In physics, motion means a continuous change in the location of a body. Change in motion is the result of applied force. Motion is typically described in terms of velocity, acceleration, displacement, and time. An object's momentum is directly related to the object's mass and velocity, and is conserved within a system, as described by the law of conservation of momentum.
Everything in the universe is moving. As there is no absolute reference system, absolute motion cannot be determined and only motion relative to a point of reference can be determined; this is emphasized by the term relative motion.
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.
Relative motion is a change in location relative to a reference point, as measured by a particular observer in a particular frame of reference. Essentially, an object is in relative motion when its distance from another object is changing. However, whether the object appears to be moving or not depends on the 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
The place or object used for comparison to determine the change in position of an object is known as the reference point. Thus, if it is assumed that the reference point is stationary, an object can be said to be in motion if it changes position relative to a reference point. A classic misinterpretation of relative motion was the incorrect assumption that the Sun moved around the Earth rather than the other way around.