d
\(u \) = velocity of bullet

\(\frac{{dm}}{{dt}} = \) Mass thrown per second by the machine gun

\(= \)Mass of bullet \(×\) Number of bullet fired per second

\( = 10\;g \times 10\;bullet/\sec \)\( = 100\;g/\sec \; = \;0.1\;kg/\sec \)

\(\therefore \) Thrust \( = \frac{{udm}}{{dt}} = 500 \times 0.1 = 50\;N\)

Acceleration of the car \( = \frac{{{\rm{Thrust \,on \,the \,car}}}}{{{\rm{Mass \,of\, the\, car}}}}\)

\( = \frac{{50}}{{2000}} = \frac{1}{{40}} = 0.025\;m/{s^2}\)