Class 8 General Science Chapter 7 Question Answer | Friction | English Medium | ASSEB

Friction

Welcome to HSLC Guru! In this lesson we explore Chapter 7 — Friction from the ASSEB Class 8 General Science textbook. Friction is the invisible force that lets us walk, write, hold a pencil, and stop a moving vehicle. At the same time, it wears out machine parts and wastes energy as heat. This chapter explains the nature of friction, the factors that influence it, the different types of friction, its useful and harmful effects, and the methods used to increase or reduce it. You will also meet fluid friction (drag) and learn why aeroplanes, fish and birds have streamlined shapes. The notes follow the ASSEB syllabus and provide complete textbook Q&A along with extra practice questions for revision.

Summary

Friction is the force that opposes the relative motion between two surfaces in contact. Whenever a body slides, rolls or tries to move over another surface, friction acts in a direction opposite to the motion or the tendency of motion. Friction arises because no surface is perfectly smooth — even a polished surface, when seen under a microscope, has tiny bumps and grooves. When two surfaces are pressed together, these irregularities lock into one another and resist motion. The two main factors that decide the magnitude of friction are the nature of the surfaces in contact (rough or smooth) and the normal force pressing them together (which depends on the weight of the body). A heavier body or a rougher surface produces more friction.

Friction is classified into three types. Static friction acts on a body at rest and prevents it from starting to move. Sliding friction (also called kinetic friction) acts when a body is sliding over another surface. Rolling friction acts when a body rolls over a surface, as in the case of wheels and ball bearings. For the same pair of surfaces, static friction is greater than sliding friction, and rolling friction is the least. That is the reason why it is easier to roll a heavy drum than to slide it, and why wheels and ball bearings are used in machines and vehicles.

Friction has many advantages. We can walk on the ground because of friction between our shoes and the floor; on a polished or oily surface our feet slip. Writing with a pen or chalk is possible because of friction between the tip and the paper or board. Friction allows us to hold objects, light a matchstick, fix nails in a wall and stop moving vehicles using brakes. However, friction also has serious disadvantages. It causes wear and tear of machine parts, soles of shoes, tyres of vehicles and edges of tools. A large amount of energy is lost in overcoming friction in machines, and this energy appears as heat, which can damage delicate components.

Friction can be increased where needed and reduced where it is harmful. To increase friction, treads are made on shoe soles and tyres, sand is sprinkled on slippery roads, and grooves are cut on the handles of tools. To reduce friction, surfaces are polished, lubricants like oil and grease are applied, and ball bearings are fitted in machines so that sliding friction is replaced by rolling friction. Fluid friction, also called drag, is the friction offered by liquids and gases on bodies moving through them. To minimise drag, vehicles, aeroplanes, ships and even the bodies of fish and birds have streamlined shapes that allow fluids to flow smoothly around them.

Textbook Questions and Answers

A. Very Short Answer Type Questions (1 Mark)

Q1. What is friction?

Answer: Friction is the force that opposes the relative motion between two surfaces in contact.

Q2. Name the type of friction that has the least magnitude.

Answer: Rolling friction has the least magnitude among the three types.

Q3. Which type of friction acts on a body at rest?

Answer: Static friction acts on a body at rest.

Q4. Give one example of a useful effect of friction.

Answer: Friction between our shoes and the ground enables us to walk safely.

Q5. Name two substances commonly used as lubricants.

Answer: Oil and grease are two commonly used lubricants.

Q6. What is fluid friction also called?

Answer: Fluid friction is also called drag.

Q7. Why are ball bearings used in machines?

Answer: Ball bearings are used to convert sliding friction into rolling friction, which is much smaller, so the machine runs smoothly.

Q8. What kind of shape is given to aeroplanes and ships to reduce fluid friction?

Answer: They are given a streamlined shape.

Q9. Why are treads made on the soles of shoes?

Answer: Treads are made on the soles of shoes to increase friction so that we do not slip while walking.

Q10. Name the force that opposes the motion of a ball rolling on the ground.

Answer: The force is called rolling friction.

B. Short Answer Type Questions (2-3 Marks)

Q1. State the two main factors that affect the magnitude of friction.

Answer: The magnitude of friction depends on:

Nature of the surfaces in contact: Rougher surfaces produce greater friction, while smoother surfaces produce less friction.
Normal force pressing the surfaces: The greater the weight or pressing force on the surfaces, the greater the friction.

