KSEEB Solutions For Class 8 Science Chapter 8 Friction

KSEEB Solutions Class 8 Science Chapter 8 Friction Points To Remember

• Friction: A force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force.
• Causes of Friction: Friction exists between two surfaces due to irregularities on the surfaces of the objects in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces. Factors That Affect Friction
  1. Roughness of the surface
  2. The extent to which the two surfaces pressed together
  3. Nature of the surface
• Types of Friction:
  1. Static Friction: When a body is at rest, the force of friction is called static friction and is always equal and opposite to the applied force. The force of friction which acts when the body is just on the verge of sliding on the surface is called limiting friction.
  2. Sliding friction: The friction force which opposes the actual relative sliding motion between two contact surfaces. Sliding friction is smaller than static friction.
  3. Rolling Friction: The frictional force that exists between two surfaces when a body rolls over the other. Rolling friction is smaller than sliding friction.
• Effects of Friction
  1. Friction produces heat.
  2. Friction causes wear and tear
  3. Friction opposes motion.
• Advantages of Friction
  1. Friction between pen and paper enables us to write on paper.
  2. Friction between our feet and the ground allows our movements like standing, walking and running.
  3. Friction between the surface of the road and the tyres of our vehicles allows the vehicles to move without slipping.
• Disadvantages of Friction
  1. Friction causes moving objects to stop or slow down.
  2. Friction produces heat causing waste of energy in machines.
  3. Friction causes wear and tear on moving parts of machinery, soles of shoes, etc.
• Friction is a necessary Evil: As friction is advantageous to us it is considered a friend but due to its disadvantages it is a foe. Depending on the circumstance, friction can be a help or a hindrance. Thus it is a necessary evil.
• Increasing Friction: By pressing the surfaces together more strongly. For Example: when brakes are applied on a bicycle or car, the brake pads press
  against a moving part of the wheel and the force of friction increases. Friction can be increased by increasing the roughness of the surfaces in contact.
  For example, The Treading of shoes and tyres is done to increase friction.
• Reducing Friction: Friction between the sliding surfaces of two objects can be reduced by making the surfaces in contact smooth by polishing them. Sliding friction between the moving parts of vehicles and machinery can be reduced by using oil, grease, graphite or any other lubricant. Rolling friction is less than sliding friction. Hence, sliding friction is replaced by rolling friction by using rollers, like ball bearings between the hub and the axles in the moving parts of machines and vehicles. Friction is reduced by providing wheels:
  Example: suitcase, school bags of kids, etc.
  Aeroplanes, boats, fishes, and birds which move through fluids have bodies of special shape called streamlined shape, so as to reduce the friction due to fluid and avoid energy loss.

Kseeb Solutions For Class 8 Science Chapter 8 Friction Pdf

Class 8 Science Friction KSEEB Ncert Textbook Exercises

Question 1. Fill in the blanks.
Answer:
1. Friction opposes the relative motion between the surfaces in contact with each other.
2. Friction depends on the nature of surfaces.
3. Friction produces heat.
4. Sprinkling of powder on the carom board reduce friction.
5. Sliding friction is less than static friction.

Question 2. Four children were asked to arrange forces due to rolling, static and sliding frictions in decreasing order. Their arrangements are given below.
1. Rolling, static, sliding
2. Rolling, sliding, static
3. Static, sliding, rolling
4. Sliding, static, rolling
Answer: 2. Rolling, sliding, static.

Question 3. Alida runs her toy car on the dry marble floor, wet marble floor, and newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
1. Wet marble floor, dry marble floor, newspaper and towel
2. Newspaper, towel, dry marble floor, wet marble floor
3. Towel, newspaper, dry marble floor, wet marble floor
4. Wet marble floor, dry marble floor, towel, newspaper
Answer: 1. Wet marble floor, dry marble floor, newspaper and towel.

Question 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of the frictional force acting on it.
Answer: The frictional force will act parallel to the inclined surface, opposite to the direction of the sliding of the book.

