Answer:
![F = [P_a + \rho g(H-x)]Ldx](https://tex.z-dn.net/?f=F%20%3D%20%5BP_a%20%2B%20%5Crho%20g%28H-x%29%5DLdx)
here
= atmospheric pressure
L = length of dam wall
H = height of water in the dam
= density of water
Explanation:
Let the length of the wall is "L"
Now here we know that pressure due to liquid filled in the dam at given position is given as
Let say the height of water in the dam is "H"
now the wall height where we took a small element is at height "x" from bottom
So pressure at that position is given due to water is

here we know
= density of water
H - x = height measured from the top surface
= atmospheric pressure
now we have to find the Force on the wall
so here area of the element is given as

now force is given as
![F = [P_a + \rho g(H-x)]Ldx](https://tex.z-dn.net/?f=F%20%3D%20%5BP_a%20%2B%20%5Crho%20g%28H-x%29%5DLdx)