Momentum exchange rate between the stellar population and an idealised thin-edged SB with constant expansion and contraction velocities v_exp = 20 km s⁻¹ and v_col = 10 km s⁻¹, respectively, and expansion phase duration of 30 Myr. The ambient gas density is 0.015 M_⊙ pc⁻³. The stellar density is 0.1 M_⊙ pc⁻³, with a Maxwellian velocity distribution with σ = 25 km s⁻¹. The relative velocity between the SB and the stellar distribution is v_b = 10 km s⁻¹. The solid line shows the semi-analytical solution given by Eq. (A.21); the symbols correspond to the full numerical solution.