I need help with the following problem. I have attached a rough sketch showing what i'm looking at. Basically a popppet valve is closed over a vent. It is then pushed open by a force applied to the valve by the pushing rod. Their is a spring on the rod resisting this and providing a force labeled Fspring. The air inside the container at P1 is then expanding from rest through the valve to Pa (atmospheric). What i am trying to work out, and can't find a formula anywhere for, is what the force on the poppet valve will be due to the expanding air (Ffluid). I'm trying to model this system and currently the valve won't stay open as i can't compute the force on the poppet valve accuratley.