Center of mass rigid body, the gravity forces will not cause the body to rotate, which means the weight of the body can be considered to be concentrated at the center of mass. The linear and angular momentum of a collection of particles can be simplified by measuring the position and velocity of the particles relative to the center of mass. Let the system of particles "P", "i"=1...,"n" of masses "m" be located at the coordinates r with velocities v. Select a reference point R and compute the relative position and velocity vectors, The total linear and angular momentum vectors relative to the
Mass in special relativity derived form formula_44 is not valid because formula_45 in formula_46 is generally not a constant (see the section above on transverse and longitudinal mass). Even though Einstein initially used the expressions "longitudinal" and "transverse" mass in two papers (see previous section), in his first paper on formula_47 (1905) he treated "m" as what would now be called the "rest mass". Einstein never derived an equation for "relativistic mass", and in later years he expressed his dislike of the idea: Okun and followers reject the concept of relativistic mass. Also Arnold B. Arons has argued against teaching the concept of relativistic