Friction
What You Will Learn
What is Friction?
Types of Friction
Factors Affecting Friction
Advantages & Disadvantages
Reducing & Increasing Friction
Fluid Friction
SECTION 01
What is Friction?
Friction is a force that opposes the relative motion between two surfaces in contact. It acts in the direction opposite to the direction of motion or attempted motion.
When you push a book across a table, you feel resistance — that resistance is friction. It arises because no surface is perfectly smooth; at a microscopic level, every surface has tiny hills and valleys called irregularities. When two surfaces touch, their irregularities interlock and resist relative motion.
INFOGRAPHIC — FRICTION AT THE MICROSCOPIC LEVEL
SURFACE A (moving)
SURFACE B (stationary)
Interlocking irregularities create friction force
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ROUGH SURFACES
More interlocking = MORE friction force
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SMOOTH SURFACES
Less interlocking = LESS friction force
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NORMAL FORCE
Heavier object = MORE friction force
KEY DEFINITION
Friction is the force that opposes the relative sliding motion of one body over another body with which it is in contact. The direction of frictional force is always opposite to the direction of motion.
SECTION 02
Types of Friction
INFOGRAPHIC — 4 TYPES OF FRICTION
1 — STATIC FRICTION
The friction that acts on a body before it starts moving. It keeps a stationary object at rest when a force is applied. Static friction adjusts itself up to a maximum value.
Example: Pushing a heavy box that doesn’t move
2 — SLIDING (KINETIC) FRICTION
The friction that acts when one body slides over another. It is less than maximum static friction. Also called kinetic friction.
Example: A book sliding across the table after a push
3 — ROLLING FRICTION
The friction that acts when a body rolls over a surface. It is much less than sliding friction — this is why wheels were invented!
Example: A ball rolling on the floor; bicycle wheels
4 — FLUID FRICTION (DRAG)
The friction exerted by fluids (liquids and gases) on objects moving through them. Also called drag. Depends on the shape and speed of the object.
Example: Water resistance on a swimmer; air on an aircraft
MAGNITUDE COMPARISON (same surfaces and load)
Static (Max) — HIGHEST
Sliding (Kinetic)
Rolling — LOWEST
IMPORTANT ORDER TO REMEMBER
Static Friction > Sliding Friction > Rolling Friction
This is why it is harder to start sliding an object than to keep it sliding, and why wheels (rolling) make transport so much easier than dragging.
SECTION 03
Factors Affecting Friction
INFOGRAPHIC — FACTORS THAT CHANGE FRICTION
FACTOR 1: NATURE OF SURFACES
The rougher the surfaces in contact, the greater the friction. A rough road creates more friction than a smooth ice rink.
| Surface Type | Friction Level |
| Sandpaper | VERY HIGH |
| Concrete | HIGH |
| Polished Wood | MEDIUM |
| Wet Ice | LOW |
| Oiled Surface | VERY LOW |
FACTOR 2: WEIGHT (NORMAL FORCE)
The heavier the object pressing the surfaces together, the greater the friction. More weight = more interlocking of irregularities.
1 kg
Low Friction
5 kg
High Friction
NOTE
Friction does NOT depend on the area of contact between surfaces (for solid surfaces). A large flat box and a small box of the same weight experience the same friction on the same surface.
SECTION 04
Advantages & Disadvantages of Friction
ADVANTAGES OF FRICTION
Walking
Friction between feet and ground lets us walk without slipping.
Vehicle Braking
Brakes work because of friction between brake pads and wheels.
Writing
Friction between pen/pencil and paper allows us to write.
Fire from Matchstick
Rubbing a matchstick against a rough surface produces heat to ignite it.
Nuts and Bolts
Friction prevents nuts and bolts from loosening on their own.
DISADVANTAGES OF FRICTION
Wear and Tear
Machine parts and shoe soles wear out due to constant friction.
Heat Generation
Friction converts kinetic energy to heat, wasting energy in engines.
Noise Pollution
Friction between machine parts produces unwanted noise and vibrations.
Energy Loss
Engines must work harder to overcome friction, consuming more fuel.
Reduced Efficiency
Friction reduces the efficiency of machines and mechanical systems.
