We explain what friction or friction force is and what its causes are. Also, static friction and kinetic or dynamic friction.
What is friction?
Friction, friction force or friction force is a force existing between two surfaces that are in contact, and that opposes the movement, that is, it has the opposite direction to the movement. This force can be of two types: static (when it opposes the initiation of a slide) or dynamic (when it opposes relative movement).
The friction force is not one of the fundamental forces in the universe, as is the gravityRather, it is due to the complex interaction between the surface of two objects in physical contact. Generally speaking of friction for objects solid, but there is also friction in the liquids: the effect of friction between the layers of a liquid substance defines its viscosity.
To set an object in motion at rest, the force that pushes it to move must precisely overcome the endurance exerted by friction, which is greater between rough and uneven surfaces than on smooth and polished surfaces.
On the other hand, if an object that is in contact with a surface, for example, a table, is pushed and as a result begins to move in the direction of the push, its speed will decrease as the friction force that the table exerts on him, beat that of the initial push. This friction force depends on the mass of the bodies, so that heavier objects show more friction than light ones.
Friction also has an effect on the surfaces in contact. It is often imperceptible, however the Kinetic energy that is lost through friction becomes heat, that is to say the surfaces are heated by friction. Both can even suffer wear and tear.
Causes of friction
Air also causes friction on objects moving at high speedsFriction may be due, in the first place, to micro imperfections between the surfaces in contact, which make it difficult for one to slide over the other, even if they cannot appear to be perceived. This is why some surfaces have more friction than others.
Thus, friction can be mechanically reduced, by adding lubricants, for example, or it can be increased, degenerating the surfaces in some way.
Static friction
Static friction (Fe) is the force that tends to oppose the relative displacement between two surfaces in contact. It is about the force that needs to be overcome to initiate the movement of an object. It is always less than or equal to the static friction coefficient (which has an empirical value and depends on the material) between the two surfaces multiplied by the normal force.
This static friction is usually greater than the dynamic, which explains why it is more difficult to start pushing a heavy piece of furniture over rough ground, than to keep pushing it once it is already in motion.
Kinetic or dynamic friction
Dynamic friction (Fd) 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 necessary to keep the motion going does not change as long as the acceleration is constant. Therefore, it is equal to the coefficient of dynamic friction, denoted by the Greek letterμ, multiplied by the normal force.
The differences between static and dynamic friction are not fully understood on a physical level, but the static force is believed to be greater due to electrical attractions and micro-welds between surfaces at rest.
Examples of friction
The examples of friction are innumerable. Friction is that force that opposes us to move an old piece of furniture by pushing it with our forces. It is also the force that will stop and jump a flat stone that we throw sideways on the surface of the Water.
The friction force is what the brakes use to hold a car in place when we park it on a steep hill, and it is the same as how if we slam on the brakes while driving the same car at high speeds, it will reduce the speed and displacement of the car. car beating the inertia.
Other common examples of friction have to do with the heat that is generated by friction, such as when we light a fire by rubbing two wooden sticks against each other, or when we suffer a burn from the friction of a rope in sudden movement when we were holding it in our hands.