/sci/ - Science & Math

Someone who's taken fluids pls help:
I don't understand this equation. I understand every part of its derivation, but it does not make sense to me.
For example, imagine a rocket ship in space with no gravity, no body forces. It turns on its engine, shooting a flow of hot gas behind it. This equation suggests that the force vector on the rocket would be parallel and pointing in the same direction as the velocity vector of the gas that is leaving the rocket. But this makes no sense. Why is there not a negative sign in front of this equation?
>>10984707
Because you don't always have to work with an arbitrary coordinate system that's fixed to the ground. If you have a_t and a_n, you can work with a coordinate system that is fixed to the body that is in motion)