SECTION 05
Reducing and Increasing Friction
Methods of Reducing Friction
HOW WE REDUCE FRICTION
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LUBRICANTS
Oil, grease, and graphite reduce friction between moving parts in machines.
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BALL BEARINGS
Replace sliding friction with rolling friction in motors and cycles.
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STREAMLINING
Streamlined shapes reduce fluid friction (drag) on vehicles and aircraft.
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SMOOTH SURFACES
Polishing and smoothing surfaces reduces interlocking of irregularities.
DIAGRAM — STREAMLINED vs NON-STREAMLINED SHAPES
High turbulence — high drag
BLUNT / FLAT SHAPE
Creates turbulence; high drag/fluid friction; more energy needed to move forward.
Smooth flow — low drag
STREAMLINED SHAPE
Flow lines smoothly over body; minimal turbulence; low drag; less energy needed.
Methods of Increasing Friction
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TREADS ON TYRES
Grooved tyres grip wet roads better; athletes use spiked shoes.
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ROUGH SURFACES
Stairs have rough anti-slip strips; playgrounds use rough rubber mats.
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GRIP WRAPPING
Handles of cricket bats are wound with tape for better grip.
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SAND ON ICE
Sand or gravel is sprinkled on icy roads in winter to prevent skidding.
SECTION 06
Fluid Friction & Drag
When objects move through a fluid (liquid or gas), they experience a resistive force called fluid friction or drag. The drag depends on the shape of the object, the speed of motion, and the nature of the fluid.
REAL-WORLD EXAMPLES OF FLUID FRICTION
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SWIMMER IN WATER
Water exerts drag on the swimmer. Streamlined swimsuits reduce this.
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AIRCRAFT IN AIR
Air drag acts against flight. Aircraft are given streamlined shapes to cut through the air.
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FISH IN WATER
Fish have spindle-shaped bodies to minimise fluid friction as they swim.
Quick Revision — Key Points Summary
| Concept | Key Point |
|---|---|
| Definition of Friction | Force opposing relative motion between surfaces in contact |
| Cause of Friction | Interlocking of microscopic irregularities on surfaces |
| Order of Friction | Static > Sliding (Kinetic) > Rolling |
| Factors Affecting | Nature of surfaces (roughness) and weight (normal force) |
| Methods to Reduce | Lubricants, ball bearings, streamlining, polished surfaces |
| Methods to Increase | Rough treads on tyres, spikes on shoes, sand on icy roads |
| Fluid Friction | Drag force from liquids/gases is reduced by streamlined shapes |
| Area of Contact | Does NOT affect friction between solid surfaces |
PRACTICE EXERCISES
Exercise Zone
6 Question Types | 10 Questions Each | Answers Included
EXERCISE TYPE 1
One-Sentence Answer Questions
Answer each question in one complete sentence.
Q1. What is friction?
Answer: Friction is a force that opposes the relative motion between two surfaces in contact.
Q2. Why does friction arise between surfaces?
Answer: Friction arises because the microscopic irregularities (hills and valleys) of two surfaces interlock when they come in contact.
Q3. Define static friction.
Answer: Static friction is the friction that acts on a body at rest when an external force tries to move it, preventing the body from moving.
Q4. What is rolling friction?
Answer: Rolling friction is the friction that acts when a body rolls over a surface, and it is much smaller than sliding friction.
Q5. Name one method to reduce friction in machines.
Answer: Lubricants such as oil and grease are applied between moving machine parts to reduce friction.
Q6. What is fluid friction?
Answer: Fluid friction is the resistive force exerted by a fluid (liquid or gas) on an object moving through it, also called drag.
Q7. Why do tyres have grooved treads?
Answer: Tyres have grooved treads to increase friction with the road surface, improving grip and preventing skidding, especially on wet roads.
Q8. State one advantage of friction in daily life.
Answer: Friction between our feet and the ground allows us to walk and run without slipping.
Q9. Why do fish and birds have streamlined bodies?
Answer: Fish and birds have streamlined bodies to minimise fluid friction (drag) as they move through water or air.
Q10. What happens to friction if a surface is made smoother?
Answer: If a surface is made smoother, friction decreases because there are fewer irregularities to interlock with the other surface.
