Hydrostatics Pressure - Finding thrust and resultants

[maxisman]

Homework Statement

I'm getting stuck with this one and in need of some direction! Obviously I'm not looking for a direct solution, but maybe a outline of what I should be doing? (I'll attach an image in a few minutes)

<Image here incoming!>

Known data:

d2 = d1 / 2
F = thrust force (N)
Pa = ρ . g . ha g = gravitational constant 9.81N/m^2
ρ = rho, water density 1000kg/m^2
g = gravitational constant 9,81N/m^2
b = 8m (lock gate width

Homework Equations

I think I've answered part a) correctly, see section 3.

b)
Finding the resultant: would it be Ft (total force) = F1 - F2 (F1 being the greater force)?

c)
Find the acting point: All I know is that the centroid of a RH triangle is 2/3 from the base, so somehow a combination of the two?

The Attempt at a Solution

My attempt so far:

a)
Thrust at F1 = 1/2 . ρ . g . d1^2
= 1/2 . 1000kg m^3 . 9,81 . d1^2 . 8
F1 / d1^2 = 39240N (39,24 . 10^3N) I've moved the d1^2 over to left and worked out the rest.

Thrust at F2 = 1/2 . ρ . g . (d2^2 / 2) . 8
= 1/2 . 1000kg m^3 . 9,81 . (d2^2 / 2) . 8
F2 / (d2^2 / 2) = 39240N (39,24 . 10^3N)

Any direction would be wonderful

Also any good books for me purchase too!

Thank you

 

[Chestermiller]

And that figure is where?