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 and the other surfaces (dynamic friction force). Friction is also said to be the force that opposes the onset of sliding (static friction force). This force originates thanks to imperfections, especially microscopic ones, that appear between the surfaces in contact. Said imperfections cause the perpendicular force R between the two surfaces to not be perfectly so, but instead to form an angle with the normal N (the angle of friction). This word is derived from the Latin "frictio".
When it is said that there is friction it is because two surfaces that cause a force are rubbing, for this reason two types of friction are possible, one is static and the other is dynamic. The first is a resistance that needs to be transcended to mobilize one thing against another with which it has contact, in the world there are infinities of objects that make use of friction, an example of this are some toys such as friction carsthat manage to work, thanks to that force that can overcome static friction, by pushing the toy in a backward direction. For its part, dynamic friction, which is less than static, appears at the moment when a body is already in motion. When the surfaces come into contact, if they are not totally smooth, presenting generally small imperfections, a force is produced which opposes the movement at an angle and which is opposite in its direction to the movement. It is a resistance to that movement that will be successful if the applied force is necessary.
Although not all the differences between the two types of friction are known exactly, the general idea is that the static is slightly greater than the dynamic; Since the surfaces on which the friction will occur are at rest, it is very likely that ionic bonds or micro-welds will be generated that join them, which does not take place once it is in motion.
