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Eulers equation is:

dp = -qVdV, where p is pressure, q is density and V is velocity. And that's clearly a differential equation. But i'd like to know how its derived. I know i have to start with Newtons 2nd law:

F=ma

my textbook says that

The force in the x direction acting on a fluid particle is

F = p dydz - (p + dp/dx dx) dydz (1)

Hence F = dp/dx dxdydz = force on fluid element due to pressure. (d)

This is where my confusion begins... I don't understand why the Force would be pressure multiplied by dydz and then minus pressure plus dp/dx multiplied by dydz. In other words i really don't understand what's going on with equation one, or how it becomes equation (2).

Can any1 help? I think once i understand this example i'll be able to work out others. Thanks.