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MIND MAP, , Advantages, , , , , Prevents from slipping, Stops a moving vehicle, Enables us to walk, , Disadvantages, Definition, Type of force that opposes the, motion of an object, , , , , , Wear and tear of objects, Heat is generated, wastage of energy, , FRICTION, , Ways to increase, friction, , , , , increasing the roughness of, surface in contact, changing the shape of object, increasing the weight of object, , Types, , , , , Static friction, Sliding friction, Rolling friction, , Static friction > sliding friction, >rolling friction, , Ways to reduce, friction, , , , , Smoothening the, surface in contact, Applying oil, grease, or wax, Using ball bearings,, wheels, air cushions

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EXPLANATION, Friction: When one surface moves over another surface, a force comes into play and opposes, their relative motion. This force is called friction or force of friction., , , , , Friction always opposes motion because it acts in the opposite direction to motion., Force of friction arises due to contact between the two surfaces. If there is no contact, between the two surfaces, there would be no friction., All types of surface like solid, liquid and air offer the force of friction., , Example:, (a) A moving wheel on the ground stops after covering a certain distance because surface, of ground offers friction in the opposite direction of motion., (b) Gently push a ball on a table. It stops after moving for some distance because of, friction between the surfaces of ball and table., , Factors Affecting Friction:, Nature of surfaces in contact : Friction is created because of irregularities on the surfaces. The, irregularities on surfaces result in interlocking of the two surfaces. The interlocking between two, surfaces creates friction. Hence, the force of friction will be greater in case of rough surface than, in case of smooth surface., , Surface irregularities

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Pressure Between two Surfaces: The force of friction increases with increase in pressure, between the two surfaces. Moving a heavy box is difficult as compared to moving a light box., Pressure is higher in case of a heavy box. Irregularities between surfaces get interlocked tightly, because of pressure, and the surface offers more friction., , Static Friction: The force required to overcome friction at the instant an object starts moving, from rest is a measure of static friction., Sliding Friction: The force required to keep the object moving with the same speed is called, sliding friction. Sliding friction is always lesser than the static friction. Due to this, it is easier to, push a moving box compared to pushing a box at rest., Rolling Friction: When an object rolls over the surface of another object, the resistance to its, motion is called rolling friction. Rolling friction is lesser than sliding friction., , Friction: A Necessary Evil, Friction is a necessary evil because it has both beneficial and harmful effects., , Advantages of Friction, , , , , , , It is not possible to walk on the road in the absence of friction. Walking on ice is almost, impossible because of lack of friction., Friction is necessary to keep a ladder against a wall. The ladder will not find grip against, the wall in the absence of friction., Soles of shoes have grooves on them. Grooved soles give more friction to the ground, which gives better grip when we walk. Shoes with worn out soles can be slippery., The tyres of vehicles have treads for better grip over road. It provides friction to the, surface of road. When treads are worn out, the tyres need to be replaced with new one., Writing with pen is possible because paper provides friction to the tip of pen.

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Disadvantages of Friction, , , , It results in wear and tear of shoes and tyres, machine parts, clothes, etc., When a machine is operated, heat is generated by friction, which results in wastage of, energy. Moreover, a significant amount of energy is wasted in overcoming friction., , Increase of friction:, Friction is desirable in some situations. Therefore, friction is increased to accomplish some of the, important tasks., Ways to Increase Friction:, , , , , , , , , Treaded tyres of vehicle increase friction. Vehicle with treaded tyres provide more, friction which gives better grip with the road., Soles of shoes are treaded to increase friction between sole and the ground. This helps in, providing better grip on surface while walking., Javelin thrower rubs some powder on his palm to get better grip on the javelin., Kabaddi players rub their hands with soil for a better grip of their opponents., Rubber grips are put on bicycle handles, electrical tools and on many other tools., Increased friction helps in giving better grip on these objects., A surface can be made rough with the help of sand paper in order to increase friction., Holding a bottle cap with a thick towel helps in increasing friction. Increased friction, makes it easier to open the bottle cap., , Reduction of friction:, In Some situations, friction is undesirable. We need to reduce it.

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Ways to Reduce Friction, , , , , , , , In the engines of vehicles, use of lubricants increase the efficiency as parts are not, rubbed directly with each other because of thin layer of lubricant., In many machines, graphite powder, oil, grease are used as lubricant for reducing, friction., Few drops of oil are poured on the hinges of a door for reducing friction, the door moves, smoothly., While playing carrom, sprinkle fine powder over it to reduce friction., Some machines use air cushion for reducing friction because use of oil is not ideal in, such machines., Ball bearing converts sliding friction into rolling friction. Rolling friction is less than, sliding friction. Thus, ball bearing helps in reducing friction., , Fluid Friction:, Gases and liquids are generally known as fluids. Similar to solids, fluids also provide friction.,  The frictional force offered by fluids is called drag.,  The frictional force on an object in a fluid depends on:,  Speed of object with respect to the fluid,  Shape of object,  Nature of fluid,  Objects which are made to move in fluids are made streamlined shape to reduce friction, offered by fluid.,  Birds and fishes are naturally endowed with streamlined body. Boats and airplanes are, also made streamlined so that they can easily move through air or water.,  A shape which is pointed on both ends and wide at the middle is called streamlined shape, which facilitates easier movement through fluids.

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Note: Do the following work in Science Notebook, QUESTION-ANSWERS:, Q1. 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., A1. When a book slides on the writing desk, a frictional force acts between the book and the, surface of the desk. The direction of frictional force on the book is opposite to the, direction of its motion and acts in upward direction, as shown in the following figure:, , Q2. 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?, A2. We are able to walk because of the friction present between our feet and the ground., In order to walk, we push the ground in the backward direction with our feet. The force, of friction pushes it in the forward direction and allows us to walk. The force of friction, between the ground and feet decreases when there is soapy water spilled on the floor., Hence, it becomes difficult to walk on the soapy floor., Q3. Explain why sportsmen use shoes with spikes., A3. Sportsmen use shoes with spikes because these shoes give them a better grip while, running. This is because, the force of friction between the shoes and the ground, increases with the help of spikes., Q4. 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?, A4. Force of friction arises because of interlocking of irregularities on the two surfaces in, contact. When a heavy object is placed on the floor, the interlocking of irregularities on the, surfaces of box and floor become strong. This is because, the two surfaces in contact are, pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push, the heavier box, Seema has to apply a greater force than Iqbal.

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Q5. Explain why sliding friction is less than static friction., A5. Friction comes into play when irregularities present in the surfaces of two objects in contact, get interlocked with each other. In sliding, the time given for interlocking is very small., Hence, interlocking is not strong. Therefore, less force is required to overcome this, interlocking. Because of this reason, sliding friction is less than static friction., Q6. Explain why objects moving in fluids must have special shapes., A6. When a body moves through a fluid, it experiences an opposing force which tries to oppose, its motion through the fluid. This opposing force is known as the drag. This frictional, force depends on the shape of the body. By giving objects a special shape i.e., streamlined, shape, the force of friction acting on them can be minimized. Hence, it becomes easier for, the objects to move through the fluid.