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€ 8 (23).pdf, , , , up due to friction with air, so a heat, shield is required to protect the space, ship., , . Treads (grooves) on vehicle tyres, increase friction and provide a better, grip to vehicles on road and prevent, them from skidding., , . By using sandpapers friction increases, which makes the surface of walls and, doors smooth to be painted again., , . The main cause of friction is the, presence of irregularities (peaks and, valleys) on the two surfaces in contact., It can be explained by the following, example:, , When a book is placed on a table, the, actual contact between the table and, the book is made at a much smaller, number of points, that is, at the peaks, only., , Since the entire load of the book falls on, such points only, the book presses the, table very hard at these contact points., The peaks and valleys of the surfaces, of book and table top interlock. When, the book is made to slide on the table,, these interlocked points try to hold the, book and hence oppose the motion., , , , Table Peak Valley, Irregularities, , 2. There are three types of friction:, , (i) Static (ii) Sliding (iii) Rolling, , (i) Static friction: When we apply a, force on a body to make it move, and it does not move the force, applied is balanced by the force of, friction. Thus, “the force of friction, , , , 41, , 3. (a), , 41/100, , in static (stationary) situation, is self adjusting. Such force of, friction is called static friction”, , (ii) Sliding friction: The force of, friction acting between the two, surfaces when one of them just, slides over the another with a, uniform speed is called sliding, friction., , (iii) Rolling friction: When a body is, , placed over the other bodies like, , roller or a wheel and is allowed to, move this is called rolling friction., , e.g., suitcases and travelling bags, , are provided with small wheels, , which help in easy movement of, suitcases and travelling bags., , Walking: When we walk, there is, , friction between our foot (or the, , sole of our shoe) and the ground., , This prevents us from slipping., , (6) Writing: When we write there is, , friction between our pen and the, paper which prevents the pen, from slipping on the paper., Also if there is no friction, we will, not be able to hold a pen vertically., It is friction that opposes slipping, and helps us to hold the pen., , (c) Chewing the food : While chewing, food saliva helps to soften our, food. Food is rolled into a ball, by chewing and with the help, of tongue. Saliva then acts as a, lubricant, helping us to swallow, this ball of food., , .. A matchstick lights up due to friction, , against the side of matchbox when we, strike it. This can destroy houses when, catches fire. A tyre of wheel has rough, surface to have more of a friction.