Important Questions for CBSE Class 8 Science Chapter 9 - Friction

Very Short Answer Questions: 1 Mark

1. Which one of the following is not a lubricant? 

A. Oil B. Water

C. Grease D. Graphite

Ans: B. Water

2. The frictional force on an object in a fluid depends on its _______ with respect to the fluid. 

A. Speed B. Weight 

C. Volume D. Mass

Ans: A. Speed

3. Which of the following statements is false about friction? 

A. Friction is undesirable. 

B. Friction is caused by the irregularities on the two surfaces in contact. 

C. The force of friction is lesser if a rough surface is involved. 

D. Friction is a necessary evil.

Ans: C. The force of friction is lesser if a rough surface is involved.

4. Lubricants are the substance which 

A. Increase friction B. Decrease friction 

C. Natural D. All of the above

Ans: B. Decrease friction.

5. The force of friction on a smooth surface is _____________. 

A. Greater B. Lesser 

C. Higher D. None of the above

Ans: B. Lesser

6. Friction is a type of a __________. 

A. Contact force B. Magnetic force 

C. Non-contact force D. Gravitational force

Ans: A. contact force

7. The force of friction is always _______ the applied force.

Ans: Opposes. 

8. What is used in the brake system of a bicycle to increase the friction?

Ans: Brake pad

9. In many machines, friction is reduced by using ______________.

Ans: Ball bearing.

10. What are lubricants?

Ans: Lubricants are substances which help in reducing friction between two surfaces.

11. Friction is sometimes ______________.

Ans: Undesirable

12. Define drag.

Ans: The frictional force exerted by fluids is called drag.

Short Answer Questions: 3 Marks

13. Give reason for the following:

a. Sole of the shoes are grooved. Why?

Ans:  The soles of the shoes are grooved in order to increase the friction between the shoes and the surface. It prevents the person from slipping.

b. The Tires of any vehicle are threaded. Why?

Ans: Tyre of vehicles is threaded in order to prevent the vehicle from skidding by increasing the friction.

14. Give some examples that friction is necessary for everyday activities.

Ans: Friction is part of everyday activities such as:

1. When we walk there is fiction between surface and our shoes

2. While driving there is friction between wheels and the road.

3. When we write there is friction between the pen/pencil tip and the paper.

15. Can we eliminate friction completely?

Ans: No, we can never eliminate friction completely. In any situation, we can only increase or decrease the friction between two surfaces. Even when a surface seems smooth from naked eyes it has many irregularities on a microscopic level.

16. Write a few examples where sliding friction is replaced by rolling friction.

Ans: Sliding friction is generally replaced with rolling friction in circular objects since it helps in body movement. Few such examples are as follows:

1. In car tyres: Rolling friction helps in movement of the car and static friction is only used when we apply breaks.

2. Use of ball bearings in bicycles.

17. How can we minimise fluid friction?

Ans: Fluid friction can be reduced by giving bodies moving in fluids appropriate shapes known as streamline.

18. How can you say that rolling reduces friction?

Ans: It is always easier to move a heavy object by rolling instead of sliding or lifting. Rolling takes much less force and effort. Thus, we can conclude that rolling reduces friction. 

19. Differentiate between static friction and sliding friction.

Ans:

20. It is difficult to move on a wet floor. Why?

Ans: Walking on a wet floor is difficult because the water layer makes the surface smooth. The water coating reduces friction, making it difficult for the foot to maintain a firm grasp on the floor, causing it to slip.

21. What happens when a book is gently pushed on the table? Why?

Ans: When we push the book gently it moves for a distance and then suddenly stops due to the friction caused by the table surface and the book. 

22. Write a short note on spring balance.

Ans: A spring balance is a device that measures the force exerted on a certain object. It is made up of a coiled spring that expands when a force is applied to it.

A pointer on a graded scale moves to gauge the spring's stretching. The magnitude of the force is determined by the reading on the scale. The scale has a unit Newton meter.

Long Answer Questions: 5 Marks

23. Explain increasing and reducing friction with examples.

Ans: 

Example of Increasing Friction:

a)  When we put a brake on the vehicle for it to stop.

b)  Gymnasts use a gritty material to increase friction on their hands, which helps them grip better.

c)   We increase force to stop a moving ball.