Karnataka Board Class 8 Science Chapter 8 Solutions

Question 5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer: Soapy floor reduces the frictional force applied by the floor to a maximum. That is why; it is difficult to walk on a soapy floor.

Question 6. Explain why sportsmen use shoes with spikes.
Answer: Sportsmen use shoes with spikes because spikes produce the desired frictional force and thus help in holding the ground firmly.

Question 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer: Seema will have to apply a larger force because her box is heavier and heavier objects will have to undergo greater frictional force from the surface of contact.

Question 8. Explain why sliding friction is less than static friction.
Answer: Suppose a box kept on the surface has to be pushed. If the box is slided then, when the box starts sliding, the contact points on its surface do not get enough time to lock into the contact point on the floor. So, the sliding friction is slightly less than the static friction and we find it easier to move the box already in motion than to get it started.

Question 9. Give examples to show that friction is both a friend and foe.
Answer:
1. Friction as a friend:

• To hold a glass, we have ridges on our palms, which increase the friction between palm and the glass.
• We do write anything with a pen or pencil because there is friction between the surfaces of paper and the point of the pen or pencil.
• The teacher writes on black —the board with chalk because of friction between the blackboard surface and the chalk.
• If there is no friction, then a moving body would never stop.

2. Friction as a foe:

• Friction wears out materials, whether they are screws, ball-bearing or soles of shoes.
• Friction can also produce heat, which increases the wear and tear of machine parts. It also causes much wastage of energy because this heat is not utilized.

Question 10. Explain why objects moving in fluids must have special shapes.
Answer: Birds and fishes have to move about in fluids all the time. Their bodies must have evolved to shapes which would make them not lose much energy in overcoming friction. Such shapes are called streamlined. Cars are also designed so that their shapes resemble the streamlined shapes of birds, fishes, etc.

Friction Class 8 Kseeb Solutions With Answers

Friction Activities

Question 1. Gently push a book on a table. You observe that it stops after moving for some distance. Repeat this activity pushing the book from the opposite direction Does the book stop this time, too? Can you think of an explanation? Can we say that a force must be acting on the book opposing its motion? This force is called the force of friction.

Answer: Yes, the book stops this time too. After sliding for some time, the book stops. The surface of the table provides the frictional force to oppose the sliding of the book. Yes, we can say that a force must be acting on the book opposing its motion.

Kseeb Class 8 Science Friction Exercise Solutions

Question 2. Tie a string around a brick. Pull the brick by a spring balance You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.

Now wrap a piece of polythene around the brick and repeat the activity. Do you observe any difference in the readings of the spring balance in the above two cases? What might be the reason for this difference? Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?
Answer:

• It is observed that the reading of the spring balance in the case when a brick is wrapped with a piece of polythene is less than in the other cases.
• The reason for this difference could be that in the first case, since the brick surface in contact with the ground is rough, the surface gets the greater force of friction. But in the second case, when the brick is wrapped with polythene, the surface gets The pencil cell covers different distances on smoother.
• Hence, less frictional force is required for different surfaces Repeat this activity by moving it. spreading a thin layer of sand over the table.
• By wrapping a piece of jute bag around the hi Von on the same slope throughout the brick, it is observed that the reading of spring balance is more than the second case when the brick was wrapped with polythene.

Question 3. Make an inclined plane on a smooth floor, or on a table. You may use a wooden board supported by bricks, or books. Put a mark with a pen at any point A on the inclined plane. Now let a pencil cell move down from this point. How far does it move on the table before coming to rest? Note down the distance, Now spread a piece of cloth over the table. Make sure that there are no wrinkles in the cloth. Try the activity again.
The pencil cell covers different distances on different surfaces Reapt this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.
Answer: The distance covered by the pencil cell is
brick wrapped with polythene. different every time because of the different

Question 4. Take a few pencils which are cylindrical down from this point. How far does it move shape? Place them parallel to each other on
the table before coming to rest. Note a table. Place a thick book over it down the distance. Now spread a piece of Now push the book. You observe the pencil cloth over the table. Make sure that there roll as the book moves. Do you feel it are no wrinkles in the cloth? Try the activity easier to move the book in this way than to again slide it. Do you think that resistance to the motion of the book has been reduced? Have you seen heavy machinery being moved by placing logs under it?


