Model evolution for a case with log[M_gas,0(M_⊙)] = 6.3, R_cl = 25.7 pc, ϵ_ff = 0.3, and for the IZw 18 metallicity. First plot from top to bottom: shell radius (left y-axis), shell velocity (right y-axis). Second: shell mass (M_sh), stellar mass formed from core star formation (M_sc,c), and total stellar mass (M_sc). Third: the outward force produced by the thermal energy (${\dot{U}}_{\text{th}}$, dashed) and the total inward forces (${\dot{U}}_{\text{in}}$, solid). Fourth, bottom right panel: mechanical power (solid) produced by the stellar feedback and the cooling rate (dashed).