Friction as a Contact Force | Tutorvista.com
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Friction as a Contact Force
In real life an object which is moving over a horizontal surface does not continue with constant velocity when the accelerating force is removed. Instead the object slows down and eventually stops. The deceleration is caused by a frictional force between the object and the surface on which it travels. A frictional force always acts to oppose the motion of an object over a surface and is an example of a contact force.
When we try to slide one object over another, we find that there is a force which opposes the motion. This force is called friction.
Whenever there is relative motion of two surfaces in contact, there is frictional resistance. When a body moves over another, it has to overcome friction. This needs energy. Hence, some energy is always wasted in overcoming friction. When two parts of a machinery rub against each other heat is produced, and the efficiency of the machine decreases. There will be a greater wear and tear in the different parts of a machine due to friction.
If the surfaces that are in contact are rough, there is more friction. Hence to reduce friction, the surfaces in contact are kept smooth and polished. It is worth noting that if the two surfaces in contact are of the same metal, the friction increases on polishing the two surfaces. This is why the bearings and the axle are made of different metals.
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From what has been discussed above, it may appear that if there were no friction, our life would have been much more easier and we would not have to spend energy in overcoming the frictional resistance. This is not true. As a matter of fact, it is only because of friction that we are able to move around. Try to run on a very smooth surface and see what happens. As we step forward, friction opposes the forward force, so that the foot does not slip forward.
When we raise our heel again to take another forward pace, the friction prevents the front part of the shoe moving back. On account of the friction between the ground and the rubber type of a bicycle, the latter can move forward. In the absence of friction between the wheels and ground, the wheels are unable to roll. This is why the wheels of bicycle and other vehicles are treaded to increase friction. To stop the bicycle, we use the opposing frictional force between the brake blocks and the wheel rim. When the driver applies the brakes of an automobile, the friction of the brake lining on the brake drum slows down the wheels and enable him to bring the car to a stop. Friction holds a screw in wood. It also enables us to use nails to fix boards together. If friction were eliminated, dishes would slide off the table unless the table were perfectly level. Thus without friction, our life would have been, in reality, very inconvenient.
Types of Friction
When steel slides over an alloy of lead, the friction is less than when steel slides on steel. Bearings are sometimes packed with such alloy.
Air cushion
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By separating the surfaces by an air cushion e.g., a hovercraft, can travel over rough land, swamp or sea. Here, as the moving object does not come in contact with the other solid surface, the frictional forces are reduced to a very great extent.
Hovercraft Principle
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When an object moves through a fluid, there will be friction due to the resistance of the material medium. This resistance increases with the velocity of the body as in the case of a rocket and aeroplane. Hence to minimize this friction due to the resistance of the medium, the bodies of aeroplanes, missiles etc. are streamlined. In nature also, we find the same adaptation of shape to suit the medium, specially in aquatic animals. Compared to air, resistance to movement in water is much more. To overcome this resistance, fish etc. have a shape that offers least resistance. Shapes of the birds also manifest this adaptation.
By polishing