CBSE Class 8 Science Chapter 12 Friction NCERT Solutions
This chapter delves into the fundamental concept of friction, a force that opposes motion between surfaces in contact. The NCERT Solutions for Class 8 Science, Chapter 12, provide a comprehensive understanding of friction. It covers different types of friction, including static, sliding, and rolling friction, and explains how friction depends on the nature of the surfaces in contact and the force pressing them together. The solutions also explore methods to increase or decrease friction, such as using lubricants or designing treads on tires. These solutions are designed to help students grasp the principles of friction, analyze its effects in various scenarios, and prepare effectively for their examinations by offering clear explanations and step-by-step problem-solving.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 8 |
| Subject | Science |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 12 |
Chapter summary
Chapter 12 of NCERT Class 8 Science focuses on Friction. These solutions explain the nature of friction as a force opposing motion, its dependence on surface roughness, and its effects like producing heat. The chapter differentiates between static, sliding, and rolling friction, highlighting their relative magnitudes. It also addresses practical applications, such as why sports shoes have spikes and the effect of lubricants like soap water on friction. The exercises cover fill-in-the-blanks, ordering friction types, and explaining real-world phenomena related to friction.
Learning outcomes
- Understand that friction is a force that opposes motion between surfaces.
- Identify and differentiate between static, sliding, and rolling friction.
- Explain how the roughness of surfaces affects the magnitude of friction.
- Analyze the role of friction in everyday situations.
- Describe methods to increase or decrease friction.
- Solve problems related to the ordering and effects of different types of friction.
Topics covered
Paper topics
- Friction
- Force
- Motion
- Static Friction
- Sliding Friction
- Rolling Friction
- Surface Roughness
- Lubricants
- Heat Production by Friction
- Increasing Friction
- Decreasing Friction
- Applications of Friction
Important topics
- Types of Friction (Static, Sliding, Rolling)
- Factors Affecting Friction
- Friction Opposes Motion
- Increasing and Decreasing Friction
- Real-world Examples of Friction
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Questions and Solutions
Question 1
(a) Friction opposes the _____ between the surfaces in contact with each other.
(b) Friction depends on the ______ of surfaces.
(c) Friction produces _____.
(d) Sprinkling of powder on the carrom board _____ friction.
(e) Sliding friction is _____ than the static friction.
(a) Friction opposes the motion between the surfaces in contact with each other. Friction acts in the direction opposite to the applied force or motion.
(b) Friction depends on the roughness of surfaces. The rougher the surfaces, the greater the friction.
(c) Friction produces heat. When surfaces rub against each other, the kinetic energy is converted into heat energy.
(d) Sprinkling of powder on the carrom board reduces friction. The powder acts as a lubricant, making the surface smoother and decreasing the resistance to motion.
(e) Sliding friction is lesser than the static friction. It requires more force to start an object moving (overcoming static friction) than to keep it moving (overcoming sliding friction).
Question 2
(a) Rolling, static, sliding
(b) Rolling, sliding, static
(c) Static, sliding, rolling
(d) Sliding, static, rolling
The correct arrangement of friction forces in decreasing order is (c) Static, sliding, rolling.
Explanation:
Static friction is the friction that needs to be overcome to initiate motion. It is generally the largest among the three.
Sliding friction is the friction experienced when one object slides over the surface of another. It is usually less than static friction.
Rolling friction is the friction experienced when a round object rolls over a surface. It is the smallest among the three types of friction.
Therefore, the order from greatest to least friction is static > sliding > rolling.
Question 3
(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
The correct arrangement of the force of friction in increasing order is (a) Wet marble floor, dry marble floor, newspaper and towel.
Explanation:
Wet marble floor offers the least friction because the water acts as a lubricant, reducing the contact between the car's wheels and the floor.
Dry marble floor offers more friction than a wet marble floor because there is direct contact between the surfaces without a lubricant.
Newspaper and towel surfaces are rougher than a marble floor. A towel, being more textured, generally offers more friction than newspaper. Thus, the friction increases in the order: wet marble < dry marble < newspaper < towel.
