Frictional force

Friction force is a force that opposes the motion of bodies. It can be static, if the body is at rest, or dynamic, for bodies in motion.

The simple act of walking is only possible because of the frictional force
When we want an object to move, we apply a force on it (pulling or pushing), however, this object does not always move. This occurs because a force contrary to this movement begins to act on it, the friction force, which can be defined as:

“ The force of friction is a force that opposes the motion of bodies.”

The frictional force is opposite to the force F and the motion of the body

It can be classified in two ways:

  • Kinetic (or dynamic) friction force : is a force that arises in opposition to the motion of objects that are moving;
  • Static friction force : acts on the object at rest and makes it difficult or impossible for it to start moving.

The magnitude of the static or kinetic friction force mainly depends on two factors:

  1. from the magnitude of the normal force (N) to the surfaces in contact;

Normal Force is the force exerted by the surface on the body. It is perpendicular to the surface and has a direction opposite to the weight.

  1. of the materials that constitute these surfaces and that define the coefficient of friction ( μ) between them.

Knowing the factors that determine the friction force, we can define the expressions used to calculate it. The kinetic friction force is calculated with the formula:

at = μ c . No

Where μ c is the coefficient of kinetic friction between the two surfaces.

The static friction force is calculated by the following formula:

at = μ e . No

Where μ e is the coefficient of static friction between the surfaces.

The unit of measurement of friction force in the International System is the Newton. The coefficients of static and kinetic friction are dimensionless, that is, they are pure numbers that do not have a unit of measure.

Note in the following table some coefficients of kinetic and static friction between the surfaces of some materials.

Material 1 material 2 μ and μc _
Aluminum Carbon steel 0.61 0.47
Eraser Asphalt 0.4
Copper Cast iron 1.1 0.29
Graphite Graphite 0.1
Glass Glass 0.9 0.4
Human limb joints Human limb joints 0.1 0.1

Note in the data in the table that the coefficient of static friction is always greater than the kinetic. This means that the force of static friction will always be greater than the force of dynamic friction, and it will always be more difficult to start a body in motion than to keep it in motion.

The friction force is due to small roughnesses that exist between the surfaces and that are macroscopically imperceptible. Look at the figure:

The friction force is due to the roughness between the two surfaces in contact

These observed irregularities are responsible for the friction force, thus hindering the movement of objects.

Friction is often seen by us as something negative. As negative examples, we can mention the wear caused by friction on machine parts and on the soles of shoes; the greater consumption of fuel so that the automobiles overcome the friction force between the parts; the waste of energy due to the friction between the electrical charges in the conductors, among others.

However, if there were no friction, it would be impossible to perform simple everyday tasks, such as walking or putting a car in motion. But do you know why this happens?

  • When we walk, we push the ground backwards with our feet, and the ground, in turn, exerts a frictional force on the person, pushing him forward. If there were no friction, when trying to walk, we would be sliding on the floor without moving. This is what happens, for example, when we try to walk on the floor with soap or very well waxed.
  • The car engine sets the wheels in rotation, which in turn push the asphalt backwards, and the frictional force between the tire and the asphalt propels the car forward, producing the motion. If the friction force did not exist, the wheels would turn, but the car would not move.

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