Mass in special relativity what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference, The invariant mass is the ratio of four-momentum (the four-dimensional generalization of classical momentum) to four-velocity: and is also the ratio of four-acceleration to four-force when the rest mass is constant. The four-dimensional form of Newton's second law is: The relativistic expressions for "E" and "p" obey the relativistic energy–momentum relation: where the m is the rest mass, or the invariant mass for systems, and "E" is the total energy. The equation is
Earth mass value) in the Schiehallion experiment in the 1770s, and within 1% of the modern value in the Cavendish experiment of 1798. The mass of Earth is estimated to be: which can be expressed in terms of solar mass as: The ratio of Earth mass to lunar mass has been measured to great accuracy. The current best estimate is: The "G" product for the Earth is called the geocentric gravitational constant and equals It is determined using laser ranging data from Earth-orbiting satellites, such as LAGEOS-1. The "G" product can also be calculated by observing the motion of the Moon or