Concept in Definition ABC

Evelyn Maitee Marin | June 2022
Industrial Engineer, MSc in Physics, and EdD

In the universe, all bodies are in constant interaction, and as a consequence, there is an infinity of forces that are responsible for of all the physical and chemical phenomena that have existed: the combination of elements is an interaction, and in it forces are produced intermolecular. Also, at the macroscopic level there is evidence of forces as a result of interactions, for example, to lift a briefcase, it is required to apply a force.

In order for the Moon to orbit the Earth, the Earth must exert a force on it, and for the Earth and other planets in the Solar system can orbit the Sun, there must be forces that allow this movement. From the above, two types of interactions can be generally distinguished: by contact and by distance.

contact interactions

They are those that involve direct contact between bodies. Some examples of contact forces are:

Normal reaction (n): is a force that is generated when a body rests on or touches a surface. Its name is due to the fact that this force always acts perpendicular to the tangent plane of contact and is directed from the surface to the body. Examples of this force occur all the time when a person is standing on a flat surface. horizontal, since the ground exerts an upward vertical force to support the body and prevent it from falling down. weight action.

Tension (T): this type of force is exerted by flexible bodies (can be bent) such as ropes, cables, springs or chains, among others. The term tension is due to the fact that the only way in which a flexible element such as a rope can exert force is pulling, since if you try to push with a rope, it will bend and no force is applied some. The tension is represented parallel to the cable and always leaving the body on which it acts.

Friction force (Ff): it is a force that originates from the roughness that all the surfaces present, which generates a resistance to the relative movement between them. No matter how smooth a surface may appear to the naked eye, there are always, at least at a microscopic level, irregularities that cause a kind of grip that opposes the slip between two surfaces in contact, therefore, the friction force is represented tangent to the contact surface and opposite to the movement (or to the tendency of the same). Two types of friction forces are distinguished: static and the kinetics (²).

The static friction force (F_fs): acts when the body is at rest but with a tendency to move. The magnitude of this force is equal to the force (or component of the force) that generates the tendency to move, and reaches its maximum value at instant where impending motion occurs, the point at which the friction force is directly proportional to the normal reaction of the surface. the constant of proportionality is called the coefficient of static friction (μ_s).

On the other hand, the kinetic friction force (F_fk), is exerted when there is relative motion between the surfaces. This force is approximately constant and its magnitude is determined by multiplying the coefficient of kinetic friction (μ_k) for the normal reaction.

Friction coefficients are dimensionless quantities whose value depends on the nature of the surfaces in contact. Its value is between zero and unity (0 < μ < 1), and experimentally it has been shown that the static coefficient of friction is greater than the kinetic one (μ_s > μ_k).

distance interactions

These types of interactions arise without the need for the interacting bodies to be in physical contact with each other. To justify this phenomenon, Physics has developed a whole theory called "field theory", being the field a representation in space and time of a physical quantity associated to some property (dough, electric charge, magnetic materials). In general, three types of remote interactions can be distinguished:

Gravitational force: it is a force of attraction generated by the interaction at a distance of two bodies with mass, and its magnitude obeys the Law of Universal Gravitation:

Where:

F: magnitude of the attractive force between the masses
G: universal gravitational constant (G ≈ 6.67x10^-11 N•m²/kg²)
m, M: masses of bodies
r: separation distance between the masses

Electric force: this force occurs between particles or bodies that are electrically charged, and It can be attractive or repulsive, depending on whether the signs of the charges are different or the same. respectively. For point charges, the magnitude of the electric force can be determined from Coulomb's Law:

where:

F: magnitude of the attractive force between the charges
k: Coulomb's constant (k ≈ 9x10⁹ N m2/C²)
what₁ and what₂: values ​​of point charges
r: separation distance between the charges

Magnetic force: is the result of the electromagnetic force as a result of moving charges. The value of the magnetic force can be determined from Lorentz's Law:

F ⃗=q∙v ⃗×B ⃗

Where:

F ⃗: magnetic force
q: moving load
v ⃗: speed of cargo movement
B ⃗: magnetic field

Determination of forces

In classical mechanics, Newton's Laws offer an explanation of the interactions between bodies and the determination of the forces that are generated as a result of those interactions. In particular, Newton's Second Law expresses that the acceleration experienced by a body (a) is directly proportional to its mass (m) and inversely proportional to the applied force (F):

F = m • a

It is important to note that forces are vector quantities, so they have magnitude, direction and senses. The magnitude is determined by the expression above, and the direction and direction will be the same as those of the acceleration. The units of force in the international system are equivalent to kg m/s2, that is, Newton (N).

1 N = 1 kg•m/s2