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.

How is friction helpful?

 

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

 

You must have noticed that when you have to slide a heavy box, it is easy to slide it once the box has started sliding. You may also have noticed that it is much more easy to roll a heavy drum than to slide it. This is because there are three types of friction. They are:

• the Static Friction

• the Kinetic or Dynamic Friction, and

• the Rolling Friction.

The friction that exists between the two surfaces in contact when there is no relative motion between them is called static friction.
The force which makes the surface just start to slide over another, is called limiting friction. The friction that exists between a surface sliding on another surface is called kinetic or dynamic friction.
When a body rolls on the surface of another, the friction that exists between the surfaces is known as rolling friction. In the example described above, when the box just starts moving, the friction is the static friction. Once it has started moving, the friction that exists is the sliding friction. In the case when the drum is rolling on the surface, the friction is the rolling friction. Experimentally, it has been proved that
Static friction > Kinetic or Dynamic friction > Rolling friction

Laws of Friction

 

• Friction always acts in such a direction that it opposes the motion. Force of friction is directly proportional to normal reaction.

• Until motion takes place, the static friction adjusts itself so as to be equal to the force tending to produce motion.

• In any given case, there is a limiting value to the frictional force.

• Limiting friction is independent of the area of contact.

• Limiting friction is directly proportional to the normal reaction.

Ways to Reduce Friction

 

Lubricants

  Use of lubricants makes the surface smooth. In machines, friction is reduced by using lubricants such as oil, grease or graphite powder. The lubricant fills the minute unevenness of the two surfaces and separates them by forming a very thin layer in between. This layer offers very little resistance and as a result, the friction gets reduced.

Ball bearings or roller bearing

 

They are made in various designs to reduce friction. They reduce friction because rolling friction is smaller than sliding friction. These bearings are also lubricated to further improve their function.

Ball-bearing and rolling friction

Anti-friction metals

  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

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

Streamline flow

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

Polishing the surfaces reduces the ruggedness between them. In an automobile when the piston moves in the cylinder the friction is reduced by polishing the surfaces, which are in contact. The jewel bearing and polished agate edges reduce the friction to a minimum. This is because by polishing the unevenness is reduced to minimum. In a chemical balance the beam rests on an agate knife-edge, where the friction is reduced. Roads are made smooth by concreting and cementing the surface.
The speed of a body moving through the fluid also depends upon the size and shape of the body [Figure (a) shown below] shows the streamlines of the fluid through which a body in the shape of a sphere is moving. Such smooth flow is known as 'Laminar flow' and occurs at relatively low speeds. As the speed increases, the resistance and friction increases. When such a stage is reached, 'turbulence' sets in and the force of friction increases to a great extent. The turbulent flow is accompanied with eddy currents which decreases the speed of the body. As a result, a lot of energy is wasted in overcoming this resistance [Figure (b) shown below]. The shapes of the bodies are designed in such a way that they offer minimum resistance while they move in air or water [Figure (c) shown below]. This reduces the tendency to form eddy currents and the body moves through the fluid with minimum resistance. Such shapes are called streamline shapes and the resultant is called streamline flow.