Vacuum formation behind a receding piston
= Vacuum formation behind a receding piston
The gas state at a receding piston reaches vacuum when $\dot X_p=-2c_0/(\gamma-1)$, because then the <sound speed> and <mass density> both vanish. If the piston subsequently recedes faster, a vacuum gap opens between it and the gas boundary, which continues with velocity $-2c_0/(\gamma-1)$.