The concept of friction is used to indicate the force that exists between two surfaces in contact and which opposes the relative movement between one surface and another (dynamic friction force also known as kinetic friction). It is also said that friction is the force that opposes the start of the slip (static friction force). This force originates thanks to the imperfections, especially the microscopic ones, that appear between the surfaces in contact. These imperfections cause the perpendicular force R between the two surfaces is not perfect, but on the contrary form an angle with the normal N (the friction angle). This word is derived from the Latin “frictio”.
The physicist Arthur-Jules Morin determined the concept and coined the coefficient of friction which is an empirical measurement that dates the ratio between normal and friction forces.
The main reasons for the emergence of the friction force are the characteristics of the contact surfaces, as well as the action of the normal force. Although some bodies may appear smooth, seen in a microscopic way, they necessarily have imperfections or roughness in the contact surfaces, which is what, together with the action perpendicular to the weight produced in the shock, generate resistance to displacement. It is common to differentiate two types of friction force, depending on the moment of movement in which it occurs:
When the two surfaces are at rest, the force that opposes the start of movement is called static friction. As it prevents movement, it can be said that it is equal to the net force applied to the body, only in the opposite direction.
Static friction is always less than or equal to the coefficient of friction . However, when friction experiments are carried out on soft metal blocks, carefully cleaned, the difference between the static and kinetic coefficients tends to disappear.
Here are some examples of static friction :
- A box of much weight against the ground, difficult to lift and move.
- A table lamp resting on a light table.
- A dry plastic and a wet plastic, where the second has less friction than the first.
- Frictional toys that mimic the behavior of force in the case of vehicles, but in a static way.
- The rest of the body when a person leans against the wall.
The dynamic friction also known as kinetic friction is the one that exists in a body that is already in motion and has a constant magnitude. The difference with static friction can be seen in the fact that bodies at rest are very difficult to move (static friction), but when that force has been overcome is much easier (dynamic friction).
The coefficient of friction, here, is less than static and dimensionless, because it is the result of dividing two forces: kinetic and normal friction. The number that indicates the level of dynamic friction is the coefficient referred to when talking about the generality of the coefficient of friction since it is the most reliable number.
The following are examples of dynamic friction or kinetic friction:
- The feet against the ground, when walking.
- The wheels of a bicycle against the ground.
- The friction between an airplane and the air.
- Submarine vehicles, with the friction exerted on the water.
- The skates on an ice rink or concrete.