= Shock speed in a symmetric cloud collision
{title2=$S=(\gamma-1)V/2$}
Two identical cold clouds approach at speeds $\pm V$ in their center-of-momentum frame. <Reflection symmetry> makes the uniformly compressed gas stationary. A shock moving outward at $S$ sees upstream speed $V+S$ and downstream speed $S$; their ratio equals the strong adiabatic compression $(\gamma+1)/(\gamma-1)$. Solving gives the displayed speed. For a <monatomic gas> $S=V/3$. Finite-cloud breakout ends the steady upstream supply and requires a subsequent boundary-release calculation.
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