: The measure of mass is not the same in all structures, that is, mass is not absolute. If there is a relative motion between two inertial structures, an object of the same mass will not be equal to the mass of the measured object of the object to two observers located in two structures. The mass of the object is greater than the mass of the object measured by the observer of the moving inertial structure to the observer of the static inertial structure. This change in mass due to the velocity of the object is called mass relativity.