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Science, , 1 Mark Questions and Answers:, , , , 1., , 10., , 8th Std, 8. Friction, , , , Define friction., Friction is the force which resists the motion of a body, while, moving on a surface., , What is meant by sliding friction ?, When one surface is sliding over the other, sliding friction comes, into play., , When does static friction come into play ?, Static friction comes into play to counter balance the applied force, on the body., , What are lubricants ?, The substances which reduce friction are called lubricants., , What is rolling friction ?, When one body rolls over the surface of another body, the, resistance to its motion is called the rolling friction., , What are fluids ?, Fluids is the common name given to gases and liquids., , Do fluids exert friction ?, Yes, fluids also exert friction., , What is meant by air resistance ?, There is friction between a moving object and the air through which, it moves, known as air resistance., , What is meant by drag ?, The frictional force exerted by fluids is known as drag., , Is friction a contact force or non-contact force ?, Contact force., , il., , 12., , , , Why is it difficult to walk on ice ?, It is difficult to walk on ice because the friction between feet and, ice is less, , When a boat is moving in the river, does the water offer force of, friction to its movement ?, Yes, the water opposes the movement of the boat., , , , Mark Questions and Answers :, , , , When your writing desk is tilted a little, a book kept on it starts, sliding down. Mention the direction of the frictional force acting on it., , The frictional force acts in the upward direction., , You spill a bucket of soapy water on a marble floor accidently., Would it make easier or more difficult for you to walk on the, floor ? Why ?, , It would be more difficult to walk on the floor with soapy water, because when we walk on a soapy floor, there is little friction between the floor and the feet, so it becomes difficult to walk., , Put a tick mark on the correct choice :, , When one object moves over the surface of other object., , * surface of only lower object exerts frictional force on the up, per object., , * surface of only upper object exerts frictional force on the lower, object., , * surfaces of both the objects exert force on each other., , * surfaces of both the objects exert force in a direction op, posite to each other., , Four children have got similar toy cars. They run the car on the, dry marble floor, wet marble floor, newspaper and towel, simultaneously. The force of friction between the car and, , .Adifferent surfaces in increasing order will be :, Wet marble floor, dry marble floor, newspaper and towel.

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Newspaper. towel. dry marble floor, wet marble floor., Towel, newspaper, dry marble floor, wet marble floor., Dry marble floor, wet marble floor, towel, newspaper., , 5. Explain why sportsmen use shoes with spikes., Sportsman use shoes with spikes to increase the friction. In this, way, the sportsman can run faster., , 6. Iqbal has to push a lighter box and Seema has to push a similar, heavier box on the Who will have to apply a larger force and, why ?, , Seema will have to apply more force because the weight of the box, increases and the friction increases. For a heavier box the ridges of, the surfaces get interlocked more., , 7. Explain why sliding friction is less than static friction., Sliding friction is less than static friction because it comes into play, once the body has started sliding. To make the body slide, more, force is required to overcome the irregularities in the surface of the, , body., , 8. Explain why objects moving in fluids must have special shapes., Fluids also exert a frictional force on the body moving through them., Therefore, the shape of the body is streamlined to cover the frictional force of fluids and to move smoothly through them., , 9. Why does the force of friction exist ?, Friction is caused by the irregularities on the two surfaces in, , contact., , 10.Can we have surfaces with zero friction ?, No, the friction can be less in some surfaces, but it can never be, zero., , 11. Which surface have more friction — rough or smooth ? Why ?, Rough surfaces have more friction because they have larger, number of irregularities., , 12., , 13., , 14., , 15., , 16., , 17., , 18., , 19., , ‘d with chan = =, is i i ite on rough blackboar' he ., Why is tk Do the rough blackboard saul ring ne, ee a chalk particles get stuck to — = ariti, blackboard susfare making the writing visible., , ill i i 2 Why ?, A ball is set rolling 0 the ground. will it papel bel is, The ball will stop after sometime because tne, acting from the opposite direction., Why do the soles of shoes wear out ?, The soles of shoes wear out due to the, and the ground., , friction between the soles, , Why are the soles of shoes grooved ? ; ; +, Soles of shoes are grooved so as to provide better grip between the, , shoes and the ground., , Give two examples where rolling friction is utilized., Rolling friction is utilized by using ball bearings between the hub, and the axle of a ceiling fan and the bicycle., , What is function of ridges on your palm and bottom of your, , feet ?, The function of the ridges is to increase the friction., , Name the two factors on which the magnitude of frictional force, , depends., , Frictional force depends on, , * — The weight of the body and, , * The nature of the surface in contact., , Give two methods by which friction is reduced in the wheels of, , the car., , * — The wheels of the car are provided with ball bearings along, the axle which reduce the friction., , * — The tube and tyres are inflated which reduce friction.

