The friction is the force that opposes the sliding movement is produced by contact between two surfaces. There are 4 main types of friction: static friction, sliding friction, rolling friction, and fluid friction. The table lamp lying on a table and A box of much weight against the ground are some examples of static friction.
The formula, then, is F = Ц * N, where f is the friction or friction force, Ц is the coefficient of friction and N is the normal force. This, however, is restricted to the case of static friction, where the phenomenon occurs below the movement threshold. When that threshold is exceeded, the object begins to slide on the surface, and then the friction becomes less, changing to be called dynamic friction. In everyday life, the concept of friction is directly related to rubbing, scrubbing or rubbing something: the friction force is what makes the opposition, and is what allows people or automated bodies that the human being created to stop a once launched.
The friction force is present to the extent that when it comes to moving a heavy object horizontally, at first the force that must be exerted to remove it from rest is much greater than that which must be done once it is started, where it is much easier to get it moving. This happens because once the static friction has expired, the microscopic joints that kept the surfaces in contact welded break.
Finally, it is common for industries concentrated in the production of certain goods to manipulate the friction force, which tends to optimize it by increasing or reducing it depending on the case: there are times when it is necessary for the surface in contact with the ground to be rough, so as to increase this force. Others, on the other hand, are supported on another surface that sees the reduction of friction necessary, for which tools such as grease or oil are usually used, such as lubricants to reduce friction between components and the energy losses they carry.
When the two surfaces are at rest, the force that opposes the start of the movement is called static friction. Since it prevents movement, it can be said that it is equal to the net force applied to the body, only in the opposite direction.
The static friction is always less than or equal to the friction coefficient. However, when friction experiments are carried out on carefully cleaned, smooth metal blocks, the difference between static and kinetic coefficients tends to disappear.
Here are some examples of static friction :
- A heavy box against the ground, difficult to lift and move.
- A nightstand resting on a light table.
- A dry and a wet plastic, where the second has less friction than the first.
- Friction toys that mimic force behavior in vehicles, but statically.
- The rest of the body when a person leans against the wall.
The dynamic 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 already been overcome it 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 friction and normal. The number that indicates the level of dynamic friction is the coefficient that is referred to when talking about the generality of the coefficient of friction, as it is the most reliable number.
The following are examples of dynamic friction :
- Feet against the ground, when walking.
- The wheels of a bicycle against the ground.
- The friction between an airplane and the air.
- Underwater vehicles, with the friction it exerts on water.
- Skates on an ice or concrete rink.