Estimate of the reflection shock velocity and the density of the interacting cloud in the case of Cassiopeia A, solving the fluid equation in Hester et al. (1994). Black and red curves show the velocities of the transmitted shock (that is propagating through the cloud) and reflection shock, respectively. The blue area shows the range of the velocities of the western filaments in this work: ∼2100–3800 km s⁻¹.