Imagine a rock rolling down a mountainside. It continues rolling down, till it meets a boulder which is quite large and is lying motionless on the path of the rock. The rock collides with the boulder and comes to rest. Its motion stops. There is a loud noise during the collision. What stops the rolling rock? This phenomenon, which is responsible for stopping objects in motion, is known as friction. It generally acts in the opposite direction with respect to the direction of motion.  Friction – Friction is the force, which resists the relative motion of surfaces or bodies sliding against each other. The surfaces can be solid, or between layers of fluids, or material/elements.

Friction is said to be a necessary evil. Without friction, motion once started would not cease at all and would continue perpetually.

The Law of Conservation of Energy is however not violated due to friction.

There are different types of friction. They are:

• Dry friction is a force which opposes the relative lateral motion of two solid surfaces which are in contact. Dry friction can be subdivided into static friction (“stiction”) between immobile surfaces, and kinetic friction which operates between moving surfaces. With the exception of atomic or molecular friction, dry friction generally occurs due to the interaction of surface features, which are termed as asperities.
• Fluid friction is the friction between the various layers of a viscous fluid that are moving relative to each other.
• Lubricated friction is that type of fluid friction where a lubricant fluid separates two solid surfaces.
• Skin friction is the force resisting the motion of a fluid across the surface of a body, constituting a part of drag.
• Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.

The study of friction forms an important aspect in the study of mechanics. The ancient Greeks were interested in studying friction. The laws of friction were elucidated by Leonardo da Vinci, but these were formally rediscovered and published by Guillaume Amontons and the laws of dry friction became known as Amonton’s three laws of dry friction. Other scientists who have taken the understanding of the role of friction forward are Coulomb, van Musschenbroek, Leslie, Morin, Jenkin and Ewing, Tabor, etc.

Friction manifests as heat energy generally and sometimes as noise. When objects are moving against each other, there is a generation of kinetic energy. Friction causes this kinetic energy to change into thermal energy. Thus, work is converted into heat. It is a non-conservative force. Mechanical energy is not conserved and some part of the mechanical energy is dissipated as heat.

The biggest disadvantage of friction is wear and tear that happens as a result of friction. Wear is the gradual removal of material at solid surfaces occurring as a result of damage caused by friction and/or corrosion. Wear leads to loss of functionality. Tribology is the study of wear of materials.

Any machine with moving parts is subject to the deleterious effects of friction and wear. Wear has an economic impact as it necessitates regular servicing and replacement of parts or whole of machines. Thus, accelerated friction harms industries and there is always a search for lessening friction which would lead to better usage. This leads us to the concept of efficiency. Friction leads to a decreased efficiency of machines, and no machine is 100% efficient because of the presence of friction. In a frictionless machine, input would be equal to output. This is never the case.

Friction causes an increase in fuel consumption of vehicles. Thus, a vehicle with better fuel consumption will have less friction. Vehicles with more wear and tear or less efficiency will have more friction and will need more fuel to operate. This again has an economic impact.

Friction also causes overheating of engines. This accelerates breakdown and decreases efficiency. Increased efficiency demands need increased fuel or energy consumption due to friction.

Heat generated due to friction can cause expansion of materials as well. This would lead to mechanical instabilities which impact the functioning of machines even more than normal wear and tear as this has a structural impact as well.

## Methods to decrease the disadvantages of friction

There are various methods used to decrease the disadvantages caused by friction. Some of these are:

• Using Lubricants: A liquid with low viscosity, for example, oil, or grease, placed between two surfaces is used to reduce friction between two surfaces. When sound is used as a lubricant, it is known as acoustic lubrication.
• Devices to reduce friction: Wheels, roller bearings, ball bearings, air cushions etc are devices which can reduce friction between two surfaces in contact with each other.
• Using thermoplastic materials between surfaces can also reduce the deleterious effects of friction because they help in dissipating the heat energy generated due to friction.

To know more about friction, and its various other impacts on our daily lives, visit here.

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