why we need to take velocity of air in tunnel with respect to train **&** velocity of train with respect to earth (inertial frame) (here we are comparing pressure difference between air inside and outside train)? 
>A train with cross-sectional area $𝑆_𝑡$ is moving with speed $𝑣_𝑡$ inside a long tunnel of cross-sectional area $𝑆_0 (𝑆_0 = 4𝑆_𝑡)$. Assume that almost all the air (density $\rho$) in front of the train flows back between its sides and the walls of the tunnel. Also, the air flow with respect to the train is steady and laminar. Take the ambient pressure and that inside the train to be $𝑝_0$. If the pressure in the region between the sides of the train and the tunnel walls is $𝑝$, then $𝑝_0 − 𝑝 = \frac{7}
{2𝑁}
𝜌𝑣_𝑡^2$
. The value of 𝑁 is _______.

>Answer: 9

Solution: https://youtu.be/RXmdziWqI6I?t=540

Question Source: JEE Advanced 2020

Such thing is also applied to find pressure difference above and below airplane wings by NCERT. Do we need to take velocities of fluid with respect to moving object?