# Frictional force

So far, to calculate the force, or acceleration of a body, we consider that the surfaces on which it moved did not exert any force against the movement, that is, when applied a force, it would move without stopping.

But we know that this is an idealized case. As smooth as a surface is, it will never be completely free of friction.

Whenever we apply a force to a body on a surface, it will stop.

This is what characterizes the frictional force:

• Opposes the movement;
• It depends on the nature and surface roughness (coefficient of friction);
• It is proportional to the normal force of each body;
• It transforms the body's kinetic energy into another type of energy that is released in half.

Frictional force is calculated by the following ratio: Where:

μ: coefficient of friction (dimensionless)

N: Normal force (N)

## Static and Dynamic Friction

When pushing a car, it is easy to see that until the car is in motion it is necessary to apply a force greater than the force required when the car is already moving.

This is because there are two types of friction: static and dynamic.

### Static Friction

It is the one that acts when there is no slip of the bodies.

The maximum static frictional force equals the minimum force required to initiate movement of a body.

When a body is not in motion the friction force must be greater than the applied force, in this case a static friction coefficient is used in the calculation: .

So: ### Dynamic Friction

It is the one that acts when the bodies slip.

When the static frictional force is exceeded by the force applied to the body, it will move, and we will consider its dynamic frictional force.

The dynamic frictional force is always smaller than the applied force, in its calculation the kinetic friction coefficient is used: So:  