Class 8 Science Chapter 9 HOTS Questions - Friction

Q1: Will force of friction come into play when a rain drop rolls down a glass window pane?
Ans: Yes

Q2: Will an object experience any force of friction which is falling through a long straight glass having no air in it?
Ans: No

Q3: Cartilage is present in joints of our body, which helps in their smooth movement. If cartilage wears off, how would this affect the movement of joints?
Ans: Wearing off cartilage will make its surface rough and increase the friction. As a result the movement of joints will become difficult which may lead to joint pains.

Q4: Why handle of cricket bat or badminton racquet is rough?
Ans: To have a better grip by increasing friction between handle of the bat and hands.

Q5: Two identical metal sheets, A and B, are rubbed with paper and sand paper respectively. Which one of them will shine more?
Ans: Metal sheet B will shine more because friction between sand paper and metal sheet is larger than friction between paper and metal sheet. Thus, sand paper will be able to remove the outer dull layer of metal more effectively than other.

Q6: On Sunday Razak invited everybody to play carrom in his house. While playing, striker was not moving properly from one place to another. Everybody thought it is their bad luck. But Razia, one of the player, quickly got up and sprinkled some talcum powder on the board.

• Why Razia sprinkled some talcum powder?
• Can you give some other example which serve the same purpose?
• What value of Razia is shown here?

Ans:

• Razia sprinkled some talcum powder to make the surface of carrom board smooth and hence to reduce friction.
• Friction can be reduced by using lubricants, ball bearings and painting the rough surface.
• Razia is an intelligent girl with scientific aptitude.

Q7: Ram and Shyam while returning from school saw a labour pushing hard to move a heavy box. Ram immediately ran to him and started helping him in pushing the box whereas Shyam brought two iron bars and asked the labour to put them below the box and then to push the box.

• What are the factors that affect friction?
• Who will help the labour more-Ram or Shyam? How?
• What values of Ram and Shyam are shown here?

Ans:

• Nature of surfaces in contact, weight of the object, force of adhesion and deformation.
• Shyam’s suggestion of keeping iron bar below the box will help the labour more. Putting the iron bar below the box will make it to roll over the bar. As rolling friction is less than sliding friction, it will help in moving the box with much easier.
• Ram and Shyam are helping in nature. Shyam is intelligent with scientific aptitude.

Q8: What are the various types of friction? Explain.
Ans: There are mainly three types of friction:

• Static friction: The friction existing between a stationary object and the surface on which it’s resting is called static friction. This type of friction is more powerful than others. We need a lot of energy to overcome such type of friction.
• Sliding friction: The friction between the two surfaces in contact when one of them just slides , over the other is called sliding friction. It is also known as kinetic friction. It is less than static friction.
• Rolling friction: When one body rolls over the surface of another body, the resistance of its motion is called rolling friction. Rolling friction is less than sliding friction. It is thus the least friction of three types of friction, i.e., Static friction > Sliding friction > Rolling friction.

Q9: What are the causes of friction? Explain in detail.
Ans: Friction is mainly caused by the following factors:

(i) Interlocking of surfaces: The force of friction arises due to interlocking of the irregular projections on the two surfaces. All surfaces have many irregularities. When we attempt to move one object over the other, we have to apply a force to overcome interlocking of the irregularities in their surfaces and friction is produced. The roughness varies from surface to surface due to varying irregularities on the surfaces. So the more is the roughness of the surface, the larger is the number of irregularities and hence the greater will be the friction.

(ii) Force of adhesion: When the two rough surfaces come in contact, it leads to the formation of various contact points. The atoms or molecules present at such points of contact give rise to attractive forces of adhesion that opposes the relative motion of the bodies. Hence, the force of adhesion acting between two surfaces cause friction.
(iii) Deformations: Soft materials will deform when under pressure. This also increase the resistance of motion. For example, when you stand on a sand bed, you sink slightly, which causes resistance when you try to drag your feet along the sand surface. In this case, you must plow through to move, thus creating resistive force.
(iv) Weight of the body: The greater is the weight of the body, the greater will be the force of friction. If we push a heavy box, it will require greater force to move as the force of friction is greater. On the other hand, it is easier to move a lighter box because it produces less friction.

Q10: Explain increasing and decreasing friction with suitable examples.
Ans: In our daily life, in many cases we increase or decrease friction for our own convenience. Some examples are given below:

• Soles of our shoes are grooved to provide the shoes better grip on the floor so that we can move safely. Similarly 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 of their opponents.
• Roads are made rough to prevent from slipping.
• Handle of cricket and tennis are made of rough materials to get 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, sprinkling powder on carrom board, we reduce friction.
• Wheel and ball bearings are used in machines and vehicles to reduce friction.

Q11: What do you mean by fluid friction? How can fluid friction be reduced?
Ans: The gases and liquids are called fluid. All of them exert force of friction on solid bodies moving in either of them. The force exerted by fluid is called drag. It depends on speed of the object with respect to 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 shapes of aeroplanes, ships and all vehicles are designed to have shapes which minimise fluid friction.

Q12: Write advantages and disadvantages of friction.
Ans: Friction as friend:

• It allows us to grip and catch any object.
• It helps us to walk comfortably on the floor.
• It helps to minimise the speed or to stop any moving object.
• It helps us to write.

Friction as foe:

• It causes wear and tear in objects.
• It causes damage to the parts of machines and tools which further require money to get them repaired.
• It reduces the speed of moving objects, so more force is required.
• It produces hurdles in moving any object freely.