Class-8, Science, Chapter:- 12 Friction, Notes, NCERT Solutions & Frequently Asked Questions

Science

Class - 8,  Chapter - 12

Friction 

Notes, Ncert Solutions & Frequently Asked Questions 

-- Notes --

⭐ Friction force:- 

• The external force that opposes relative motion between two surfaces in contact. 

• Friction acts on the surface of contact of both the bodies.

⭐ Relative motion:- When one object moves relative to another it is called a relative motion.

⭐ Cause of friction:-

• Friction occurs due to surface irregularities of the two objects in contact.

• Adhesive forces between surfaces in contact.

• Plowing effect 

a) Surface irregularities:-

• All surfaces when zoomed into a microscopic level contain hills and valleys that interlock when they move or rub on top of each other.

• This unevenness of the surface is called as surface irregularities or roughness.

• Rough surfaces have larger irregularities while smoother surfaces have lesser irregularities.

b) Adhesive forces:- 

• When two surfaces are in contact they start to form bonds and begin to stick to each other. This phenomenon is called as Adhesion.

• When we try to move objects that are on top of another, we are basically breaking the bonds or overcoming the adhesive forces.

c) Plowing effect:- 

• When surfaces are soft or can change their shape easily, they get deformed when they come in contact with another object. 

• Eg.- carpets, when a heavy object is placed on them, it looks like a valley that is caused by the deformation of the shape.

• This effect of the surfaces sinking into each other is known as Plowing effect.

⭐ Factors Affecting Friction:- 

• Friction depends on the nature of surfaces in contact.

• For a given pair of surfaces, friction depends upon the state of smoothness or roughness of those surfaces. It is less for smooth surfaces.

• Friction is independent of the area of contact.

• Friction depends on how hard the two surfaces pressed together.

⭐ Interlocking:- Irregularities on two surfaces in contact may clasp with each other, increasing friction.

⭐ Calculating frictional force using a spring balance:- 

• Using a spring balance we can find the frictional forces opposed by different materials.

• Sandpaper gives a higher reading as compared to stainless steel. 

⭐ Polishing surfaces in contact to change friction:- Polishing surface reduces irregularities and therefore makes the surface smooth.

⭐ Lubricants:- 

• The substances which reduce friction are called lubricants.

• Eg.- grease, graphite powder, oil etc. 

⭐ Normal reaction force:-

• Force applied that is exactly perpendicular to the surfaces in contact is called normal reaction force.

• It increases the frictional force.

⭐ Static Friction:- The force required to overcome friction at the instant when an object starts moving from rest is called static friction.

⭐ Kinetic Friction:- The friction that comes into play when objects are in motion is called as kinetic friction.

• Kinetic friction is of two types:- 

a) Rolling friction:- When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

b) Sliding Friction:- When one surface is sliding over the other surface, sliding friction comes into play.

Note:- 

• Sliding friction is less than static friction.

• Rolling friction < Sliding friction.

⭐ Friction - a Frenemy:- As friction involves breaking bonds, they make the particles vibrate → increase kinetic energy and therefore increase heat.

⭐ Applications of friction:- 

• Writing, walking, running, tyres on a car, a nail stays in the wall due to friction, usage of a matchstick.

⭐ Friction can be reduced by:- 

• using grease, oil, powder, ball bearing, and cushion of dry air between the moving surfaces.

• using anti-friction alloys.

Note:- 

• Friction can be increased by making a surface rough.

• The sole of the shoes and the tyres of the vehicle are treated to increase friction.

⭐ Ball Bearing:- Ball bearings reduce friction. They are used between hubs and the axles of ceiling fans and bicycles.

⭐ Drag force:- 

• Frictional force exerted by fluids is called drag.

• The drag force on an object depends on speed as well as the shape of the body and nature of the fluid.

• Fluid Friction/Drag can be minimised by giving suitable shapes to bodies moving in fluids.

• To overcome fluid friction bodies of fish and birds are streamlined. 

• Similarly, the ships and aeroplanes are also made streamlined.

-- NCERT Solutions --

Question 1.- Fill in the blanks.

a) Friction opposes the ______ between the surfaces in contact with each other.

b) Friction depends on the ______ of surfaces.

c) Friction produces ______

d) The sprinkling of powder on the carrom board _______ friction.

e) Sliding friction is _______ than the static friction.

