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
Magnitude (astronomy) which the reference is a source with a constant flux density per unit frequency. Another is the STMAG system, in which the reference source is instead defined to have constant flux density per unit wavelength. The human eye is easily fooled, and Hipparchus's scale has had problems. For example, the human eye is more sensitive to yellow and red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude. Apparent magnitude can also be affected by factors such as dust in the atmosphere or light cloud cover absorbing