# Confusion with pressure I have a few questions:

1. At any point inside a fluid like a gas, the net force is zero although the pressure is not. If this is true it should mean that if I place a surface( with no thickness) at some point in the fluid, there will be no net force on it. Am I wrong here?
2. Why in the book does it say that there is a net average force on the front and back surface of the cylindrical volume element they have considered? As the volume element enters the high-pressure region don't both sides of the front face experience equal and opposite force so the net force is zero and same applies the rear face, so why is there an average force? ( Right bottom of the image I provided shows that they have considered a net force on each of the two faces and also in the first paragraph they state it explicitly.)
• In the 2nd question, isn't there a pressure gradient due to "moving air"? – Samuel Weir Mar 30 '19 at 0:07

• They are not determining the net force on the surface. They are determining the net force on the slug of fluid. The force exerted on the slug by the fluid behind it is +pA. The force exerted on the slug by the fluid ahead of it is $-(p+\Delta p)A$. So the net force exerted on the slug is $-A\Delta p$. The pressure is changing with distance along the pipe. – Chet Miller Mar 30 '19 at 12:09