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Friction - Class 8 Science Textbook Solution

 Class 8 Science Chapter 8 Friction
1. Fill in the blanks.
1.    Friction opposes the _______ between the surfaces in contact with each other.
2.    Friction depends on the _______ of surfaces.
3.    Frcition produces _______
4.    The sprinkling of powder on the carrom board _______ friction.
5.    Sliding friction is _______ than the static friction.

1.    Friction opposes the relative motion between the surfaces in contact with each other.
2.    Friction depends on the nature of surfaces.
3.    Friction produces heat.
4.    The sprinkling of powder on the carrom board reduces friction.
5.    Sliding friction is less than static friction.
2. Four children were asked to arrange forces due to rolling, static, and sliding frictions in 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
(c) static, sliding, rolling
3. Alida runs her toy car on the 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
(а) 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
(a) 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.

Fig. Frictional force is acting opposite to the movement of book i.e. upwards.
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?
Soapy floor reduces the frictional force applied by the floor to the maximum. That is why it is difficult to walk on a soapy floor.
6. Explain why sportsmen use shoes with spikes.
Sportsmen use shoes with a spike to increase the friction so, that their shoes do not slip while they run or play.
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?
Seema will experience more frictional force because the heavy object will be pressed hard against the opposite surface and produces more friction.
8. Explain why sliding friction is less than static friction.
Suppose a box kept on the surface has to be pushed. If the box is slid then, when the box starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So the sliding friction is slightly less than the static friction and we find it easier to move the box already in motion than to get it started (i.e., static friction.)
9. Give examples to show that friction is both a friend and a foe.
Friction is a friend because:
•    It allows us to walk comfortably on the ground.
•    It allows us to grip and catch different objects.
•    Things don’t move from their places because of friction.
•    Speeds of moving objects can be minimized or stopped by friction only.
Friction is a foe because:
•    It wears out materials, whether they are screws, ball- bearings, or soles of shoes.
•    It also produces heat, which increases the 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.
When objects move through fluids, they have to overcome friction acting on them. In this process. They lose energy. To minimize friction, objects are given special shapes.
Friction Additional s and Answers
1. Why the vehicle slows down when brakes are applied?
When brakes are applied frictional force acts on the vehicle which slows down the motion of the vehicle and finally brings it to rest.
 2. Why does a ball moving on the ground stopping after some time?
A ball moving on the ground stops after some time because the ground applies a force of friction, which slows down the motion of the ball, and finally the ball stops.
3. What is a force of friction?
Force of friction opposes the relative motion between two surfaces in contact and it acts on both surfaces.

 4. What is spring balance?
Spring balance is a device used for measuring the force acting on an object.

 5. How is the force measured by spring balance?
Spring balance consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.

 6. Is it easier to hold a kulhar (earthern pot) or a glass tumbler?
It is easier to hold a kulhar than a glass tumbler as the glass tumbler surface is smooth and provides lesser friction.

 7. Suppose the outer surface of the tumbler is greasy or has a film of cooking oil on it. Would it become easier or more difficult to hold it?
It would become more difficult to hold a glass tumbler with a film of cooking oil on its surface because this surface will provide no friction or least friction.
8. How does friction cause problems for us?
Why is friction called evil?
Friction is evil. It causes a lot of problems for us. It wears out the objects or materials on which it acts. The soles of our shoes get worn due to friction. The floors, machines, their parts, clothes, metals, etc. get worn due to the friction they bear. Friction spoils the parts of the machines. They are, therefore, regularly lubricated to minimize friction. Friction produces heat, which further damages the objects on which it is exerted, The grooves of the tires are finished due to regular friction they bear, which is offered by their regular motion on roads. Except for this wear and tear, friction utilize a large amount of energy spent to overcome it.

 9. Why the sole of the shoe is grooved?
The sole of the shoe is grooved to provide the shoes better grip on the floor so that we can move safely.

10. What is rolling friction?
When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.

11. Rolling increases or reduces friction?
Rolling reduces friction.
12. Why the wheel is said to be one of the greatest inventions of mankind?
Wheels have reduced friction and it becomes easier to take things from one place to another.
13. What is drag?
The frictional force exerted by fluids is called drag.
14. Does air have friction?
Yes, the air has friction.
15. Do you find any similarity in the shape of a bird and airplane?
Yes, both the airplane and bird have streamlined bodies to minimize the force of friction provided by air while flying in the sky.

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