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1., , 3 Mark Questions and Answers :, , Give examples to show that friction is both a friend and a foe., , 1. Friction is a friend in the following ways-it helps us to walk,, , to write on the blackboard and to apply the brakes of a car., Friction isa foe in the following ways -it causes wear and tear, of machinery and shoes, it produces heat in the machinery., , 2., , Why does friction increase if the two surfaces are pressed ?, , Since the friction is due to the interlocking of irregularities in the, , two surfaces which slide with respect to each other, it increases 8., when the two surfaces are pressed harder., , Why it is easier to move a box which is already moving than one, which is at rest ?, , When the box starts sliding, the contact points on its surfaces, do, , not get enough time to lock, into the contact points on the floor. So,, the sliding friction is slightly less than static friction. 9., , Why does a car stop when brakes are applied ?, , In cars, the friction is increased by using brake pads in the brake 10., system. The brake pads do not touch the wheels but when we press, , the brake level, these pads arrest the motion of the run due to fric, tion and the wheels stop moving., , Give three factors on which the frictional force in fluids de- 11., , pends., , The frictional force in fluids depends on :, , * — The speed of object with respect to the fluid, * — The shape of the object., , * — Nature of the fluid., , A labourer has to push a heavy trunk. What should be done to, reduce his effort in pushing it and why ?, , The labourer should put some cylindrical logs of wood under the, , trunk. In this way the area of contact between the two surfaces is, reduced. Rolling friction is less than sliding friction., , Give three harmful effects of friction., , * Friction generates heat, for e.g., between the parts of, machinery., , * — Friction causes wear and tear of the rubbing surfaces e.g., soles, of shoes wear out., , * Friction reduces the speed of the body and increases the con, sumption of energy, , Why is friction essential ? Give three examples., , * ” Friction is necessary between the soles of our feet and ground, so that we can walk without slipping., , * — Friction helps us in writing on paper., , * — Friction makes it possible to light a match stick, by rubbing it, along the rough surface of the match box., , How do trees help reduce friction ?, Trees slow down strong winds and save us from wind storms., , Why do meteors bum on entering the atmosphere ?, , The speed of the meteor is very high. Therefore, there is a large amount, of friction between the air and meteor. This generates a lot of heat, and the meteor starts burning., , Gaurav went to the market on his bicycle. He found that he had, to apply a lot of force while pedalling. Why ? What can he do z, Gaurav had to apply more force because there was more friction be, tween the parts of the bicycle. He should apply oil to the parts tc, reduce the friction.

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5 Mark Questions and Answers :, 1. Fillin the blanks :, 1. Friction opposes the Relative motion between the surfaces in, contact with each other., Friction depends on the Nature of surfaces., , , , , , , , Qi, 3. Friction produces Heat, 4. Sprinkling of powder on the carom board reduces, friction., 5. Sliding friction is more on a Surface area than the static, friction., 2. Give five examples to show that friction is increased deliberately., , Athletes wear spiked shoes to help them run fast., , 1. Mountaineers wear spiked shoes while walking on snow., , 2. The tyres of vehicles are grooved., , 3. The roads are made rough., , 4. The blackboard is made rough., , tiles laid along the surface of spaceships 2, into the space from the rockets with a very, high speed. This creates a very high frictidnal force between the air, and space ship. As result, the spaceship gets heated to red hot. The, heat resistant tiles save the spaceship from burning., , 2 Why do we lubricate the moving, d where they, , 3, Why are heat resistant, The spaceships are sent, , 4. What is meant by lubrication, parts of our vehicles ? Name some lubricants an, , are used., time to time. Lubrica, Lubrication means oiling the machines from, done to reduce friction between them. We, , tion of the moving parts is, , can use oil or grease in machines. Graphite can be used in machines, which reach a very high temperature., , ape’ ? Where and why is this, , 5, What is meant by ‘streamlined sh, provided streamlined body ?, , shape used ? To whom has nature, Streamlined shape means they are narrow in front and broader at the, , back. The bodies of aeroplanes, missiles and rockets are streamlined, to reduce friction with air. Ships and boats have a streamlined shape, , to reduce friction with air. Nature has given birds and fish, streamlined bodies.