Answer: It is observed that the book moves efficiently when placed on cylindrical pencils or rollers. It can be concluded that rolling friction is smaller than sliding friction.

Karnataka Board 8th Science Chapter 8 Important Questions And Answers

Friction Additional Questions

Question 1. What is the force of friction?
Answer: The force acting on a moving object equal and opposite to the direction of motion is called the force of friction.

Question 2. What is the cause of friction?
Answer: The irregularities on the two surfaces in contact is the cause of friction.

Question 3. Why are the wheels of vehicles made circular?
Answer: Rolling friction is less than sliding friction.

Question 4. What is the direction of the force of friction acting on a moving object?
Answer: Opposite to the direction of moving objects.

Question 5. What is the name of a special shape given to objects moving in fluids?
Answer: Streamlined

Question 6. Give one example where ball bearings are used.
Answer: Ceiling fan

Question 7. Which surface does produce more friction smooth or rough surface?
Answer: Rough surface

Question 8. What is drag?
Answer: The frictional force exerted by fluids is called drag.

Question 9. What are fluids?
Answer: Commonly gases and liquids are called fluids.

Question 10. How can fluid friction be minimised?
Answer: By giving suitable shapes to bodies moving in fluids.

Kseeb Class 8 Science Chapter 8 Textbook Solutions Pdf

Question 11. If we push the book on the table, it stops after some time. Why?
Answer: When we push a book on a table, a force is applied to it in the opposite direction of the motion called the force of friction. This force opposes the movement and the book stops.

Question 12. What is a spring balance?
Answer: A spring balance is a device which measures the force acting on an object. It contains a coded spring. When a force is applied to it, the spring gets stretched. The pointer moves on the scale showing the magnitude of the force.

Question 13. What are the two factors on which the force of friction depends?
Answer: These two factors are:

• The nature, i.e., smoothness or roughness of the surfaces in contact.
• How the surfaces in contact pressed together.

Question 14. Explain increasing and decreasing friction with suitable examples.
Answer: In our daily life, in many cases, we increase or decrease friction for our own convenience. Some examples are given below:
Increasing friction:

• The soles of our shoes are grooved to provide the shoes better grip on the floor so that we can move safely. Similarly, the tyres of vehicles are treaded to increase friction to provide better grip.
• We increase friction by using brake pads in the brake system of bicycles and automobiles.
• Gymnasts apply some coarse substance on their hands to increase friction for better grip.
• Kabaddi players rub their hands with soil for a better grip on their opponents.
• Roads are made rough to prevent from slipping.
• Handles of cricket and tennis are made of rough materials to get a better grip.

Decreasing friction:

• In some situations, however, friction is undesirable and we would want to minimise it. For example:
• A few drops of oil are poured on the hinges of a door, the door moves smoothly.
• A bicycle and motor mechanic uses grease between the parts of these machines to reduce friction.
• By polishing surfaces, and sprinkling powder on carrom board, we reduce friction.
  Wheel and ball bearings are used in machines and vehicles to reduce friction.

Kseeb 8th Standard Science Chapter 8 Notes And Solutions

Question 15. What do you mean by fluid friction? How can fluid friction be reduced?
Answer: The gases and liquids are called fluids. All of them exert a force of friction on solid bodies moving in either of them. The force exerted by fluid is called drag.

• It depends on the speed of the object with respect to the fluid.
• It also depends on the shape of the object and the nature of the fluid.
• It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process, they lose energy.
• Efforts are, therefore, made to reduce friction. So, the objects are given special shapes, Birds and fishes have to move about in fluids all the time.
• Their bodies must have evolved to shapes which would make them lose less energy in overcoming friction. Such shapes are called streamlined.
• This is why the shapes of aeroplanes, ships and all vehicles are designed to have shapes which minimise fluid friction.

