The friction It is a force that exists between two surfaces in contact and that opposes movement. When an object moves on a surface, it must overcome a force that opposes this movement. If the object is pushed for example and then left free, this force, also called friction, will be responsible for its speed to decrease or even to slow down.
Physicist Arthur-Jules Morin studied this concept, and coined the coefficient of friction. This coefficient is characteristic of each pair of materials in contact and is defined as the ratio between the friction force and the force normal to the displacement surface and is therefore dimensionless.
Although some bodies may appear smooth, if viewed microscopically they have imperfections or roughness at the contact surfaces. It is these imperfections that are responsible for the friction.
Types of friction
They are often differentiated two kinds of friction force, according to the moment of movement in which it occurs:
- Static friction. When the two surfaces are at rest, the force that opposes the start of motion 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.
- Dynamic friction. It is what 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 it is much easier (dynamic friction). The dynamic coefficient of friction is lower than the static one.
Examples of static friction
- A very heavy box resting on the ground is difficult to start to move due to static friction.
- It is easier to start sliding a wet plastic than a dry one since the second has more friction than the first.
- Friction toys that imitate the behavior of force in the case of vehicles, but in a static way.
- The feet against the ground, when walking. Static friction makes the foot not move with respect to the ground, when it moves, it is when the friction is dynamic or does not exist and we skid.
- The wheels of a bicycle against the ground. All wheels work thanks to static friction. Each point of the wheel is stationary with respect to the ground. It does not slip, allowing the wheel to turn and the bike to advance.
Examples of dynamic friction
- When we push an object on a table, it slides but eventually slows down due to the action of dynamic friction.
- When we push any object in order to move it, we slide it on the floor. We have to use more force than we would if the floor were made of ice, because there is friction that opposes the movement of the object.