Basic analytical descriptions of the evolutions of the radius (blue curve) and the terminal velocity (red curve) of a 1D SNR (e.g. Draine 2011; Vink 2020), characterised by an ejecta mass of M_ej = 1.4 M_⊙ and an initial kinetic energy energy of E = 10⁵¹ erg, and expanding in a homogeneous medium with a density of 1 cm⁻³. The idealised successive phases known as the free expansion stage, the Sedov-Taylor stage, the pressure-driven stage, and the momentum conserving (or snowplow) stage are highlighted in red, orange, green, and blue, respectively.