The concept of strength has a great many denotations in different fields, being in some cases synonymous with strength at both the physical and mental levels, resilience and resistance to events.

But beyond that, we also call force one of the main magnitudes of physics, studied from basic physics to the most complex branches of science, and which participates in a great number of phenomena, actions and reactions.

So, at the physics level we can talk about different types of force , about which we will make a brief mention in this article.

## What do we call strength?

Before starting to talk about the various typologies or categories that have been established when analysing different types of force, it is necessary to establish a brief definition of the concept.

In a generic way we can define the force as a physical magnitude of vectorial type , to which it is associated and considered the cause of the capacity to generate a displacement or movement with acceleration by a body or object, a modification in its structure or even its state of rest when to reach it a resistance to another force must be exerted. In order to be correctly defined, it should be noted that every force has a specific point of application, direction and intensity that will determine the final behaviour of the object.

How magnitude that is the force has a unit of measurement, the Newton (in honor of Isaac Newton, who is considered the first to establish a mathematical formula for its calculation), which refers to the amount of force needed to generate an acceleration of one meter per second squared in a body of one kilogram of mass. There are also other units of measurement, such as the dyne.

## Types of force

It is possible to classify the types of force according to different criteria. Let’s look at them.

### 1. According to specific parameters

We can find classifications based on aspects such as their permanence, the existence or not of direct contact between the bodies, or their way of acting. Examples of this are the following types of force.

#### 1.1. Fixed forces

Fixed or permanent forces are understood as all those forces inherent to the body or object itself and which derive from its structure or configuration, and from which it is not possible to escape. One of the most easily visible is the weight , product of the mass of the body and the gravitational attraction to which it is subjected.

#### 1.2. Variable forces

Also called intermittent, these are those forces that are not part of the structure of the object or body in which the movement or change occurs but rather come from other bodies or elements . An example would be the force applied by a person to a car to move it.

#### 1.3. Contact

Contact forces are understood to be all those that are characterized by the need for contact between bodies or elements in order to generate movement or structural change. These are the forces traditionally worked by classical mechanics , as we will see later.

#### 1.4. At a distance

Unlike the previous case, the forces at a distance are all those in which it is not necessary for there to be contact between the bodies in order to achieve an alteration of the structure or a displacement of the bodies. An example of this would be electromagnetism .

#### 1.5. Statics

Static are all those forces that do not vary in intensity, direction or place, remaining practically constant as long as they exist. An example would be the force of gravity.

#### 1.6. Dynamics

Dynamic forces are all those in which the general values that form part of the force vary constantly and abruptly , changing its direction, place of application or intensity.

#### 1.7. Action

This denomination is given to those forces that are applied to an object in order to move it or modify its structure, not arising from the object itself but from some external element. The fact of pushing something would imply to be applying an action force .

#### 1.8. Reaction

All those that are generated by the body itself in response to the application of an external force , from a given point of application, are called as such. In the previous case, the moved body would be exerting a reaction force towards us.

#### 1.9. Balanced

We understand as such those forces that oppose each other having the same intensity but whose directions are totally opposite , something that generates that the body in question is maintained in a concrete position. This type of force would be exemplified by any object that is still on the ground or by two people of the same force pushing each other at the same time.

#### 1.10. Unbalanced

We refer to those forces that when applied to a specific body generate its movement , as there is no balance or sufficient counter-force to prevent it.

### 2. In classical mechanics: contact forces

There are many and diverse types of force that can be found in nature, but generally when the concept of force begins to be studied physically it is usually used in the context of classical mechanics, referring to a type of force called contact. Within these we can find the following types of force.

#### 2.1. Normal

We understand as normal force that force which is exerted by the interaction between two bodies in contact , such as an object and the ground, exerting a force reactive to that of the weight which would go in the opposite direction to that of the latter.

#### 2.2. Applied

As applied force we understand that force that a body uses on another and that causes an accelerated movement or a change in the structure of the object. It is a direct contact force.

#### 2.3. Friction

The friction or friction force is that force that appears when two bodies come into contact and that acquires a direction directly opposite to the applied or normal force . For example, when an object is pushed, it offers a resistance produced to a great extent by the force of friction against the ground.

Another analogous form of this type of force, which is sometimes classified independently, is air resistance. This force explains, for example, why it can take two objects of the same mass thrown from the same height at the same time to reach the ground (air friction), or why an object pushed down a slight slope can end up slowing down.

#### 2.4. Elastic

We call elastic force to that which is produced when a surface or object is held in a position of non-equilibrium by a given force, appearing as a reaction that seeks to restore that initial position or equilibrium. That is, it is produced when a body subjected to a force that has deformed it tries to return to its original state . A typical example can be found in springs or stretched rubbers that seek to return to their original position.

#### 2.5. Voltage

We are in front of a peculiar kind of force, characterized by being able to transmit a force between different bodies and that is generated when two opposite forces pull a body in opposite directions without breaking it . It can be used to generate systems that distribute the force to be applied to generate the movement. The tensile force is the force that allows us to use, for example, pulleys to move heavy objects.

#### 2.6. Inertia

Inertia force or fictitious force is called that with which a body is moved by the resultant of the forces that have been previously applied to it even when the body or object that has generated that force has already stopped applying it directly. It is the force with which a body maintains its state of movement, in the same direction of acceleration. This is what happens, for example, when in the event of a crash or sudden deceleration of a car, the occupants’ bodies tend to project in the same direction as the vehicle.

### 3. The fundamental forces

In addition to those typical of classical mechanics and related to macroscopic bodies, we can find other great forces that refer to the relationships that particles of matter have with each other or the existence of forces at a distance, their study being mostly a product of modern physics and allowing us to explain a large part of the previous ones.

#### Gravitational force

We call gravitational force to that force of attraction existing between objects and whose intensity depends on their masses and the distance between them . The most studied gravitational force is that of the planet itself, which attracts the bodies that exist on it towards its surface, being one of the most known forces at a distance. It is also the force that makes the planets orbit around the stars. It is also important in magnitudes like weight.

#### 3.2. Electromagnetic force

Although in the past we spoke separately of magnetic and electrostatic forces, the progressive study of the properties of these forces has shown that they are in fact interrelated.

This is the force through which electric particles are attracted or repelled by other charged particles either with the opposite sign (force of attraction) or with the same sign (of repulsion). When these relationships occur in moving particles, electromagnetic fields are generated.

#### 3.3. Weak nuclear strength

Probably some of the most difficult forces to understand for those not versed in physics is the nuclear force. In the case of the weak nuclear force, we are dealing with a type of force which allows the disintegration of neutrons and radioactivity . Besides generating forces of attraction and repulsion it allows a particle to change.

#### 3.4. Strong nuclear force

Coming from particle physics, the strong nuclear force is one that allows two particles that by electric charge should repel each other to remain together, something that allows the existence of a nucleus of protons in most molecules.

#### Bibliographic references:

• Hellingman (1992). “Newton’s third law revisited. Phys. Educ. 27 (2): pp. 112 – 115.
• Hibbeler, R. C. (2010). Engineering Mechanics, 12th edition. Pearson Prentice Hall. p. 222.
• Newton, Isaac (1999). The Principia Mathematical Principles of Natural Philosophy. Berkeley: University of California Press.