Example for Reducing Friction:

a) We reduce the friction in the cricket ball by rubbing it continuously to increase its  spin.

b)  Drop of oil/grease is used in machines for its smooth functioning.

c)  Powdered is sprinkled on the carrom board for the coins to move better.

24. Why is ‘friction: a necessary evil’? Explain.

Ans: 

Friction is Necessary as it Helps us in Day to Day Activities Such as:

a)  It helps us in walking and running comfortably.

b)  It makes writing easy on paper.

c)   It balances the movement of vehicles on the road.

d)   Nail is fixed on the wall due to friction.

Friction at the Same Time Can be Known as Evil Because:

a)  It causes wear and tear of the objects.

b)  Makes movement of objects difficult.

c)   Reduces life of machine and tyres.

25. How do lubricants help to reduce friction?

Ans: Lubricants such as oil, grease, or graphite establish a thin film between moving parts of a machine, preventing moving surfaces from rubbing against each other. Interlocking of defects is minimised, and movement becomes more smooth and efficient. Lubricants are chemicals that help to reduce friction. It may not be advisable to use oil as a lubricant in particular machinery.

26. Explain in detail how friction occurs.

Ans: The roughness on the two surfaces in contact generates friction. Even surfaces that appear to be exceedingly smooth contain a vast number of minute defects. Irregularities on both surfaces connect with one other. To overcome interlocking, we must exert force when attempting to move any surface. On rough surfaces, the number of irregularities is higher. As a result, when a rough surface is involved, the friction force is greater. We can see that the friction created by the irregularity on the two surfaces comes in contact with each other. If the two surfaces are forced harder together, it is apparent that the friction force will grow. This friction can be reduced with the use of lubricants like oil or grease.

Download Important Questions for Class 8 Science Chapter 9 PDF

Class 8 Chapter  9 Friction Summary

Friction - It is the external force that opposes the relative motion between the two surfaces in contact. Friction acts on both surfaces which are in contact with each other. When one object moves in relative to the other object is known as relative motion. The main cause of friction is that it occurs due to surface irregularities between the two objects in contact. It also occurs due to the adhesive forces between surfaces in contact. Another cause of friction is that it occurs due to the ploughing effect.

Factors Affecting Friction - Many factors affect the friction between the two surfaces like the nature of the surface in contact, which means how hard the two surfaces are in contact pressed against each other. As more surfaces in contact with each other lead to the formation of more bonds and to break more bonds more friction is required. The normal reaction force exactly perpendicular to the surface increases the friction.

The frictional force of the object can be calculated using the spring balance. It is used to calculate the frictional force between two different material surfaces. For example, sandpaper has higher friction compared to stainless steel material. To change the friction of the material the surface of the material can be polished, which reduces the irregularities and therefore makes the surface smooth. 

Some of the important types of friction are static friction and kinetic friction. Kinetic friction is further divided into sliding friction and rolling friction. Friction produces heat by constantly breaking the bonds between two surfaces. They make the particle vibrate which increases the kinetic energy and therefore it increases the heat.

Many things would have been impossible without the application of friction like writing, walking, running, tyres on a car, a nail that stays in the wall due to friction, usage of a matchstick. And due to rolling friction, it was possible to reinvent the wheel as the rolling mechanism provides less friction compared to the sliding friction. Some of the parts of the machine use ball bearings to reduce the friction of moving parts.

Benefits of Referring to the Important Questions for CBSE Class 8 Science Chapter 9 - Friction

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• These questions and answers are provided by subject matter experts after thorough research of the questions asked in the previous years and the weightage of the topics.

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Conclusion

Friction Class 8 CBSE important questions are composed of fascinating and thrilling discoveries. Class 8 Science Chapter 9 extra questions and important questions are a great platform to provide fundamentals of the chapter and solutions to the complex questions. Students can go through these important questions and their answers to explore and visualise the concepts and applications of force and friction. The important question is one of the best study resources for students to revise their concepts during final examinations. 

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