3. A rocket is flying horizontally in the sky. At time instant $t$, the mass of the rocket is $M$; the velocity of the rocket is $U$ relative to the ground; the exhaust gas is coming out of rocket nozzle at a speed of $Vo$ and mass flow rate is $\dot{m}$. Assuming the air resistance force on the rocket is a constant value of $D$ and the rocket is accelerating at $a$. Please derive the expression of $a$. (Please choose C.V. properly and explain why)
$U, a$
$Ao, Vo, \dot{m}, P1$ (gage pressure)
$D$