Question 16. A toy to play with: Take an empty matchbox. Take out its tray.
Cut a used refill of a ball pen of the same width as the tray as shown in the figure. Fix the refill with two pins on the top of the tray in Fig. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out Insert the tray in the outer cover of the matchbox.
Suspend the matchbox by the thread. Leave the thread loose. The matchbox will start falling down due to gravity. Tighten the thread now and observe what happens. Explain your observation. Can you relate to friction?

Answer: The matchbox will not fall further due to the friction between the thread and the matchbox.

Friction Class 8 KSEEB Exercise Solutions 

Friction Multiple Choice Questions

Question 1. Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play
1. Only if the objects are solid.
2. Only if one of the two objects is liquid.
3. Only if one of the two objects is gaseous.
4. Irrespective of whether the objects are solid, liquid or gaseous.

Answer: 4. Irrespective of whether the objects are solid, liquid or gaseous.
Explanation: Force of friction acts in solids, liquids and gasses and opposes each other’s motion.

Question 2. A boy is shown pushing the box from right to left. The force of friction will act on the box
1. From right to left ( !)
2. From left to right (’!)
3. Vertically downwards (“!)
4. Vertically upwards (‘!)

Step-By-Step Solutions For Friction Class 8 Karnataka Board

Answer: 2. From left to right (*!)
Explanation: Friction always acts in the opposite direction to the motion, hence the force of friction will be from left to right.

Question 3. To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
1. Stone
2. Plastic block
3. Wooden block
4. Glass block

Answer: 1. Stone
Explanation: Stone exerts a greater frictional force which sharpens the blades upon rubbing.

Question 4. A toy car released with the same initial speed will travel farthest on
1. Muddy surface
2. Polished marble surface
3. Cemented surface
4. Brick surface

Answer: 2. Polished marble surface
Explanation: Polished marble surface has the least frictional force and least inertia, hence car will travel farther on a marble floor.

Question 5. If we apply oil to door hinges, the friction will
1. Increases
2. Decreases
3. Disappear altogether
4. Will remain unchanged

Answer: 2. Decreases
Explanation: Because oil acts as a lubricant and reduces friction.

Question 6. Which of the following statements is incorrect?
1. Friction acts on a ball rolling along the ground.
2. Friction acts on a boat moving on water.
3. Friction acts on a bicycle moving on a smooth road.
4. Friction does not act on a ball moving through air.

Answer: 4. Friction does not act on a ball moving through air.
Explanation: Friction acts on the body moving in the air as fluid friction

Question 7. A boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the same ball travel a longer distance before coming to rest, he may
1. Spread a carpet on the wooden surface.
2. Cover the ball with a piece of cloth.
3. Sprinkle talcum powder on the wooden surface.
4. Sprinkle sand on the wooden surface

Answer: 3. Sprinkle talcum powder on the wooden surface.
Explanation: Applying talcum powder will reduce the frictional force, which will make the ball to travel a longer distance.

Question 8. In a large commercial complex, there are four ways to reach the main road. One of the paths has loose soil, the second is laid with polished marble, the third is laid with bricks and the fourth has a gravel surface. It is raining heavily and Paheli wishes to reach the main road. The path on which she is least likely to slip is
1. Loose soil.
2. Polished marble.
3. Bricks.
4. Gravel.

Answer: 4. Gravel.
Explanation: The only path will have the sufficient frictional force to walk easily, hence the answer is 4. Gravel.

Question 9. Two blocks of iron of different masses are kept on a cemented floor Which one of them would require a larger force to move it from the rest position?


Answer: The larger force will be required to move the heavier block.

Question 10. Will the force of friction come into play when a raindrop rolls down a glass window pane?
Answer: Yes, the force of friction comes into play when a raindrop rolls down a glass window pane.

Question 11. Two boys are riding their bicycles on the same concrete road. One has new tyres on his bicycle while the other has tyres that are old and used. Which of them is more likely to skid while moving through a patch of the road which has lubricating oil spilt over it?
Answer: The bicycle with worn-out tyres is more likely to skid.

