# Dictionary

## mechanics, relativistic

The generalization of classical mechanics that takes into account the effects of special relativity. The basic laws are almost unchanged: First of all, bodies on which no external forces act stay at rest or move with constant speed along straight paths – in the language of special relativity: such bodies move on straight lines in spacetime. Secondly: The total force acting on a body is equal to the change of its momentum over time (but notice: this momentum is defined using the body’s relativistic mass, which depends on the bodies speed relative to the observer). Thirdly, mass and momentum are conserved quantities – their total sum is the same whenever particles interact (this is equivalent to a slightly modified version of the “action equals reaction” principle of classical mechanics).

There exists an elegant reformulation of these laws of mechanics using four-dimensional concepts adapted to the geometry of spacetime, such as the “four-momentum”.