The normal force is that exerted by a surface on the body that is at rest on it.
The concept of force has multiple uses. We can talk about physical force to refer to the magnitude that has the capacity to influence the movement and shape of bodies (elements with mass).
According to their characteristics, it is possible to distinguish between various types of force: gravity force , centripetal force , tension force , friction force , maximum force and magnetic force , among others.
What is normal force
This time we are going to focus on the notion of normal force . This is the name given to the force exerted by a surface on the body that is resting on it . The normal force has the opposite direction, but the same direction and magnitude, as the force exerted by the body itself on the surface in question.
Let's take the case of a 50-kilogram box that is placed on a chair. The weight of said box is equal to its mass (expressed in kilograms) multiplied by gravity. The result of this operation, but in the opposite direction , will be the normal force exerted by the chair on the box.
A normal force is a contact force.
Main features
It is important to keep in mind that the normal force is always a contact force. When the two bodies are not in contact (that is, when their surfaces do not touch), there is no normal force that affects another. In the case of the previous example, if someone holds the box a few centimeters above the chair, the normal force mentioned will not be applied.
The normal force is perpendicular to the surfaces that are in contact and prevents one body from passing through the other. Contact between two bodies also involves other forces, but parallel to the surfaces: friction forces .
This concept leads us to Newton's laws , a set of three principles defined by the English physicist Isaac Newton that serve to explain many of the problems posed by classical mechanics , especially those that include the movement of bodies. More specifically, we must refer to his third law, that of "action and reaction", according to which every action corresponds to an equal or opposite reaction. In this case, we can say that the normal force exerted by the surface has the same magnitude as the external force, although its direction is opposite.
Modulus of normal force
In the field of physics, the number used to represent the length of a vector when it is represented graphically is called a module . To obtain it, we must do nothing more than place the values of the vector corresponding to each of the Cartesian axes in the famous equation of the Pythagorean theorem normally used to calculate the hypotenuse of a right triangle: the hypotenuse is equal to the square root of the sum of the squares on the different sides . Another name for this concept is magnitude .
Generally, the modulus of the normal force coincides with the projection of the new force, that is, the force that results from its application on the body, on the normal of the surface. It is important to note that the projection of a force is understood as its transformation into others that are on the axes and that when added together result in the original. In the case of the weight of the aforementioned box, if we consider that the surface of the chair is a plane whose inclination forms an angle α, we can say that the calculation of the normal force is carried out as follows:
F = mgcos(α)
Here we have the variables m , which represents the mass, and g , gravity, both multiplied by each other and by the cosine of the angle that can be seen between the surface of the chair and the imaginary horizontal axis .