&quot;In general, relativistic and rest masses are equal only in systems which have no net momentum and the system center of mass is at rest&quot;.  This is the case of a system being heated.  I&#x27;m still led to believe that the mass is still increased by the increased motion of the particles in that system.  <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Mass_in_special_relativity" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Mass_in_special_relativity</a>