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Class 8 Lesson 12 (cardova), EXERCISE, A - Tick the correct options., , 1. (c) 2. (c) 3. (c) 4. (d) 5. (c) 6. (c) 7. (a) 8. (c) 9. (d), , B. Look at the figure [Fig. (a)] given alongside and tick (3) the, correct options., 1. (c) 2. (b) 3. (a), C- Fill in the blanks., 1. Less, 2. Increase, 3. Streamlined, 4. Reduce, 5. Drag (fluid friction), 6. Opposite, 7. More, Less, 8. Solid, 9. Skidding, 10. Lubricants, D- Very Short Answer Questions., , 1. The force acting along the two surfaces in contact which, opposes the motion of one body over the other is called friction., 2. Sliding Friction, 3. Gases, 1|Page
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Class 8 Lesson 12 (cardova), 4. When a wooden block is moved over the surface of a table, it, slides. The opposing force here is sliding friction., 5. A log of wood rolls over a flat surface. The opposing force here, is rolling friction., E. Short Answer Type -1 Questions., , 1. The special shape of a body or an object around which a fluid, (liquid or air) can flow, offering minimum amount of friction is, called streamlined shape., 2. The substances which reduce friction are called lubricants., For example – Oil and grease, 3. (a) The roughness of the surfaces, (b) The interlocking of the two surfaces, 4. Athletes wear spiked shoes to increase friction because spiked, shoes have better grip on the ground and prevent slipping., 5. Spring balance is a device used to measure the force acting on, an object. It works on the principle of stretching of spring, when, force is applied., F. Short Answer Type-II Questions., , 1. The different ways of increasing friction are –, (a) By making the surfaces rough: Friction can be increased by, increasing the roughness of the surfaces in contact. For example,, the surface of the head of a matchstick and the sides of a, matchbox are deliberately made rough to increase the friction to, 2|Page
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Class 8 Lesson 12 (cardova), produce more heat because of which the matchstick lights up, easily., (b) By making grooves: We can increase the friction in case of tyres, of bicycles, cars, buses, etc., by making grooves in them. Due to, greater friction, the tyres get a better grip on the road which, prevents skidding of the vehicles., 2. Rolling friction is less than sliding friction because the area of, contact of the surfaces is reduced when we use round objects, (e.g., wheels, ball bearings, roller bearings, etc.). In case of sliding, friction, the area of contact is greater and offers more friction. So,, whenever possible, sliding friction is replaced by rolling friction., For example—roller skates, luggage with wheels, vehicles, etc., 3. (a) Weight of the body : The magnitude of friction increases with, the increase in weight of a body., (b) Nature of surfaces in contact with each other : Smooth surfaces, have lessfriction and rough surfaces have more friction., 4. (a) When we apply brakes, the car stops due to the force of, friction between the brakes’ lining and the drum of the wheel., (b) We should always care for others. A person’s life is valuable, than anything in this world and we should always try to save one’s, life, , G. Long Answer Questions., 1. (a) The bodies of birds are streamlined such that they, experience least amount of friction due to air., 3|Page
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Class 8 Lesson 12 (cardova), (b) When we strike a matchstick against a rough surface, it catches, fire because the force of friction raises the temperature of the, matchstick head to such an extent that the chemicals stored in it, catch fire to produce flame., (c) Grooves are made in tyres to increase friction due to which the, tyres get a better grip on the road. This prevents skidding of the, vehicles., (d) A lubricant reduces friction by forming a thin layer between the, moving surfaces so that they do not directly rub each other., 2. Ball bearing is a hollow, circular device containing small metal, balls which are fitted around the moving part of a machine (like an, axle). The ball bearing reduces the friction by converting sliding, friction into rolling friction., For example, when the axle of a machine fitted with ball bearing, rotates, then the metal balls also roll and hence, the friction is, reduced., , 4|Page
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Class 8 Lesson 12 (cardova), H. HOTS (Higher Order Thinking Skills) Questions., 1. Friction damages the machines because of, , (a) continuous opposing force acting over the machines., (b) heat produced due to friction., 2. When we write on a glass sheet with a piece of chalk, the particles of chalk are not, left behind because glass is a smooth surface which does not apply frictional force on, the surface of chalk., 3. It is difficult to walk on wet marble floor because it offers very less frictional force, as compared to dry marble floor., , 5|Page
