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# What is friction?

We explain what friction or friction force is and what are its causes. In addition, static friction and kinetic or dynamic friction.

1. ### What is friction?

Friction, friction force or friction force is a force between two surfaces that are in contact , and that opposes the movement, that is, has a direction opposite to the movement . This force can be of two types: static (when it opposes the beginning of a slide) or dynamic (when it opposes relative movement).

The frictional force is not one of the fundamental forces of the universe, as is gravity , but it is due to the complex interaction between the surface of two objects in physical contact. There is generally talk of friction for solid objects , but there is also friction in liquids : the effect of friction between the layers of a liquid substance defines its viscosity .

To set an object at rest in motion, the force that pushes it to move must precisely overcome the resistance exerted by friction, which is greater between rough and irregular surfaces than on smooth and polished surfaces .

Likewise, an object that is in motion without a constant force on its side will see its speed diminished as the friction force overcomes the initial thrust. On the other hand, heavier objects show greater friction than light ones , in cases where gravity adds to the resistance to movement .

Friction can also cause impact on the moving surface. It is noticeable because it often generates heat, or even wear on both surfaces.

1. ### Causes of friction

The friction may be due, first of all, to micro-imperfections between the two surfaces in contact , which make it difficult to slide one over the other, even if they cannot be perceived in appearance. This is the reason that some surfaces have more friction than others.

On the other hand, friction can be reduced mechanically, by adding lubricants , for example. If you want to increase friction, the surfaces can be dried. However, it also occurs between particles due to the electromagnetic attraction between them, which is explained by one of the fundamental laws of matter .

1. ### Static friction

The first of the types is static friction (F e ): it tends to oppose the relative displacement between two surfaces in contact. It is the force necessary to overcome to start the movement of an object. It is always less than or equal to the coefficient of friction between the two surfaces multiplied by the normal force.

This static friction is usually greater than the dynamic one , which explains why it is more difficult to start pushing a heavy piece of furniture on rough ground, than to keep pushing it once it is already in motion.

1. ### Kinetic or dynamic friction

Dynamic friction (F d ) is the force that opposes the displacement of an object that is already in motion , as opposed to static friction.

It is a constant magnitude , since the amount of force needed to keep the movement going is the same (as long as it is not interrupted for some reason). Therefore, it is equal to the coefficient of dynamic friction , denoted by the Greek letter μ .

The differences between static and dynamic friction are not entirely understood at the physical level, but it is known that the prolonged action of static is more difficult to overcome, as is the case with things that have been in the same place for too long , due to attractions and microsoldaduras between the surfaces.