Point particle

In physics a point particle is defined as a body whose dimensions are negligible compared to the phenomenon under study. For example a planet can be considered a point particle in a celestial mechanics problem, an atom in a statistical mechanics problem and so on.

The gray mass can be simplified by representing it as a point particle (i.e. a point-like body with mass). In physics this approximation is used to describe the dynamics of extended bodies when it is possible to neglect the internal structure (as in the case of the rigid body approximation). In the image the body (in gray) is approximated as a point particle at its center of mass (in black). All the mass of the body is concentrated in one point.

In general a point particle is only characterized by its three spatial coordinates, its velocity and its mass. This means that the schematization of a body as a point particle is equivalent to neglect the existence of its internal degrees of freedom: a point particle can not store energy by rotating on itself, heating or compressing elastically. All these phenomena, in fact, to be described need a more detailed modeling of the body: always referring to a concrete example, a planet can be treated as a rigid body, rather than as a point particle, if we are interested in its rotation. The usefulness of the concept of point particle is to be able to associate a geometric point to the body and then to operate in Cartesian space with the methods of analytical geometry.

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