Friction is the force that exists between two surfaces in contact and that opposes movement. When an object moves on a surface, it must overcome a force that opposes this displacement. If the object is pushed, for example, and then released, this force, also called friction, will be responsible for its speed decreasing or even stopping.
The physicist Arthur-Jules Morin studied this concept, and coined the coefficient of friction. Although some bodies may appear smooth, when viewed microscopically they have imperfections or roughness on the contact surfaces. It is these imperfections that are responsible for friction.
Friction Formula
The formula of friction is
F = μN
where f = friction force
μ = coefficient of friction
N = normal force
Causes of Friction
Friction is caused by the interaction of microscopic bumps on the surface of objects as they rub together. While the smoothest surfaces are made up of particles that stick above the rest. These high points touch and rub each other as the objects move casing resistance. The main causes of friction is the resistance that moves it.
Types of Friction
Friction can be divided into main three types: sliding friction, fluid friction, and rolling friction. 
Sliding Friction
When two objects slide each other, then they produce sliding friction. Sliding friction is caused because the two objects are in contact with each other and moving in opposite directions. For Example, the book on the table. The friction between the surfaces of the two objects causes resistance and will eventually cause them to slow down and stop.
Fluid Friction
This friction occurs within layers of fluid. i.e. air, water and oil. It is caused by the internal layers of the fluid moving against each other. For Example, an object falling through the sky, like a ball. It produces air resistance which is consider as fluid. As the ball is traveling through the air, the air is applying frictional force against the ball in the opposite direction.
Rolling Friction
Rolling friction is caused by an object, such as a wheel or ball, rolling across a surface. It is considered as weakest type of friction. When a car is driving on the road, the tires are experiencing rolling friction. As each tire rolls across the surface of the road, the upright particles make contact and cause the car to slow. If the force that is being applied to the car stops, so will the car itself.
Examples Of Friction
Some examples of friction in our everyday life are listed below:
- All kinds of vehicles, such as submarines, ships, cars, all those that move on road or water produce friction.
- The wheels of a car moving on the pavement, intentionally high to give the driver more control over the vehicle.
- The friction between an airplane and the air, when it is flying. This friction is dependent on the aerodynamic design. This type of friction is considered as fluid friction.
- Slide a body on wet plastic, which gives it more travel than on dry plastic, precisely because water decreases the force of friction between surfaces.
- A match when it collides with the box, generating the combustion necessary to ignite.
- The friction between two objects such as stones or wood, which generate heat and eventually fire.
- An ice skating rink, where friction is reduced and therefore skates are used so that the displacement is greater.
- The sole of the shoes, with special engravings to increase friction.
- The rub between the two hands, or any part of the body.
