For upward motion of rocket, thrust force must be greater in magnitude than its weight and net force will be equal to their difference, i.e., Upward \\ \\ thrust - Weight = F_{net} -v_e \\dfrac{dm}{dt} - mg = ma -\\dfrac{dm}{dt}=\\dfrac{ma + mg}{v_e} Here, negative sign is implying that mass of gas is decreasing with the passage of time and v_e=980\\ m/s is the exhaust speed of fuel.Hence -\\dfrac{dm}{dt}=\\dfrac{m(a+g)}{v_e} -\\dfrac{dm}{dt}=\\dfrac{4000(19.6 + 9.8)}{980} \ herefore -\\dfrac{dm}{dt} = 120 kg/s