Given information :

Wall is vertical and submersed in a fluid.

Formula used :

  P=ρgd

Where,

  P= Pressure.ρ = Mass density of the fluid.g =Acceleration due to gravity.d = Depth / height of the fluid. 

  Force = Pressure × Area

Calculation :

First we ind the area of the small strip of dx depth at x .

Let, this area be dA .

As the dx is too small we assume that the pressure will remain same through out the area.

Now the pressure at depth x be Px=ρgd .

Therefore, the force on the sais small strip of the wall =dF=PxdA .

Now, for the getting the total force gainst the wall we intregrate,

  ∫dF=∫PxdA