Chapter 12: Friction

These NCERT Class 8 Science Chapter 12 notes explain the concept of friction in a clear and structured way. The chapter focuses on how friction acts as a force that opposes motion between two surfaces in contact.

It discusses factors affecting friction, types of friction such as static, sliding and rolling friction, and explains why rolling friction is the least. The chapter also covers lubrication, ball bearings and streamlining to reduce friction.

Chapter 12 – Friction is important for understanding motion, machine efficiency and everyday physical activities. It is highly relevant for school examinations and foundation level competitive preparation.

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1. Introduction: what is friction?

• Friction is a force that opposes the motion of an object.
• It acts when two surfaces come in contact and move or try to move over each other.
• Friction is a type of contact force.
• It always acts in the opposite direction of motion.
• When a book slides on a table and stops, it is due to friction.

2. Why does friction occur?

• No surface is perfectly smooth at the microscopic level.
• Surfaces have tiny surface irregularities.
• These irregularities interlock when surfaces touch.
• Interlocking resists movement and produces friction.
• Greater irregularities cause greater friction.

3. Factors affecting friction

• The nature of surfaces affects friction, as rough surfaces produce more friction.
• Smooth surfaces produce less friction.
• The force pressing the surfaces together also affects friction.
• Greater weight or force increases friction.
• A loaded truck experiences more friction than an empty truck.

4. Types of friction

• Static friction acts on objects at rest and prevents them from moving.
• Pushing a heavy box that does not move is an example of static friction.
• Sliding friction acts when an object slides over a surface.
• Sliding friction is less than static friction.
• Rolling friction acts when an object rolls and is the least among the three.

5. Friction as a necessary evil

• Friction is useful in many daily activities.
• It is also harmful in machines and moving parts.
• It can cause energy loss in the form of heat.
• Therefore, friction is called a necessary evil.
• Proper control of friction is important.

6. Advantages of friction

• Friction helps us walk without slipping.
• It allows us to write with a pen or pencil.
• It helps vehicles move and stop safely.
• Nails and screws hold objects due to friction.
• Without friction, shoes would slip on the road.

7. Disadvantages of friction

• Friction causes wear and tear of machine parts.
• It produces unwanted heat.
• It reduces efficiency of machines.
• It wastes energy during motion.
• Bicycle parts become hot due to friction.

8. Increasing friction

• Friction can be increased by making surfaces rough.
• Treads on tyres increase grip on roads.
• Sand is sprinkled on slippery roads to increase friction.
• Athletes use spiked shoes to increase friction.
• Increased friction prevents slipping.

9. Reducing friction

• Lubrication using oil or grease reduces friction.
• Lubricants form a smooth layer between surfaces.
• Polishing smoothens surfaces and reduces friction.
• Ball bearings convert sliding friction into rolling friction.
• Wheels are used to reduce friction in heavy loads.

10. Friction due to fluids

• Air and water also exert friction on moving objects.
• This type of friction is called drag.
• Drag slows down moving objects.
• Drag depends on speed and shape of the object.
• Aeroplanes are designed to reduce air resistance.

11. Streamlining

• Streamlining means giving objects a shape that reduces friction.
• Streamlined shape reduces drag in air or water.
• Fish have streamlined bodies to swim easily.
• Aeroplanes are designed with streamlined shape.
• Streamlining helps objects move faster.

12. Key exam-oriented keywords explained

• Friction: Force that opposes motion between two surfaces in contact.
• Static friction: Friction acting on objects at rest.
• Sliding friction: Friction acting when an object slides.
• Rolling friction: Friction acting when an object rolls.
• Drag: Friction caused by fluids like air and water.
• Lubrication: Method of reducing friction using oil or grease.
• Ball bearings: Devices that reduce friction by converting sliding into rolling friction.
• Streamlining: Designing objects to reduce friction in fluids.

13. One-page summary for quick revision

• Friction is a force that opposes motion.
• It depends on surface nature and force pressing surfaces together.
• Static, sliding and rolling are main types of friction.
• Friction is useful but also causes wear and heat.
• Lubrication, wheels and streamlining help reduce friction.

Understanding NCERT Class 8 Science Chapter 12 is essential to learn how friction affects motion and machines. The chapter clearly explains advantages and disadvantages of friction and methods to increase or reduce it.

The concepts help students understand practical applications in daily life and engineering basics. It is highly useful for school exams and foundation competitive preparation.

Continue reading NCERT Class 8 Science Chapter 13 – Sound to understand how sound is produced and heard.

FAQs

Q1. What is friction?
Friction is a force that opposes the motion of an object when it comes in contact with another surface.

Q2. What are the types of friction?
The main types of friction are static friction, sliding friction and rolling friction.

Q3. Why is rolling friction the least?
Rolling friction is the least because the surfaces do not slide over each other, reducing resistance.

Q4. What is lubrication?
Lubrication is the process of applying oil or grease to reduce friction between surfaces.

Q5. Why is this chapter important for exams?
It explains motion and friction concepts, which are important for school exams and foundation preparation.

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