Therefore, the increasing order of friction is wet marble floor, dry marble floor, newspaper, and towel.
Question 4
When a book starts sliding down a tilted writing desk, the frictional force acts in the direction opposite to the motion of the book. Since the book is sliding downwards, the frictional force will act upwards along the surface of the desk.
Imagine the desk tilted. The book tends to slide down due to gravity. Friction opposes this downward motion. Therefore, the frictional force acts upwards, parallel to the surface of the desk, resisting the book's slide.
Direction of Frictional Force: Upwards, along the surface of the tilted desk, opposing the downward sliding motion of the book.
Question 5
It would be more difficult for you to walk on the floor after spilling soapy water.
Reason: Soapy water acts as a lubricant. It fills the tiny irregularities present on the surfaces of both your feet and the marble floor. This filling reduces the interlocking between the surfaces, thereby significantly reducing the friction between your feet and the floor. Since friction provides the necessary grip to walk, a reduction in friction makes it harder to get a stable footing, increasing the chances of slipping and falling.
Question 6
Sportsmen use shoes with spikes primarily to increase friction and gain a better grip on the playing surface.
Explanation:
The spikes on the shoes dig into the ground (like soil, grass, or track). This digging action creates a stronger interlocking effect between the spikes and the ground compared to smooth-soled shoes.
This increased friction prevents the sportsmen from slipping, especially when they are running, changing direction rapidly, or making sudden stops.
A good grip is crucial for sports like athletics, football, cricket, and tennis, allowing athletes to perform with more power, agility, and safety.
Therefore, spikes enhance performance and reduce the risk of injuries caused by slipping.
Common mistakes
- Confusing the order of static, sliding, and rolling friction.
- Incorrectly identifying whether friction increases or decreases in specific scenarios (e.g., soapy water).
- Not understanding that friction acts opposite to the direction of motion.
Revision tips
- Focus on the definitions and differences between static, sliding, and rolling friction.
- Pay close attention to the examples provided, such as carrom boards and sports shoes.
- Review the fill-in-the-blanks questions to reinforce key concepts about friction's properties.
- Visualize the direction of friction acting on objects in motion or at rest.
Practice MCQs
Q1. Which type of friction is generally the greatest?
Explanation: Static friction is the friction that needs to be overcome to start an object moving. It is generally greater than sliding friction, which in turn is greater than rolling friction.
Q2. Friction is a force that:
Explanation: Friction is defined as a force that opposes the relative motion or tendency of motion between surfaces in contact.
Q3. Sprinkling powder on a carrom board is done to:
Explanation: Powder acts as a lubricant, reducing the friction between the carrom board surface and the striker/coins, allowing them to move more easily.
Q4. Why do sportsmen wear shoes with spikes?
Explanation: Spikes dig into the ground, increasing the friction between the shoe and the surface, which provides a better grip and prevents slipping.
Q5. Which of the following surfaces would offer the least friction to a sliding object?
Explanation: Soapy water acts as a lubricant, significantly reducing friction. Therefore, a wet marble floor offers the least friction among the given options.
Frequently asked questions
What is friction?
Friction is a force that opposes the relative motion or tendency of motion between surfaces in contact with each other.
What are the different types of friction covered in Class 8 Science Chapter 12?
The chapter covers static friction (when surfaces are at rest relative to each other), sliding friction (when surfaces slide over each other), and rolling friction (when one surface rolls over another).
How does the roughness of surfaces affect friction?
Friction depends on the roughness of the surfaces in contact. Rougher surfaces generally produce more friction than smoother surfaces.
Why is it harder to start moving a heavy box than to keep it moving?
It is harder to start moving a heavy box because static friction (the friction when the box is at rest) is generally greater than sliding friction (the friction when the box is moving).
How can friction be reduced?
Friction can be reduced by making the surfaces smoother, using lubricants like oil or grease, or by using ball bearings or rollers.
Why do we need friction?
Friction is essential for many activities, such as walking, gripping objects, and the functioning of brakes in vehicles. Without friction, it would be difficult to move or stop.
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