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Chapter review:

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• Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.
• The force of friction always opposes the applied force.
• Spring balance is a device used for measuring the force acting on an object.
• Friction can also produce heat. Vigorously rub your palms together for a few minutes.
• Friction depends on the nature of surfaces in contact.
• The substances which reduce friction are called lubricants.
• When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.
• When one body slides over the surface of another body, the resistance to its motion is called the sliding friction.
• Static friction comes into play when we try to move an object at rest.
• Rolling friction is smaller than the sliding friction.
• Sliding friction is smaller than static friction.
• The frictional force exerted by fluids is also called drag.
• Friction can be increased by making a surface rough.
• Friction can be reduced by using lubricants.

Exercise - Quesstion:

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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.

 

Additional - Question:

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Que: What is the force of friction?

Ans: The force acting on a moving object equal and opposite to the direction of motion is called force of friction.

Que: What is the cause of friction?

Ans: The irregularities on the two surfaces in contact is the cause of friction.

Que: What is the force responsible for wearing out of bicycles tyres?

Ans: Frictional force.

Que:  Why are wheels of vehicles made circular?

Ans: Rolling friction is less than sliding friction.

Que: What is the direction of force of friction acting on a moving object?

Ans: Opposite to the direction of moving objects.  

Que: Why do pieces of luggage fitted with rollers?

Ans: To reduce friction and to pull them easily.

Que: Why does a matchstick catch fire when rubbed on the rough surface of the box?

Ans: Due to friction between the stick and rough surface a matchstick catches fire.

Que: What is the name of special shape given to objects moving in fluids?

Ans: Streamlined.

Que: Which surface does produce more friction—smooth or rough surface?

Ans: Rough surface

Que: Why is fine powder sprinkled on the carrom board?

Ans: In order to reduce friction powder is sprinkled on carrom board.

Que: Why is it difficult to walk on the wet marble floor?

Ans: The wet marble floor has less friction. So it is difficult to move on it.

Que: Why does the jar of a mixer become hot when it is run for few minutes?

Ans: It becomes hot due to friction.

Que: Name the types of friction.

Ans:

• Static friction
• Sliding friction
• Rolling friction
• Fluid friction

Que: Which force helps us to write on the paper or blackboard?

Ans: Frictional force

Que: What is drag?

Ans: The frictional force exerted by fluids is called drag.

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