= Rest mass of a coalescing relativistic collision
{title2=$m_3^2=m_1^2+m_2^2+2m_1m_2\gamma_1\gamma_2(1-u_1u_2/c^2)$}
When two massive particles coalesce without additional outgoing radiation, <four-momentum conservation> makes the final <four-momentum> their sum. For collinear incident <velocities>, its <rest mass> is the positive square root of the displayed invariant, and its final <velocity> equals the <center-of-momentum frame> velocity $c^2p_{\rm total}/E_{\rm total}$. Relative <kinetic energy> can become internal <rest energy>, so the final <rest mass> is generally larger than the sum of the incident <rest masses>.
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