EXERCISE TYPE 2
Two-Sentence Answer Questions
Answer each question in exactly two complete sentences.
Q1. Explain why it is harder to push a heavy box than a light box on the same floor.
Answer: A heavier box presses down on the floor with greater force, increasing the interlocking of surface irregularities. This results in a greater friction force, making it harder to push the heavy box.
Q2. Explain how ball bearings help reduce friction in machines.
Answer: Ball bearings replace the sliding motion between machine parts with rolling motion. Since rolling friction is much less than sliding friction, ball bearings significantly reduce the total friction in machines like motors and bicycle wheels.
Q3. Describe two disadvantages of friction in machines.
Answer: Friction between moving machine parts generates unwanted heat, which wastes energy and can damage components. It also causes wear and tear of parts, reducing the machine’s lifespan and increasing maintenance costs.
Q4. Why is sliding friction less than static friction?
Answer: When a body is stationary, the surface irregularities have more time to interlock deeply, making static friction high. Once the body starts moving, the contact points shift rapidly, and irregularities have less time to interlock, so sliding friction is lower.
Q5. How does a lubricant reduce friction? Give one example.
Answer: A lubricant forms a thin layer between two surfaces, preventing them from directly touching and reducing the interlocking of irregularities. For example, oiling the chain of a bicycle reduces friction and allows it to run smoothly.
Q6. Explain the term drag and give one example from nature.
Answer: Drag is the resistive force that a fluid (liquid or gas) exerts on a body moving through it, opposing its motion. For example, a fish experiences water drag as it swims, which is why nature has given fish a streamlined shape to reduce this drag.
Q7. Friction is both a friend and an enemy. Explain with one example each.
Answer: Friction is our friend when it helps vehicles stop safely through brakes, which rely on friction between brake pads and wheels. Friction is our enemy when it wears out shoe soles or causes machine parts to overheat and break down.
Q8. Why do athletes wear spiked shoes?
Answer: Athletes wear spiked shoes because the spikes dig into the ground and greatly increase the friction between their feet and the ground. This prevents slipping and provides a better grip, allowing athletes to run faster and change direction more easily.
Q9. How does the shape of an aeroplane help it to fly efficiently?
Answer: Aeroplanes are designed with a streamlined (tapered) shape so that air flows smoothly around the body without creating turbulence. This reduces air drag significantly, allowing the aeroplane to fly at high speeds with less fuel consumption.
Q10. What happens to friction when sand is sprinkled on an icy road, and why?
Answer: When sand is sprinkled on an icy road, friction between vehicles’ tyres and the road increases because sand particles are rough and create more interlocking with the tyre surface. This prevents tyres from slipping on the smooth ice and makes the road safer to drive on.
EXERCISE TYPE 3
Yes / No Questions
Write YES or NO, then give one reason for your answer.
| # | Question | Answer | Reason |
|---|---|---|---|
| 1 | Does friction always act opposite to the direction of motion? | YES | Friction is always a resistive force that opposes the relative motion between surfaces. |
| 2 | Does friction depend on the area of contact between two solid bodies? | NO | Friction between solid surfaces depends only on the nature of the surfaces and the normal force, not the area of contact. |
| 3 | Is rolling friction greater than sliding friction? | NO | Rolling friction is much less than sliding friction, which is why wheels make transport far easier than dragging. |
| 4 | Can friction exist between a body and a fluid (liquid/gas)? | YES | Fluids exert a resistive drag force on moving objects, which is a form of fluid friction. |
| 5 | Is friction always harmful and should be completely eliminated? | NO | Friction is necessary for walking, writing, braking, and gripping — without it, daily life would be impossible. |
| 6 | Does a heavier object experience more friction on the same surface? | YES | Greater weight increases the normal force pressing surfaces together, increasing friction. |
| 7 | Does polishing a surface reduce friction? | YES | Polishing reduces the size of surface irregularities, so there is less interlocking and thus less friction. |
| 8 | Is static friction always equal to the applied force? | YES | As long as the body remains stationary, static friction adjusts itself to exactly equal the applied force, up to its maximum value. |
| 9 | Do streamlined shapes reduce fluid friction? | YES | Streamlined shapes allow fluid to flow smoothly around a body with minimal turbulence, greatly reducing drag. |
| 10 | Can we write on paper without friction? | NO | Friction between the pen/pencil tip and paper deposits ink or graphite; without it, the pen would just slide, and writing would be impossible. |
EXERCISE TYPE 4
True / False Statements
Write TRUE or FALSE. If False, also write the corrected statement.
