(b) The accelerating force of the rocket
$= upward\ thrust = \frac{\mathrm{\Delta}m}{\mathrm{\Delta}t} \bullet v$
$Given,\ \frac{\mathrm{\Delta}m}{\mathrm{\Delta}t} = 50 \times 10^{- 3}\ {kgs}^{- 1},\ \ v = 400\ {ms}^{- 1}$
So, accelerating force $= 50 \times 10^{3} \times 400 = 20\ N$