From The Art and Popular Culture Encyclopedia
In physics, motion means a continuous change in the position of a body relative to a reference point, as measured by a particular observer in a particular frame of reference. Until the end of the 19th century, Isaac 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 absolute motion is meaningless.
An object is in motion when its distance from another object is changing.Whether the object is moving or not depends on your point of view. For example, a woman riding in a bus is not moving in relation to the seat she is sitting on, but she is moving in relation to the buildings the bus passes. A reference point is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point. You assume that the reference point is stationary, or not moving.