I'm reading a textbook, chapter entitled "Fluid Mechanics".
This section is entirely devoted to fluid statics.
Assuming an element of fluid in equilibrium,
Consider a thin element of fluid with thickness dy. and density & gravity constant.
let p be pressure upward on the bottom of the surface, then total pressure on bottom of the surface is pA (pressure * Area).
Then, the pressure on the top surface is p+dp, and the total pressure on top of the surface is (p + dp) * A.
I have been looking into the web, but I am quite not sure what dp is meant when finding the top surface pressure.
My only guess is that dP is the difference of pressure due to change in altitude.
If my guess is right, is it notable to include such small change in altitude (considering the material has small dy).