Q2. Differentiate between static and sliding friction.

Answer: Static friction acts on a body that is at rest and prevents it from starting to move when an external force is applied. Sliding friction acts on a body that is already moving and slides over the surface of another body. For the same pair of surfaces, static friction is always greater than sliding friction, which is why it takes more force to start moving an object than to keep it moving.

Q3. Why is it easier to roll a heavy barrel than to slide it on the floor?

Answer: When a barrel is slid, sliding friction acts between the barrel and the floor, and this is large. When the same barrel is rolled, rolling friction acts on it, which is much smaller than sliding friction. Therefore less force is needed to roll the barrel, and it is easier to move it by rolling than by sliding.

Q4. Mention three advantages of friction in our daily life.

Answer: Important advantages of friction are:

It enables us to walk on the ground without slipping.
It allows us to write with a pen, pencil or chalk on paper or a board.
It helps us to hold objects firmly in our hands and to apply brakes to moving vehicles.

Q5. What are lubricants? How do they reduce friction?

Answer: Lubricants are substances such as oil, grease and graphite that are applied between two surfaces in contact. They form a thin layer that fills the tiny irregularities of the surfaces and prevents them from coming into direct contact. As a result, the rough projections do not interlock with each other and friction is greatly reduced. This also reduces wear and tear of machine parts.

Q6. Why do fish and birds have streamlined bodies?

Answer: Fish swim through water and birds fly through air. Both water and air offer fluid friction or drag on the bodies moving through them. A streamlined body is pointed in the front and tapering at the back, so that the fluid flows smoothly around it without forming eddies. This reduces drag, allowing fish to swim and birds to fly with minimum effort and high speed.

C. Long Answer Type Questions (5-6 Marks)

Q1. What is friction? Explain the cause of friction and the factors that affect its magnitude.

Answer: Friction is the force that opposes the relative motion between two surfaces in contact. It always acts in the direction opposite to the direction of motion or the tendency of motion of a body.

Cause of friction: No surface is perfectly smooth. Even a polished marble or glass surface, when seen under a microscope, has many tiny bumps, grooves and pits. When two surfaces are placed in contact, these irregularities of one surface lock into the irregularities of the other. To make one surface slide over the other, these locked projections must be pushed and broken. The resistance offered by these interlocked irregularities is what we call friction.

Factors affecting friction:

Nature of the surfaces: Rough surfaces such as a wooden plank or a sand-covered floor have larger irregularities and produce greater friction. Smooth surfaces like polished marble produce less friction.
Normal force: The force pressing the two surfaces together (usually the weight of the body) decides how deeply the irregularities interlock. A heavier body presses the surfaces more strongly, so friction is greater. A lighter body produces less friction.

Q2. Describe the three types of friction with examples and arrange them in order of magnitude.

Answer: The three main types of friction are static, sliding and rolling friction.

Static friction: This friction acts on a body that is at rest and prevents it from starting to move when a small force is applied. Example: a heavy box lying on the floor does not move when we push it gently because static friction balances the push.
Sliding friction: Also known as kinetic friction, it acts on a body that is sliding over the surface of another body. Example: a book sliding across a table experiences sliding friction.
Rolling friction: This friction acts when a body rolls over a surface instead of sliding. Example: a ball rolling on the ground or the wheels of a car rolling on the road experience rolling friction.

Order of magnitude: For the same pair of surfaces, the magnitudes are related as: Static friction > Sliding friction > Rolling friction. This is why it is easier to keep a body moving than to start its motion, and easier still to roll it than to slide it.

Q3. Explain the advantages and disadvantages of friction with suitable examples.

Answer: Friction has both useful and harmful effects in everyday life.

Advantages of friction:

Friction between our shoes and the ground helps us to walk and run without slipping.
It allows us to write with a pen, pencil or chalk because of the friction between the writing tip and the surface.
It helps us to hold a glass, a pen or a tool firmly in our hands.
Brakes on vehicles work because of friction between the brake pads and the wheels.
A matchstick lights only when it is rubbed on the rough surface of the matchbox.
Nails and screws stay fixed in walls and wood due to friction.

Disadvantages of friction:

Friction causes wear and tear of moving parts of machines, soles of shoes and tyres of vehicles.
A large amount of useful energy is lost in overcoming friction in machines, which lowers their efficiency.
Friction produces heat, which can damage delicate parts of machines if it is not controlled.
Vehicles need extra fuel to move against friction, which increases running cost.

