$2 x 10^6 \mathrm{~N}$
Step 1:Given
The rate of change of mass of the rocket due to the consumption of fuel is $\frac{d m}{d t}=40 \mathrm{~kg} / \mathrm{s}$
The velocity of the gases ejected from the rocket $v=5 x 10^4 \mathrm{~m} / \mathrm{s}$
Step 2: Formula used
We know that the accelerated mass in a particular direction causes an upthrust force that is equal in magnitude and opposite in direction.
For a changing mass, we get,
$\therefore F=m a=m \frac{d v}{d t}=v \frac{d m}{d t}$
Step 3:Calculate the thrust on the rocket:
Upon substituting the values we get,
$\begin{aligned}
& F=40 \times 5 \times 10^4 \\
& =200 \times 10^4 \\
& =2 \times 10^6 N
\end{aligned}$