depends on the surface's nature, as smooth surfaces have fewer irregularities and thus less friction. As a result, moving an object to a smooth surface is easier than moving an object to a rough surface.

Which one is easier to hold in hand: an earthen pot or a glass tumbler. Why?

Because of the smoother surface, the glass tumbler creates less friction and it is not easier to hold. On the other hand, an earthen pot has a rougher surface which creates more friction and thus makes it easier to grip.

Spring Balance

friction notes

Spring balance is a device used for measuring the force (or weight) acting on an object.

Christian Huygens, invented the first balance springs. Spring balance measures the weight. Spring balances are used commercially.

Working of spring balance

Spring balance consists of a coiled spring that gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of force or weight of the object.


Types of friction

There are four types of friction namely

1.Static friction

2.Rolling friction

3.Sliding friction

4.Fluid friction

1. Static friction

  • The word static means at rest.
  • Static friction can be defined as the friction that acts when we try to move an object at rest or the object which is not moving. It is the strongest type of friction.
  • For example –
    • Cup placed on a table
    • Scenery hanging on a wall
    • A car parked on a hill.
  • Limiting friction is the friction acting on a body when it is about to start moving.

2. Sliding friction

  • Sliding friction is smaller than static friction as a smaller force is now required to keep the object moving.
  • Sliding friction can be defined as the friction that acts when two surfaces slide over each other. If the applied force exceeds the limiting friction the irregularities on it do not get interlocked on the surface on which it is moving.

3.Rolling friction

  • Rolling friction is always less than sliding friction as rolling reduces friction. Rolling friction is the weakest type of friction.
  • Rolling friction can be defined as the force that opposes the motion when an object rolls over another object.
  • For example –
    • Ball bearings
    • Skateboards.
    rolling friction

What are the benefits of using ball bearings?

Due to the fact that rolling friction is less than sliding friction, ball bearings assist in maintaining the smallest possible surface of contact between two moving surfaces.

4. Fluid friction

  • The above three friction occur between solid surfaces whereas fluid friction occurs in fluids, that is in liquids or gases. A fluid is a substance that can flow and take the shape of a container.
  • Fluid friction can be defined as the friction that occurs when objects move across fluids. Another name for fluid friction is a drag. It resists the movement of viscous fluids relative to each other.
  • The force exerted by fluids is called drag. Drag force acts in a direction opposite to the direction of motion of the object. For example - When an aeroplane moves at a high speed it experienced air resistance.
  • Fluid friction depends on the following factors
    1. speed of the object with respect to the fluid.
    2. the shape of the object.
    3. size of the object.
    4. nature of the fluid.
Ways to reduce fluid friction
  • Firstly the question arises that why there is a need to reduce fluid friction.
  • As when objects move through fluids, they lose some of their energy in overcoming the fluid friction which decreases the efficiency of an object. To overcome this there is a need to reduce fluid friction.
  • Fluid friction can be reduced by moulding the shape of an object which offers little resistance to fluids. The streamlined shape is best suited for objects that move in air or water. The streamlined shape is like a thin wedge that cut the air or water and moves forwards.
  • For example – An aeroplane has a special shape called streamlined shape to reduce friction offered by air. Similarly, boats, birds and fishes have this special and suitable shape to reduce fluid friction and avoid loss of energy.
    shapes of objects moving in fluid
What are the types of friction?
The four types of friction are static, sliding, rolling and fluid frictions.
What is fluid friction?
Fluid friction is the frictional force exerted by fluids i.e., liquids and gases.

Friction - A necessary evil

Friction is called a necessary evil as friction has its advantages as well as disadvantages also.

On the soles of shoes, the grooves are present. Grooved soles provide increased friction to the ground, resulting in a greater grip when walking. Because the earth has friction, we can walk easily. As a result, friction is required even in simple tasks such as walking.
Moving on a wet muddy route or a damp marble floor is a challenging chore. This is due to the fact that these surfaces provide relatively little friction to a person's feet.

Advantages of friction

  • Friction helps in the generation of heat.
  • Friction helps in stopping bicycle or any vehicle by applying brakes.
  • Nails can be fixed in a wall due to friction
  • Friction helps to write, walk.
  • Jumping out of an aeroplane with a parachute.
  • Ignition of matchsticks.
  • The motion of pistons.
  • Provide support to the ladder against the wall.

Disadvantages of friction

  • Friction produces heat which may damage machines and cause the wastage of energy.
  • Friction results in the wear and tear of objects, for example, the moving parts of a machine, the tyres of a car, the soles of shoes, and so on.
  • Friction resists motion
  • The cause of forest fires is the friction between tree branches.
  • Production of noise in machines due to friction.
  • Consumption of more fuel in vehicles.

Ways to reduce friction

  • Use of lubricants (Substances like oil or grease polish on the surface of machines to reduce friction and the process of applying lubricants to machines is known as lubrication)
  • The use of wheels also helps to reduce friction.
  • The streamlined body of an object can reduce friction.
  • The use of ball bearings between machine parts greatly helps to reduce friction as rolling friction is less than sliding friction.
  • Sprinkling powder on dholak can reduce friction.
  • Polishing surfaces that come into contact with one another to way to reduce friction. Polishing the surface eliminates imperfections and thereby smooths the surface.

What is lubrication?
Lubricants are substances like oil and grease etc. They help to reduce friction by forming a thin film between the different parts of a machine.

Ways to increase friction

Under some circumstances, there is a need to increase friction

  • Make surfaces rougher- The roughness of the surfaces in contact can be enhanced to increase friction.
  • By increasing the mass of the object that is moving.
  • The treads(A series of patterns )present on the tyres of car, buses and trucks help to increase friction
  • The grooves on the sole of shoes help to increase friction and provide better grip to the ground.
  • The brake shoes in automobiles are used to increase friction.

Why are brake pads used in the brake system of automobiles and cycles?
Brake pads are used to improve friction, allowing the brake to operate more effectively. When we drive the car, the pads do not make contact with the wheels, but when we press the brake lever, the pads interrupt the rim's motion due to friction, and the wheels come to a halt.


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