Statics is the action of one body on another. A force is either a push or a pull. A force tends to move a body in the direction of its action. The action of a force is characterized by its magnitude, by the direction of its action, and by its point of application. Thus, force is a vector quantity, because its effect depends on the direction as well as on the magnitude of the action. Forces are classified as either contact or body forces. A contact force is produced by direct physical contact; an example is the force exerted on a
Absolute magnitude unit (AU) from both the Sun and the observer, and in conditions of ideal solar opposition. In fact, one has to take into account that Solar System bodies are illuminated by the Sun, therefore the magnitude varies as a function of illumination conditions, described by the phase angle. This relationship is referred to as the phase curve. The absolute magnitude is defined for the ideal case of phase angle equal to zero. To convert a stellar or a galactic absolute magnitude into a planetary one, subtract 31.57. A comet's nuclear magnitude () is a different scale and can not be