What is a force?

We explain to you what is a force for physics, its characteristics and its types according to each theory. In addition, how it is measured and various examples.

1. What is a force?

We usually talk about strength in our daily lives, without necessarily using this word as physics does . In technical terms, a force is a magnitude and an agent, capable of modifying the amount of movement or the given shape of a body or a particle. It should not be confused with the concepts of effort or energy.

Commonly, the concept of force is explained in the terms of classical mechanics established by the principles of Isaac Newton (1642-1727), known as the Laws of the Movement and published in 1687 in its Mathematical Principles .

According to classical mechanics, the force that affects a body is responsible for changes in its state of motion, such as modifying its rectilinear trajectory and its uniform displacement, printing an acceleration (or deceleration). In addition, every force acting on a body generates an identical force, but in the opposite direction.

The force is studied by physics and according to it four fundamental forces are recognized at the quantum level (that is, atomic): gravitational force, electromagnetic force, strong nuclear force and weak nuclear force.

In contrast, in Newtonian (or classical) mechanics, there are many other identifiable forces , such as friction force, gravitational force, centrifugal force, centripetal force, internal force, etc.

2. Force characteristics

In principle, force is a mathematical model that describes the intensity of interactions between objects , closely related to energy .

For classical mechanics, every force is composed of a magnitude and a direction , the latter being denoted by a vector . This means that it is a vector magnitude, not scalar, and also that it is concentrated in a point of the body or along a straight line in it.

3. Types of force

There are several types of strength, different in nature and in the physical approach that contemplates them. As follows:

According to Newtonian mechanics , we can talk about:

• Force friction . That one that opposes the change of movement of the bodies, exerting a resistance to leave the state of rest, or the one of movement, as we can perceive it when starting to move an object pushing it with the shoulder.
• Gravitational force . That which exerts the mass of the bodies on nearby objects, drawing them towards itself. Especially the planet Earth on the objects and beings that we live on its surface.
• Electric force . Both attractive and repulsive force that is generated by the interaction of electromagnetic fields.

You can also talk about:

• Contact forces . Those forces that are exerted from direct physical contact between one body and another.
• Remote force . Those forces that can be exerted without any physical contact between the bodies.

According to relativistic or Einsteinian mechanics , we can distinguish the following forces:

• Gravitational force . That which seems to exist when massive objects curve the space-time around them, forcing smaller objects to divert their trajectories and approach them.
• Electromagnetic force . That which the electromagnetic fields exert on the particles of matter , following the expression of the Lorenz force.

According to quantum mechanics , there are the four fundamental forces mentioned above:

• Gravitational force . The one that the masses exert on each other, being a weak force, in one direction (attractive), but effective over great distances.
• Force electromagnetic . The one that affects the electrically charged particles and the electromagnetic fields they generate, being the force that allows molecular binding. It is stronger than gravitational and has two senses (attraction-repulsion).
• Strong nuclear force . That which keeps the nuclei of the atoms stable, keeping neutrons and protons together . It is more intense than electromagnetic, but it has much lower range.
• Weak nuclear force . With a scope even smaller than strong nuclear forces, it is the force responsible for radioactive decay, capable of executing changes in subatomic matter.

4. Force units

Force, according to the International System , is measured in units called Newtons (N) , in honor of the great British physicist. These units correspond to 100,000 dynes, and are understood as the amount of force applied during a second to a mass of one kilogram, so that it acquires the speed of one meter per second . That is, that:

1 N = (1kg x 1m) / 1 s ²

There are other units for other metric systems, which are equivalent, in Newtons, to:

• 1 kilogram-force or kilopond is equal to 9.81 N
• 1 pound-force is equal to 4,448222 N

5. How is strength measured?

The dynamometer is the ideal device for measuring force . It also serves to calculate the weight of objects. It was invented by Isaac Newton himself, using the stretching of a spring and Hooke’s Law of Elasticity , in a manner similar to a spring scale.

Modern versions of the dynamometer follow the same principle, and have hooks or rings at the ends of its cylindrical body, within which there is a spring or spiral that acts as a spring. At some of its extremes the force measurement will be marked (in some cases it may even appear on a digital screen).

6. Examples of strength

There are examples of strength around us constantly. By exerting our muscular strength on an object to lift it we are overcoming the force of gravity. If we push a massive body with the shoulder , like a refrigerator, we must not only overcome gravity , but also the force of friction that opposes the movement.

The same happens, in other areas of life, when we paste a magnet from the refrigerator , since the magnetic force holds it in place, but if we approach it to another magnet by the same pole, instead, we will notice a faint repulsive force, which is another character of the same magnetic force.

7. Strength and movement

Strength and movement are strongly involved with each other. In the first place, because force is that capable of starting, stopping or modifying a movement .

For example, when a baseball crashes into the bat, the batter’s force is printed on the second to deflect his trajectory (the one that initially printed the pitcher’s strength, since the ball is normally at rest) and throw it away in the field.

The ability to generate, stop or alter a movement is called work in classical mechanics. Depending on the type of force and the type of movement, various mathematical formulas will be available to calculate them.

8. Force of gravity

The force of gravity is that attractive force that the masses exert on the matter around them , with an intensity proportional to their mass, even if it is effective over long distances.

In fact, the Sun attracts our planet from a distance, with the same kind of force with which it attracts us, that we live on its surface. Gravity can be overcome momentarily, as we do when we jump, but eventually we will succumb to it. Everything that goes up has to go down.

9. Intermolecular forces

They are those that hold the molecules together , forming more complex structures and of greater mass, depending directly on the nature of the atoms involved. That is why they are also known as intermolecular junctions or atomic junctions. These forces can be of two types: the Van der Waals Forces or the Hydrogen Bridges.