# Particle interaction

## Particle aggregates

We have seen that our complex world is made of small particles named atoms. We have also seen that particles can move or be static respect to certain reference system, for example ourselves. But, it is clear that we missed something. Our universe is not a set of myriad of particles going up and down independently one from another. In fact, in such a potential universe we could not exist.

Clearly, particles form aggregates, form objects, macroscopic things. We are surrounded by some objects that do not show their particle composition. They appear to have a ‘solid’ structure.

What did we forget? We did not remember that particles, atoms, interact. They are able to combine, to form complex structures. But, why atoms interact? Physicists define a new concept to justify this fact: forces. We have to suppose that atoms exert some effect (attraction, repulsion,…) on other atoms in order to explain the formation of aggregates.

## Interaction at distance

An important characteristic of particle interaction is that it occurs at distance, although it is not evident in most situations. One particle does not need to contact with another one in order to exert a force on it. Think, for example, on gravity. You do not need to be in contact with Earth to sense Earth attraction. Neither sun needs to contact with planets to attract them. This concept will be more understandable when we analyze atom structure. Then, you will be aware that even apparent contact between objects is not so real.

## Acceleration

How can we measure the result of interaction between particles? It is relatively easy, we use the concept that was explained in a previous section: velocity. As we discussed, we can measure this parameter simply watching at initial and final positions of certain particle (or object) and at our clock. When one particle interact with other particle, movement of both particles is altered. To quantify the result of that interaction, we only need to calculate the change in velocity. Change in velocity (remember that velocity is a vector magnitude) can be only in the direction of motion, in the scalar value or in both. This change constitutes a new physical magnitude named acceleration. As velocity, acceleration is a vector quantity.

• Acceleration is the change in velocity per unit of time.

## Mass and force

When some particle (particle A) interacts with another particle (particle B), a change in the velocity of both particles (or acceleration) is observed. Particle A exerts an influence on the motion of particle B and, in a similar way, particle B affects motion of particle A. These influences are named forces by physicists. Using this term, particle A exerts a force on particle B. If we duplicate the magnitude of this effect, the change in velocity also double. That is, the effect of any interaction is the acceleration of the particles implicated in that event. Force is defined as the acceleration obtained but multiplied by the quantity of matter. That is because we intuitively see that it is not the same to change the movement of something small that of something big.