# Motion

*This article is about***motion**in physics. See also motion (legal), motion (democracy) and Apple Motion.

In physics, **motion** means a change in the position of a body with respect to time, as measured by a particular observer in a particular frame of reference. Until the end of the 19th century, Newton's laws of motion, which he posited as axioms or postulates in his famous *Principia,* were the basis of what has since become known as classical physics. Calculations of trajectories and forces of bodies in motion based on Newtonian or classical physics were very successful until physicists began to be able to measure and observe very fast physical phenomena.

At very high speeds, the equations of classical physics were not able to calculate accurate values. To address these problems, the ideas of Henri Poincaré and Albert Einstein concerning the fundamental phenomenon of motion were adopted in lieu of Newton's. Whereas Newton's laws of motion assumed absolute values of space and time in the equations of motion, the model of Einstein and Poincaré, now called the special theory of relativity, assumed values for these concepts with arbitrary zero points. Because (for example) the special relativity equations yielded accurate results at high speeds and Newton's did not, the special relativity model is now accepted as explaining bodies in motion (when we ignore gravity). However, as a practical matter, Newton's equations are much easier to work with than those of special relativity and therefore are more often used in applied physics and engineering.

In the newtonian model, because motion is defined as the proportion of space to time, these concepts are prior to motion, just as the concept of motion itself is prior to force. In other words, the properties of space and time determine the nature of motion and the properties of motion, in turn, determine the nature of force.

In the special relativistic model, motion can be thought of as something like an angle between a space direction and the time direction.

In special relativity and euclidean space, only relative motion can be measured and that absolute motion is meaningless.

## See also

- Equation of motion
- Molecular dynamics
- Motion perception
- Newton's laws of motion
- Physics
- Trajectory of a projectile

bg:Движение ru:Механическое движение de:Bewegung et:Mehaaniline liikumine he:תנועה (פיסיקה) pl:Ruch sl:Gibanje th:การเคลื่อนที่ es:movimiento vi:Chuyển động zh:运动