Page 1 :
GURUKUL CENTRAL ACADMY, CHAPTER 12–FRICTION, WORKSHEET-1, A. force, interlocking, friction, B. 1. jamming of brakes, grooved tyres of race car, 2. soapy floor, carrom on a glass board, WORKSHEET-2, A, B, C – Refer book for answers., D. 1. decreases x increases 2. opposite is true, 3. incorrect definition, , TEXTBOOK EXERCISES, Page 135 Try These, 1. T 2. F 3. T 4. F 5. T, Page 137 Try These, 1. D 2. D 3. U 4. D 5. D 6. U, , EXERCISES, A. 1. a 2. c 3. d 4. d 5. a 6. b 7. b 8. a 9. d 10. D, B. 1. Friction, 5. lubricants, , 2. interlock, 3. static 4. grealer, 6. forward 7. reduces 8. reduces, , C. 1. F 2. F 3. T 4. T 5. T 6. T, , 7. T 8. T

Page 2 :
GURUKUL CENTRAL ACADMY, D. 1. d 2. a 3. b 4. e 5. C, E. 1., , The force which resists the motion when two surfaces are in, contact and move with respect to each other, 2. They become warm, 3. Friction generated when a circular object moves against another, surface, 4. Reduce friction 5. Remain unchanged, 6. Less hot, 7. Yes, , F. 1., , When two surfaces are in contact and move in opposite direction, it causes friction. This interlocking of the surfaces forbids further, movement giving rise to friction., , 2., , Frictional force depends on two main factors: the nature of the, surfaces in contact and the weight of the sliding body., , 3., , Rolling friction is less than sliding friction. Hence it is easier to roll, an object than to slide it., , 4., , The frictional force on an object in a fluid depends on, • its speed with respect to the fluid, • shape of the object, and, • nature of the fluid., , 5., , When an object moves through a fluid (a liquid or gas), its surface, comes into contact with the particles of the fluid. This causes, friction. The frictional force exerted by a fluid on an object is, called drag.

Page 3 :
GURUKUL CENTRAL ACADMY, 6., , Rough surfaces have irregularities in the form of grooves and, ridges. These irregularities in contact get interlocked giving rise to, friction. When the contact surfaces are smooth friction is less., , 7., , The friction that comes into play when the objects are at rest, relative to each other, is known as static friction. The maximum, friction that balances the force applied to a body resting on a solid, surface so that the body does not move is called limiting friction., , 8., , The friction acting on a body when it is sliding over a surface with, a constant speed is called sliding friction. The force required to, keep a body in motion is less than the force required to start the, motion. Once an object starts moving, the irregularities on it do not, get to interlock into the irregularities of the surface on which it is, moving. Thus, we can say that the sliding friction is lesser than the, static friction., , G. 1., , Depending upon the nature of surfaces and the magnitude of, forces applied on an object, the force of friction can be divided into, different categories. These are: static friction, kinetic (or sliding), friction and rolling friction., The friction that comes into play when the objects are at rest, relative to each other, is known as static friction. The friction, acting on a body when it is sliding over a surface with a constant, speed is called sliding friction. The force that resists the motion, when an object rolls over another surface is called rolling friction., For example, the resistance to the motion of a ball rolling on a, floor.

Page 4 :
GURUKUL CENTRAL ACADMY, 2., , Soles of the shoes get interlocked with the ground and generate, friction that enables us to keep balance. However the soles of the, shoes also get damaged due to this friction. We are able to cut, things due to friction between the surfaces in contact. But in this, process the knife becomes blunt. Automobiles are able to move, safely on the road because of, friction between the treaded tyres and the surface of the road friction, wears out the rubbing surfaces. Objects like, tyres of vehicles, soles, of shoes, moving parts of machines, surface of knives, etc. wear out, with time due to friction. Hence it is a necessary evil., , 3., , Friction between two surfaces can be increased by– making one of, the surfaces rough. Increasing the weight of the object Friction can, be decreased byOiling or applying grease on parts of a machinery, Use of ball bearings instead of non-moving metal-to-metal contact, Machinery, Smoothening of surfaces in contact Increasing the magnitude of, force applied, , 4., , When an object moves through air or a liquid, it has to cut through, the fluid to move forward. Hence All vehicles whether it is a, submarine or an aeroplane, are designed, to have streamlined, shapes which reduce fluid friction. A streamlined shape offers, least resistance while moving through a fluid., , 5., , The effect of drag can be reduced by—, • making the shape of the body streamlined, • increasing the speed of the object, • reducing the viscosity of the liquid

Page 5 :
GURUKUL CENTRAL ACADMY, Advantages of Friction, • We are able to walk on the road due to the friction between the, ground/road and the soles of our shoes. When we walk, our feet or, shoes get interlocked with the ground causing friction. This prevents, us from slipping. It is very difficult to walk on ice or on a very, smooth and polished floor., • We are able to write on paper because of the friction between the, pen/pencil and the paper. That is why it is very difficult to write on, a glazed/waxed paper, which is much smoother., Disadvantages of friction, • A person will fall if there is no friction between the soles of feet, and the ground.Friction makes moving or sliding heavy objects, difficult. A lot of energy is utilised in overcoming friction, 6., , 7., , When oil, grease or graphite is applied between the moving parts of, a machine, a thin layer is formed between the two parts and the two, surfaces are not able to rub against each other. Interlocking of, irregularity is avoided. Use of ball bearings instead of non-moving, metal-to-metal contact in fans and machinery further reduces, friction as sliding friction is transformed into rolling friction., , 8., , An object moving with high speed in a fluid experiences more drag., For example, a meteor enters the Earth’s atmosphere at a very high, speed. As a result, it experiences, high magnitude of drag. This in turn produces excessive heat that, burns the meteor.

Page 6 :
GURUKUL CENTRAL ACADMY, H. 1., , Soles of the shoes get interlocked with the ground and generate, friction that enables us to keep balance., 2. Straps of briefcases and bags have a comparatively rough surface, to avoid slipping from our hands., 3. Weightlifters and gymnasts apply a chalk on their hands which, increases friction and helps them to have a firm grip on the, weights., 4. to avoid wearing out due to friction, 5. Rubbing a matchstick produces fire by friction, 6. as banana peel reduces friction between the floor an shoe, , I. 1. crumbled piece, 2. because of presence of ball bearings