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 accurately calculate correct values. To address these problems, the ideas of Albert Einstein concerning the fundamental phenomena 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, Einstein's theories assumed relative values for these concepts. Since Einstein's equations yielded accurate results at high speeds and Newton's did not, Einstein's theory of relativity for bodies in motion has usurped Newton's laws of motion, based on absolute space and time. However, as a practical matter, Newton's equations are much easier to work with than Einstein's and therefore are more often used in applied physics and engineering.
It is interesting to note, however, that 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. Therefore, relative space and relative time result in relative motion, which means that the unit values of space and time can change for observers moving at high speeds relative to each other. These concepts have led physicists in general to conclude that only relative motion can be measured and that absolute motion is meaningless.
