12. Friction Science class 8 in English Medium ncert book solutions Text Book Exercise

Exercise - Quesstion:

1. 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)-relative motion, (b)-nature (c)-heat, (d)-reduces, (e)-less.

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. 

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, 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:

The fractional force will act parallel to the inclined surface, opposite to the sliding of a book.

5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer:

Soapywater reduce the frictional force applied by the floor to a great extent. That is why, it is difficult to walk on a soapy floor.

6. Explain why sportsmen use shoes with spikes.

Answer:

Sportsmen use shoes wwiith spikes because spikes produce the desired frictional and thus heelp in holding the ground firmly.    

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 large force because her box is heavier object will have to overcome greater frictional force offered by the surface of contact. 

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

Answer:

Friction is caused by the interlocking in the two surface.

When the object starts slidiing, the contact points on it surface, do not get enough time to lock into the contact points of the other surface. So, the sliding friction is slightly smaller than the static friction.

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

Answer:

 (i) Friction as a friend:

(a) To hold a glass, we have ridges on our palm, which increase the friction between palm and glass.

(b) We do write anything with pen or pencil because there is friction between the surfaces of paper and point of pen or pencil.

(c) Teacher writes on black –board with chalk because of friction between black-board surface and the chalk.

(d) If there is no friction, then a moving body would never stop.

(ii) Friction as a foe:

(a) Friction wears out materials, whether they are screws, ball-bearing or soles of shoes.

(b) Friction can also produce heat, which increases wear and tear of machine parts. It also causes much wastage of energy because this heat is not utilized.

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

Answer:

Birds and the fishes have to move about in fluids all the times. Thus,  their bodies have evolved to shapes which make them not to lose much energy jnovercome fluid frictional.Such shapes are called stremilined. Cars  are also designed so that their shapes of birds, fishes etc.

11. your parents are going to buy a house. They have been offered one on the roadside and another three houes would you suggest your parents should buy? Explain your answer.

Answer: I would suggest my parents to buy the houes which is there lanes away from the roadside. This world protect us from noise pollution which is maximum on the roadside.

12. Sketch larynx and explain its function in your own words.

Answer: The function of larynx is to produce sound.

13. lightning and thunder take place in sky and at the same didtance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

Answer: The light travels at the speed of 3 x 10^8, very large in comparsion to the speed of sound which travels at the rate of

330 ms^-1 in the air. That is why lightning is seen earlier and thunder is heard later.