| # | Statement | Answer | Correction (if False) |
|---|---|---|---|
| 1 | Friction acts in the same direction as the motion of an object. | FALSE | Friction acts in the direction opposite to the motion of the object. |
| 2 | Rolling friction is less than sliding friction. | TRUE | — |
| 3 | Friction does not produce any heat. | FALSE | Friction converts kinetic energy into heat, which is why objects become warm when rubbed together. |
| 4 | Streamlined shapes help reduce fluid friction on moving vehicles. | TRUE | — |
| 5 | A larger area of contact between two solid bodies increases the friction between them. | FALSE | The area of contact does not affect friction between solid bodies; it depends on the nature of the surfaces and the normal force. |
| 6 | Lubricants help reduce friction between moving machine parts. | TRUE | — |
| 7 | Perfectly smooth surfaces have zero friction. | FALSE | In practice, no surface is perfectly smooth; all surfaces have microscopic irregularities, so perfectly frictionless solid surfaces do not exist in reality. |
| 8 | Fish have streamlined bodies to minimise water resistance. | TRUE | — |
| 9 | Static friction is always less than sliding friction. | FALSE | Static friction (at its maximum) is greater than sliding friction; it is harder to start moving an object than to keep it moving. |
| 10 | Friction is not useful and should always be reduced. | FALSE | Friction is very useful in many situations, such as walking, writing, and braking; only unwanted friction in machines should be reduced. |
EXERCISE TYPE 5
Fill in the Blanks
Fill in each blank with the correct word or phrase.
1. Friction is a force that opposes the ____________ motion between two surfaces in contact.
2. The microscopic hills and valleys on surfaces are called ____________.
3. Among static, sliding and rolling friction, ____________ friction is the smallest.
4. The friction experienced by a body moving through a fluid is called ____________.
5. Oil and grease used to reduce friction are collectively called ____________.
6. Aeroplanes are given a ____________ shape to reduce air resistance.
7. The friction that acts on a stationary body when a force is applied is called ____________ friction.
8. ____________ bearings convert sliding friction to rolling friction in machines.
9. Friction between brake pads and wheels is useful for ____________ a vehicle.
10. Static friction is ____________ (greater / smaller) than sliding friction.
ANSWERS
1. relative 2. irregularities 3. rolling 4. fluid friction (drag) 5. lubricants
6. streamlined 7. static 8. ball 9. stopping / braking 10. greater
6. streamlined 7. static 8. ball 9. stopping / braking 10. greater
EXERCISE TYPE 6
Four-Sentence Answer Questions
Answer each question in exactly four complete sentences.
Q1. Explain the cause of friction. Why do rough surfaces produce more friction than smooth surfaces?
Answer: Friction is caused by the interlocking of microscopic irregularities (tiny hills and valleys) present on all surfaces. Even surfaces that appear smooth to the naked eye have these microscopic bumps and grooves. When two surfaces are pressed together, their irregularities interlock and resist any relative motion between them. Rough surfaces have larger and more numerous irregularities, so they interlock more deeply, producing greater friction compared to smooth surfaces.
Q2. Compare static friction, sliding friction, and rolling friction. Which is the greatest and which is the smallest?
Answer: Static friction acts on a body at rest and is the force that prevents it from moving when an external force is applied. Sliding (kinetic) friction acts when one body slides over another and is less than the maximum static friction. Rolling friction acts when a body rolls over a surface and is the smallest of the three types. The order from greatest to smallest is: static greater than sliding greater than rolling, which is why wheels were invented to replace the dragging of heavy loads.
Q3. Describe four methods used to reduce friction in machines and vehicles.
Answer: Lubricants such as oil and grease are applied between moving parts in engines to form a thin separating layer that reduces friction and prevents wear. Ball bearings are used in motors and cycles to replace sliding motion with rolling motion, since rolling friction is much smaller. Surfaces in machines are polished and made smooth to reduce the interlocking of irregularities. Vehicles and aircraft are given streamlined shapes to reduce fluid friction (drag) as they move through air or water.