Q4. Describe the different methods used to (a) increase friction and (b) reduce friction.

Answer: Friction is sometimes useful and sometimes harmful, so we need methods to increase or reduce it as the situation demands.

(a) Methods of increasing friction:

Treads are made on the soles of shoes and on the tyres of vehicles to give a better grip.
Sand or sawdust is sprinkled on a slippery floor or an oily road to increase friction.
Handles of tools, bats and bicycle grips are made rough or grooved so that they do not slip from the hand.
Brake pads are made of materials that produce more friction with the wheel.

(b) Methods of reducing friction:

Surfaces in contact are polished to make them smooth.
Lubricants such as oil, grease and graphite are applied between moving parts.
Ball bearings are fitted in axles of wheels and machines so that sliding friction is replaced by rolling friction.
Vehicles, aeroplanes and ships are given streamlined shapes to reduce fluid friction.
Air cushions are used in some sliding doors and machines to keep surfaces apart.

Q5. What is fluid friction? Why are vehicles given streamlined shapes? Explain with examples.

Answer: The friction offered by fluids — that is, by liquids and gases — on bodies moving through them is called fluid friction or drag. When a body moves through air or water, the fluid particles strike against its surface and oppose its motion. Fluid friction depends on the speed of the body, the nature of the fluid and the shape of the body.

Need for streamlined shapes: A body with a flat or irregular front pushes a large quantity of fluid in front of it, and the fluid forms eddies behind, both of which increase drag. A streamlined body is pointed at the front and tapering at the back, so the fluid flows smoothly around it without forming eddies. This reduces drag and allows the body to move with less resistance and at a higher speed.

Examples:

Aeroplanes have streamlined bodies and pointed noses to fly fast through air.
Ships and submarines have streamlined hulls to move smoothly through water.
Fish such as sharks have streamlined bodies that help them to swim quickly.
Birds have pointed beaks and streamlined bodies that reduce drag while flying.
Modern cars and trains are also given streamlined shapes to save fuel and reach higher speeds.

Additional Practice Questions

Multiple Choice Questions

Q1. Friction always acts in a direction

(a) same as motion (b) opposite to motion (c) perpendicular to motion (d) upward

Answer: (b) opposite to motion

Q2. The smallest of the three types of friction is

(a) static (b) sliding (c) rolling (d) fluid

Answer: (c) rolling

Q3. Friction can be reduced by using

(a) sand (b) treads (c) lubricants (d) rough surfaces

Answer: (c) lubricants

Q4. Friction between solid surfaces depends on

(a) colour of surface (b) nature of surface (c) size of room (d) temperature only

Answer: (b) nature of surface

Q5. Ball bearings are used to convert

(a) rolling into sliding (b) sliding into rolling (c) static into rolling (d) fluid into static

Answer: (b) sliding into rolling

Q6. Fluid friction is also called

(a) drag (b) thrust (c) tension (d) gravity

Answer: (a) drag

Q7. Treads on tyres are made to

(a) reduce friction (b) reduce weight (c) increase friction (d) look attractive

Answer: (c) increase friction

Q8. An aeroplane is given a streamlined shape to

(a) increase drag (b) reduce drag (c) increase weight (d) increase noise

Answer: (b) reduce drag

Q9. Friction in machines mostly appears as

(a) light (b) sound only (c) heat (d) electricity

Answer: (c) heat

Q10. A matchstick lights up due to

(a) gravity (b) friction (c) magnetism (d) electricity

Answer: (b) friction

Fill in the Blanks

Q1. Friction opposes the relative ________ between two surfaces in contact.

Answer: motion

Q2. Friction is caused by the ________ present on the surfaces in contact.

Answer: irregularities

Q3. ________ friction is the smallest of the three types of friction.

Answer: Rolling

Q4. Oil and grease used to reduce friction are called ________.

Answer: lubricants

Q5. The friction offered by air or water is known as ________ friction.

Answer: fluid

True or False

Q1. Friction always acts in the direction of motion.

Answer: False. Friction always acts opposite to the direction of motion.

Q2. Sliding friction is greater than rolling friction.

Answer: True.

Q3. Lubricants are used to increase friction.

Answer: False. Lubricants are used to reduce friction.

Q4. Streamlined shapes reduce fluid friction.