Solution:-

a) relative motion

b) smoothness (or irregularities or nature)

c) heat

d) reduces

e) less 

Question 2.- Four children were asked to arrange forces due to rolling, static and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.

a) rolling, static, sliding

b) rolling, sliding, static

c) static, sliding, rolling

d) sliding, static, rolling

Solution:- c) static, sliding, rolling.

Question 3.- Alida runs her toy car on a dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

a) wet marble floor, dry marble floor, newspaper and towel.

b) newspaper, towel, dry marble floor, wet marble floor.

c) towel, newspaper, dry marble floor, wet marble floor.

d) wet marble floor, dry marble floor, towel, newspaper.

Answer:- a) wet marble floor, dry marble floor, newspaper and towel.

Question 4.- Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Answer:- Frictional force will act upward, i.e., the direction opposite to that of sliding book.

Question 5.- You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer:- The layer of soap makes the floor smooth due to which the friction is reduced. This makes the floor slippery and the foot cannot make a proper grip on the floor. Therefore it is difficult to walk on a soapy floor. We may slip on the floor.

Question 6.- Explain why sportsmen use shoes with spikes.

Answer:- Sportsmen use shoes with spikes to increase the friction between shoes and the surface. So the shoes with spikes do not slip while the sportsmen run and play.

Question 7.- Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Answer:- A heavy object produces more friction as it is pressed hard against the opposite surface. So Seema will have to apply a larger force.

Question 8.- Explain why sliding friction is less than static friction.

Answer:- The sliding friction is less than static friction because the sliding object get less time to interlock into the contact points on the floor. So it is somewhat easier to move an object already in motion than to get it started.

Question 9.- Give examples to show that friction is both a friend and a foe.

Answer:- Some points are given below which show that friction is both a friend and a foe:-

•• Friction as a friend:-

- It allows us to grip and catch any object.

- It helps us to walk comfortably on the floor.

- It helps to minimise the speed or to stop any moving object.

- It helps us to write.

•• Friction as a foe:-

- It causes wear and tears in objects.

- It causes damage to the parts of machines and tools which further require money to get them repaired.

- It reduces the speed of moving objects, so more force is required.

- It produces hurdles in moving any object freely.

Question 10.- Explain why objects moving in fluids must have special shapes.

Answer:- The objects moving in fluids must have a special shape to overcome the fluid friction acting on them. Efforts are therefore made to minimise the friction, so objects are given special shape having pointed fronts with little broader middle portion which gets tapered at the back called streamlined.

-- Frequently Asked Questions --

Question 1.- Define friction.

Answer:- Friction is the force which resists the motion of a body, while moving on a surface.

Question 2.- What is meant by sliding friction ?

Answer:- When one surface is sliding over the other, sliding friction comes into play.

Question 3.- When does static friction come into play ?

Answer:- Static friction comes into play to counter balance the applied force on the body.

Question 4.- What are lubricants ?

Answer:- The substances which reduce friction are called lubricants.

Question 5.- What is rolling friction ?

Answer:- When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

Question 6.- What are fluids ?

Answer:- Fluids is the common name given to gases and liquids.

Question 7.- Do fluids exert friction ?

Answer:- Yes, fluids also exert friction.

Question 8.- What is meant by air resistance ?

Answer:- There is friction between a moving object and the air through which it moves, known as air resistance.

Question 9.- What is meant by drag ?

Answer:- The frictional force exerted by fluids is known as drag.

Question 10.- Is friction a contact force or non-contact force ?

Answer:- Contact force.

Question 11.- Why is it difficult to walk on ice ? 

Answer:- It is difficult to walk on ice because the friction between feet and ice is less,

Question 12.- When a boat is moving in the river, does the water offer force of friction to its movement ?

Answer:- Yes, the water opposes the movement of the boat.

Question 13.- What is friction?

Answer:- Friction is referred to as a force acting between two moving or sliding surfaces.

Question 14.- What are the benefits of friction?

Answer:- 

1. Objects can be piled up without sliding 

2. Enables us to walk with grip 

3. Helps in transfer of enerygy from one form to another

Question 15.- What are some of the examples of adhesive forces?

Answer:- 

1. Painting 

2. Icing on cakes 

3. Make up 

4. butter/jam on bread