Q4. Discuss four ways in which friction is useful in daily life.
Answer: Friction between our feet and the floor allows us to walk and run without slipping, making movement possible. Vehicles rely on friction between tyres and the road to start, stop, and steer safely. Writing on paper is possible because of friction between the pen or pencil tip and the paper surface. Matchsticks are ignited by friction when rubbed against a rough surface, producing enough heat to start a flame.
Q5. Explain fluid friction (drag). How do animals and vehicles adapt to minimise it?
Answer: Fluid friction, also called drag, is the resistive force exerted by a fluid (liquid or gas) on an object moving through it; it opposes the object’s motion. The faster an object moves and the larger its cross-sectional area, the greater the drag it experiences. Animals such as fish and dolphins have naturally streamlined (spindle or torpedo) shaped bodies, which allows water to flow smoothly around them with minimal turbulence. Human-designed vehicles like aeroplanes, submarines, and racing cars are given similar streamlined shapes to reduce drag and improve speed and fuel efficiency.
Q6. Why is friction called a ‘necessary evil’? Explain with two advantages and two disadvantages.
Answer: Friction is called a ‘necessary evil’ because it is both unavoidable and has both beneficial and harmful effects. On the beneficial side, friction helps us walk safely without slipping and allows vehicles to brake effectively to prevent accidents. On the harmful side, friction causes wear and tear of machine parts such as brake pads and gears, increasing maintenance costs. It also converts useful kinetic energy into heat, wasting energy and reducing the efficiency of engines and mechanical systems.
Q7. Describe four methods used to increase friction where it is needed.
Answer: Tyres of vehicles are made with deep-grooved treads that dig into the road surface, increasing friction and preventing skidding on wet roads. Athletes wear spiked or studded shoes that penetrate the ground, greatly increasing grip during running and jumping. Sand or gravel is sprinkled on icy or slippery roads in winter to create a rough surface that increases friction for vehicle tyres. Cricket bat handles are wrapped with tape, and the surface of equipment like gym bars is roughened to provide a better, safer grip for users.
Q8. Explain how ball bearings work and why they are preferred in machines over plain surfaces.
Answer: Ball bearings are small spherical balls placed between rotating machine parts such as axles and their housings. Instead of two flat surfaces rubbing and sliding against each other, the ball bearings roll, converting sliding friction into rolling friction. Since rolling friction is far smaller than sliding friction, much less energy is wasted as heat, and the machine operates more smoothly and efficiently. Ball bearings also reduce wear on machine parts, extending their lifespan and lowering the need for frequent replacement or maintenance.
Q9. What are the factors that affect friction? Explain each with an example.
Answer: The first factor is the nature of the surfaces in contact: rougher surfaces have more and larger irregularities, resulting in greater friction; a rubber-soled shoe on concrete has much more friction than on a polished marble floor. The second factor is the normal force (the weight pressing the surfaces together): a heavier object presses surfaces more firmly, increasing interlocking and friction; a heavy boulder is far harder to push than a light cardboard box. Friction does not depend on the speed of the moving object — increasing speed does not significantly change kinetic friction on solid surfaces. It also does not depend on the area of contact — a flat box and a tall box of the same weight experience equal friction on the same surface.
Q10. How does friction affect the efficiency of machines? What steps are taken to manage it?
Answer: Friction in machines converts useful mechanical energy into heat energy, which is wasted and cannot be used to do useful work, thereby reducing the overall efficiency of the machine. It also causes progressive wear and tear of moving parts such as pistons, gears, and bearings, leading to gradual degradation and eventual failure if left unchecked. To manage this, engineers apply lubricants (oil and grease) between moving parts to reduce direct surface contact and minimise friction losses. Additionally, ball bearings and polished surfaces are used in high-precision machines, and regular maintenance is performed to replace worn parts and replenish lubricants, keeping friction at manageable levels.
CBSE CLASS 8 SCIENCE
Chapter: Friction — Complete Tutorial
Tutorial | Infographics | Diagrams | 1-Sentence | 2-Sentence | Yes/No | True/False | Fill-in-Blanks | 4-Sentence Answers
