Class 8 Science Chapter 9 Question Answers - Friction

Q1: If we push the book on the table, it stops after sometime. Why?
Ans: When we push the book on a table, a force is applied on it in the opposite direction of the motion called force of friction. This force opposes the movement and the book stops.

Q2: Why are the worn-out tires discarded?
Ans: The worn-out tires are discarded because their treads are worn off, limiting their effectiveness in providing the required amount of friction.

Q3: Why is it easier for fish to swim in water?
Ans: Fish can easily swim in water because they have a streamlined body which produces less friction. This streamlined shape allows them to cut through water and move efficiently.

Q4: Give some examples to show that friction produces heat.
Ans: Examples include:

• Rubbing our hands together to keep warm in winter.
• Rubbing matchsticks on a rough surface to create fire.
• Q5: Give some examples of smooth and rough surfaces.
• Ans: Smooth surfaces: Surface of a mirror, surface of wet soaps, glazed tiles, surface of an oily pot, etc.
• Rough surfaces: Surface of plywood, unpainted wall, brick, tires, etc.

Q6: What is a spring balance?
Ans: A spring balance is a device used to measure the force acting on an object. It contains a coiled spring that stretches when a force is applied. The pointer on the scale indicates the magnitude of the force.

Q7: How does friction depend on the nature of the surface?
Ans: The nature of the surface affects friction. Rough surfaces have more irregularities and result in greater frictional force. Smoother surfaces have fewer irregularities and lower friction.

Q8: What are the two factors on which the force of friction depends?
Ans: The force of friction depends on:

• The nature (smoothness or roughness) of the surfaces in contact.
• The force pressing the surfaces together.

Q9: When the two surfaces are pressed harder, friction increases. Explain why?
Ans: Friction is caused by the interlocking of irregularities on the surfaces. When the surfaces are pressed harder, these irregularities interlock more tightly, resulting in greater friction.

Q10: What is a measure of static friction?
Ans: The force required to overcome friction when an object starts moving from rest is a measure of static friction.

Q11: What is a measure of sliding friction?
Ans: The force required to keep an object moving at a constant speed is a measure of sliding friction.

Q12: Why is it easier to move a box already in motion than to get it started?
Ans: The sliding friction is slightly smaller than the static friction. When a box starts sliding, the contact points do not lock into the surface as they do in static friction. Hence, it's easier to maintain motion than to initiate it.

Q13: Why are the soles of shoes and tires of vehicles grooved?
Ans: The grooves on the soles of shoes and tires of vehicles provide better grip on surfaces, enhancing traction and safety by increasing friction.

Q14: We use ball bearings between the hub and axle of ceiling fans and bicycles. Why?
Ans: Ball bearings are used between the hub and axle of ceiling fans and bicycles to reduce friction. Rolling friction in ball bearings is smaller than sliding friction, making the movement smoother.

Q15: Why are lubricants applied between the moving parts of a machine?
Ans: Lubricants are applied to reduce friction between moving parts. They create a thin layer that prevents direct contact between surfaces, reducing wear and making movement smoother.

Q16: How do brakes stop moving bicycles and automobiles?
Ans: When brake pads are pressed against the wheel's surface, friction is created. This friction generates a force that opposes the motion, gradually reducing the speed and bringing the bicycle or automobile to a stop.

Q17: People often rub their hands in winter. Why?
Ans: Rubbing hands together generates friction, which produces heat. This friction-generated heat warms the hands, making rubbing hands together a common way to keep warm in winter.

Q18: 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.

Q19: 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.

Q20: 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:
Increasing friction:

• 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.