UCSB Science Line Why does the mass of a particle changes when its speed changes? Question Date: 2014-07-30 Answer 1: The answer has to do with special relativity, and what you mean by the "mass" of a particle. You'll often hear this explanation: when the speed of a particle changes, its kinetic energy changes, and therefore so does its total energy. But special relativity, and Einstein's famous formula E = mc2 tell us that energy and mass are equivalent, so changing the speed of the particle changes its mass. This new mass is often called the relativistic mass M. Even though that explanation is popular, many physicists (myself included) don't like it. The reason is because the formula E = mc2 is really only valid when a particle isn't moving. When a particle is moving with some momentum p, the correct formula is E2 = (mc2)2 + (pc)2. This formula gives you the total energy E of a particle in terms of its rest mass m (that is, the mass of the particle when it's not moving) and its momentum p. Because it's only the rest mass m that appears in the formula, most physicists will only use the term "mass of the particle" to refer to the rest mass, and don't really care about the relativistic mass. So my short answer to your question would be: the mass of a particle doesn't change when it's moving! But its energy certainly does. Answer 2: This is something we know from relativity. Energy and mass are equivalent (hence Einstein's famous equation, E = mc2), and velocity contains energy as you probably already know. Therefore, changing the energy has the side-effect of changing the mass. Click Here to return to the search form.    Copyright © 2017 The Regents of the University of California, All Rights Reserved. UCSB Terms of Use