NCERT Solutions for Class 8 Science Chapter 12 Friction

1. Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact with each. other.

Answer: motion

(b) Friction depends on the _____________ of surfaces.

Answer: nature

(c) Friction produces __________.

Answer: heat

(d) Sprinkling of powder on the carrom board ________ friction.

Answer: decreases

(e) Sliding friction is ___________ than the static friction.

Answer: less than

2. 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

Friction occurs when irregularities of two objects in contact get interlocked with each other. Static friction occurs when we try to move object which is at rest. Sliding friction occurs between the sliding object and the surface. In this friction time of interlocking between the surface and object is small. Thus, it is weak friction. Therefore, static friction is greater than sliding friction. In rolling friction interlocking is much weaker as area of contact between the object and surface it extremely small. Also, area of contact between object and surface keeps on changing due to rolling. Thus, this is weakest friction force among all three. Therefore, correct decreasing order is static, sliding and rolling.

3. 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.

Force of friction depends on the nature of the surface. Friction is due to roughness of the surface. Roughness of surface can be arranged in ascending order as wet marble floor, dry marble floor, newspaper and towel. Thus, the right sequence in which force of friction acting on the car on different surfaces in increasing order is wet marble floor, dry marble floor, newspaper and towel.

4. 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: If book is sliding down then friction force will act in opposite direction i.e. in upward direction.

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: Frictional force allows us to walk on the floor. While walking we push floor in backward direction. It will be difficult to walk on floor having soapy water. This is because, soapy water reduces friction of the floor.

6. Explain why sportsmen use shoes with spikes.

Answer: Spikes increases force of friction between ground and shoes. Thus, it provides grip while running. Therefore, sportsmen use shoes with spikes.

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 require larger force. This is because, with heavier objects interlocking between surfaces increases. Thus, to push or pull an object it requires more force.

8. Explain why sliding friction is less than static friction.

Answer: Static friction occurs when we try to move object which is at rest. Sliding friction occurs between the sliding object and the surface. In this friction time of interlocking between the surface and object is small. Thus, it is weak friction. Therefore, static friction is greater than sliding friction.

9. Give examples to show that friction is both a friend and a foe.

Answer:

• Friction as friend:
  • Friction helps in walking
  • Friction helps in writing on books
• Friction as foe:
  • Tires, soles of shoes and slippers wears out due to friction
  • Due to friction, part of machines get heated, thereby causing damage

10. Explain why objects moving in fluids must have special shapes.

Answer: Drag force acts on the objects moving in the fluid. By giving special shapes to objects moving in the fluids, this force of friction can be minimised, which helps object to move easily in fluids.

Check other lessons NCERT Solutions for Class 8

Download free NCERT textbook class 8 Science – Chapter 8 Cell Structure and Functions