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In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement , distance , velocity , acceleration , speed , and frame of reference to an observer, measuring the change in position of the body relative to that frame with a change in time.
Relative velocities between two particles in classical mechanics. The figure shows two objects A and B moving at constant velocity. The equations of motion are: = +, = +, where the subscript i refers to the initial displacement (at time t equal to zero).
SR states that motion is relative and the laws of physics are the same for all experimenters irrespective of their inertial reference frames. In addition to modifying notions of space and time , SR forces one to reconsider the concepts of mass , momentum , and energy all of which are important constructs in Newtonian mechanics .
Relative velocity is fundamental in both classical and modern physics, since many systems in physics deal with the relative motion of two or more particles. Consider an object A moving with velocity vector v and an object B with velocity vector w ; these absolute velocities are typically expressed in the same inertial reference frame .
The laws of physics are the same for all observers in any inertial frame of reference relative to one another (principle of relativity). The speed of light in vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source. The resultant theory copes with experiment better than classical mechanics.
Mach put forth the idea in his book The Science of Mechanics (1883 in German, 1893 in English). Before Mach's time, the basic idea also appears in the writings of George Berkeley. [4] After Mach, the book Absolute or Relative Motion? (1896) by Benedict Friedlaender and his brother Immanuel contained ideas similar to Mach's principle. [page needed]
Unlike relative time, Newton believed absolute time was imperceptible and could only be understood mathematically. According to Newton, humans are only capable of perceiving relative time, which is a measurement of perceivable objects in motion (like the Moon or Sun). From these movements, we infer the passage of time.
Nevertheless, the Lorentz equations allow one to calculate proper time and movement in space for the simple case of a spaceship which is applied with a force per unit mass, relative to some reference object in uniform (i.e. constant velocity) motion, equal to g throughout the period of measurement.