# Sources of Mass in Special Relativity

Let, we have an object with mass (previously called rest mass) m. It starts moving with a velocity v, and its relativistic mass (not a preferred term nowadays) or total energy becomes $$m_{\mathrm{Rel}} = \gamma m$$. This is the mass we get if we measure it on a scale. Now if we collide this object with another object having the same relativistic mass, assuming that no energy escapes, the combined rest mass of the new object would be $$M = 2 \gamma m$$ (Resnick/Special Relativity). Should we assume that the increase in mass is because of the temperature increase of the new body, meaning it is solely due to the kinetic energy of its particles? I guess the answer is no, some of the energy should be used as binding energy or other forms. How does special relativity explain those other energies, like binding energies? I mean, they simply have mass even though nothing is 'moving'.

• Where the mass precisely comes from depends on the details of the system. Kinematics cannot tell you something like this. I also don't think $\gamma m$ is what a scale would see. Nov 8, 2021 at 6:36