Friction is a force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force.
Friction is due to irregularities on the surfaces of the objects in contact. Friction depends on the smoothness of the surfaces in contact. The force of friction depends on the nature of the surfaces in contact. The force of friction increases if the surfaces are pressed harder.
Fig: Simulated blocks with fractal rough surfaces,
exhibiting static frictional interactions
(a) A moving wheel on the ground stops after covering a certain distance because surface of ground offers opposite direction friction to the surface of wheel in motion.
(b) After applying breaks to a moving bike, the surface of brakes offers friction to the surface of moving wheel and the bicycle stops because of force of friction.(c) Gently push a book on a table. It stops after moving for some distance because of friction between the surfaces of book and table.
Reduction of Friction:
(i) In Some situations, friction is undesirable. We need to reduce it. Friction between the sliding surfaces of two objects can be reduced by making the surfaces in contact smooth by polishing the surfaces.
(ii) Sliding friction between the moving parts of vehicles can be reduced by using oil, grease or graphite.
(iii) In electrical plugs, graphite is used to reduce friction between the plug pins and the corresponding socket. In certain machines, like the drill used by a dentist, a layer of air is used as a lubricant to reduce friction between the moving parts.
Fig: Minimizing Friction
The interlocking of irregularities on the surfaces of two bodies in contact, which cause friction is overcome to great extent by the use of lubricants, the material used to reduce friction, and friction is reduced.
Friction and its Types
(1) Static Friction
Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is static friction between your shoes and the trail each time you put down your foot. Without this static friction, your feet would slip out from under you, making it difficult to walk. In fact, that’s exactly what happens if you try to walk on ice. That’s because ice is very slippery and offers very little friction.
Fig: Static Friction
(2) Sliding Friction
Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. That’s why it’s easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful.
Fig: Sliding Friction
(3) Rolling Friction
Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including bicycles, cars, 4-wheeler, roller skates, scooters, and skateboards.
Fig: Rolling Friction
(4) Fluid friction
Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If you’ve ever tried to push your open hand through the water in a tub or pool, then you’ve experienced fluid friction.
Objects which are made to move in fluid are made streamlined shape to reduce friction offered by fluid. The shape of aeroplane, ship, boat, birds and fish are made streamlined shape. A shape which is pointed on both ends and wide at the middle is called which facilitates easier movement through fluids. The frictional force offered by fluids is called drag.
Friction: A Necessary Evil:
Friction is a necessary evil because it has both harmful and beneficial effects.
(i) We can walk easily because ground offers friction. Without this force, you would not be able to walk at all. With your first step out of the bed, you’d slip and fall. Therefore, friction is necessary even for a simple task like walking.
(ii) 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.
(iii) 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.
(iv) Friction can also produce heat. Vigorously rub your palms together for a few minutes.
(v) Writing with pen is possible because paper provides friction to the tip of pen.
(vi)When a machine is operated, heat generated causes much wastage of energy.