Question 12. Shows two boys applying force on a box. If the magnitude of the force applied by each is equal, will the box experience any force of friction?


Answer: Force of friction will be zero as the net force on the box is zero.

Question 13. Imagine that an object is falling through a long straight glass tube held vertically; the air has been removed completely from the tube. The object does not touch the walls of the tube. Will the object experience any force of friction?
Answer: No, the object will not experience any force of friction.

Question 14. You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.
Answer: After using slippers for a long time, the soles become smooth. This decreases the friction between the sole and the floor. Therefore, slippers become slippery.

Important Questions For KSEEB Class 8 Friction 

Question 15. Is there a force of friction between the wheels of a moving train and iron rails? If yes, name the type of friction. If an air cushion can be introduced between the wheel and the rail, what effect will it have on the friction?
Answer: Yes, there exists a rolling force between the wheels of a moving train and iron rails. If an air cushion is introduced between the wheel and the rails, the friction will decrease.

Question 16. Cartilage is present in the joints of our body, which helps in their smooth movement. With advancing age, this cartilage wears off. How would this affect the movement of joints?
Answer: With advancing age, this cartilage wears off. This increases friction, which will lead to difficulty in movement and will cause joint pain.

Question 17. While playing tug of war Preeti’s hands. Suggest a way out for her to prevent this.


Answer: She may rub soil to increase friction between the rope and her hand.

Question 18. The handle of a cricket bat or a badminton racquet is usually rough. Explain the reason.
Answer: To have a better grip on the bat it should be rough because a rough surface decreases friction and allow you to have a firm grip on the bat.

Question 19. Explain why the surface of the mortar and pestle (ciabatta) used for grinding is etched again after prolonged use.
Answer: To increase friction to make it more effective for grinding again.

Question 20. A marble is allowed to roll down an inclined plane from a fixed height. At the foot of the inclined plane, it moves on a horizontal surface
1. Covered with silk cloth
2. Covered with a layer of sand and
3. Covered with a glass sheet. On which surface will the marble move the shortest distance? Give the reason for your answer.
Answer: A marble moves to the shortest distance on the floor covered with sand as it offers maximum friction against its motion.

Question 21. A father and son pushed their car to bring it to the side of the road as it had stalled in the middle of the road. They experienced that although they had to push with all their might initially to move the car, the push required to keep the car rolling was smaller, once the car started rolling. Explain.
Answer: Initially they had to apply force to set the card in motion but once the car started rolling, they had to apply force only to balance the rolling friction of the car, the value of which is very less.

Question 22. When the cutting edge of a knife is put against a fast-rotating stone to sharpen it, sparks are seen to fly. Explain the reason.
Answer: Heat is produced as a result of friction between a knife and the rotating stone. As the speed of rotation increases the amount of heat generated also increases which results in the sparks.

Question 23. We have two identical metal sheets. One of them is rubbed with sandpaper and the other with ordinary paper. The one rubbed with sandpaper shines more than the other. Give reason.
Answer: The friction between plane paper and the metal sheet is less compared to the friction between sandpaper and metal. This friction acts on the top layer of the metal sheet which will remove the top layer to make the metal shine.

Question 24. While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when brakes are applied suddenly. Explain.
Answer: If the seat cover is very smooth then the friction between the seat and our body is very less. Because of this, we tend to slip when brakes are applied in a rickshaw.

Question 25. Two friends are trying to push a heavy load Suggest a way which will make this task easier for them.

Answer: Their work can be made easy by putting rollers below the heavy load because the rolling friction is smaller than the sliding friction.

Karnataka State Board Syllabus for Class 8 Textbooks Solutions

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Conservation of Plants and Animals
• Chapter 7 Force and Pressure
• Chapter 8 Friction
• Chapter 9 Sound
• Chapter 10 Combustion and Flame
• Chapter 11 Cell Structure and Functions
• Chapter 12 Reproduction in Animals
• Chapter 13 Reaching the age of Adolescence
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water