CBSE Class 8 - Science - CH 12 - Friction

Friction

Rolling Friction

NCERT Chapter Solutions, MCQs, Q & A

Q1(MCQ): Friction is a:
(a) contact force
(b) non-contact force
(c) magnetic force
(d) none of these

Answer:  (a) contact force

Q2(NCERT): Fill in the blanks:(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.

Answer:
(a) motion
(b) irregularities/roughness
(c) heat
(d) reduces
(e) less than

Q3: How do you define friction? What is the SI unit of friction?

Answer: Friction is 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. Friction is due to irregularities on the surfaces of the objects in contact. Friction depends on the smoothness of the surfaces in contact. The force of friction depends on the nature of the surfaces in contact.
Since friction is a type of force, it is measured in Newton (N).

Q4(NCERT): Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below.
Choose the correct arrangement.

(a) Rolling, static, sliding
(b) Rolling, sliding, static
(c) Static, sliding, rolling
(d) Sliding, static, rolling

Answer: (c) Static, sliding, rolling

Q5(MCQ):  Name the force that always opposes motion.
(a) electrostatic force
(b) frictional force
(c) magnetic force
(d) muscular force

Answer: (b) frictional force

Q6(MCQ): Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be :

(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

Answer: (a) Wet marble floor, dry marble floor, newspaper and towel.

Q7: Name four types of frictional forces.

Answer: Three types of frictional forces are:

1. Static friction 
2. Dynamic friction 
3. Rolling friction
4. Fluid friction (drag)

Types of Friction

Q8(NCERT): Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Answer: Frictional force acts opposite to the movement of the book i.e. upward direction.

Q9(NCERT): 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: It would be difficult to walk on the floor. Soapy water fills the floor irregularities, thus reduces the friction considerably. Feet do not make necessary grip with the floor surface, thus increasing the chances of falling.

Q10: What are the causes of frictions?

Answer: Following are the causes of friction:

1. roughness of two surfaces in contact and interlocking of the irregularities between the surfaces.
2. nature of surface i.e. smooth or rough
3. shape of surface (e.g. flat or curved)
4. impurities between surface (e.g. adding lubricant between two rough surface decrease friction)
5. pressure of two surfaces i.e. how hard the two surfaces are pressed together.
6. mass of the body of surfaces in contact

Q11(MCQ): The frictional force exerted by fluids is called:
(a) thrust
(b) rolling friction (c) dynamic friction
(d) drag

Answer: (d) drag

Q12(NCERT): Explain why sportsmen use shoes with spikes.

Answer: Spikes increase friction and give tighter grip with ground. It avoids slipping of sportsmen while playing or running.

Q13(NCERT): 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: Friction is a self-adjusting force. Increase in weight will put more force on the floor surface and therefore will experience more opposite force (friction). Seema will have to apply larger force.

Q14(NCERT): Explain why the sliding friction is less than the static friction.
Answer: When the objects are at rest, the interaction or interlocking between the surfaces of the objects is higher than that of when objects are moving. When objects are moving, there is less interaction between their surfaces. That's why sliding friction is less than static friction.

Q15: A cart has to pull harder during the start of the motion than later. Explain?

Answer: See answer of Q14.

Q16(MCQ): Which device is used to reduce friction in the axles of cars and shafts of electric motors?

(a) Screws
(b) Ball-bearings
(c) Nuts
(d) Bolts

Answer: (b) Ball-bearings

Q17(MCQ): A brick slides on a horizontal surface. Which of the following increases the magnitude of the frictional force?

(a) by putting a second brick on top
(b) by decreasing the surface area of contact
(c) by decreasing the mass of the brick
(d) by polishing the surface in contact

Answer: (a) by putting a second brick on top

Q18(NCERT): Give examples to show that friction is both a friend and a foe.
Answer: Friction is a necessary evil acting both as friend and foe.

Examples of friction as friend:

• We are able to walk because of frictional forces between ground and our feet.
• We are able to write because of friction between ball point and the paper surface.
• Nails and screws stick to wall surfaces because of friction.
• Lighting a match stick is because of friction between match stick and the side surface of match box.

Examples of friction as foe:

• Wear and tear of soles of our shoes is due to friction.
• When a tyre deflates, it is difficult to move the the vehicle because of increased friction between the tyre and road surface.
• Machines gets heated up and produce noise because of friction.
• Due to friction, machines and vehicles consumes more fuel and increase maintenance cost.

Q19(MCQ): Powder is used in carom board ____________.
(a) to increase friction
(b) to decrease friction
(c) to decorate the board
(d) to add fragrance

Answer: (b) to decrease friction

Q20(NCERT): Explain why objects moving in fluids must have special shapes.
Answer: The frictional force exerted by fluids is also called drag or fluid friction.To overcome or minimize air fluid friction, the bodies are given special shapes e.g. streamlined shape. Aeroplanes, bullet trains have streamlined curve surface to reduce air drag.