
Concept explainers
To Calculate: The hydrostatic force on a semicircular gate at the bottom of a dam with water

Answer to Problem 40E
The total hydrostatic force on the gate is 11263467 N .
Explanation of Solution
Given: The tank contains water.
- The diameter of the gate is 4 m .
- The height of the gate below water surface is 12−2=10 m .
Concept Used:
- The hydrostatic pressure of a water of density ρ at a depth d from the surface of the water is P=ρgd .
- The force on a surface due to pressure P exerted is F=PA , where A is the area of the surface being considered.
The radius of the gate is 42=2 m
The depth of the center of the gate from the surface of water is 10 m
So, if θ is the angle with the horizontal made by a radius passing through a point p located on the circular edge of the gate, then
- The depth of the point p is 10−rsinθ=10−2sinθ .
- The width of a horizontal strip of the gate passing through the point is 2rcosθ .
- The length of an infinitesimal section of perimeter around the point p making an angle dθ with the center of the disc is rdθ .
- The height (thickness) of the horizontal strip of disc that corresponds to this small section of perimeter is rdθ⋅cosθ
Therefore, the area of this infinitesimally thin strip is
dAθ=width×height=2rcosθ×rdθcosθ
=2r2cos2θdθ
and the hydrostatic pressure at the level of point p is
Pθ=ρgh=ρg(10−2sinθ)
Therefore, the force on this thin strip is
dF=PθdAθ=ρg(10−2sinθ)⋅2r2cos2θdθ=784000(10−2sinθ)⋅cos2θdθ
Now the total force on the gate is the integral of dF from θ=0 to θ=π . So
F=∫π0dF=∫π0784000(10−2sinθ)⋅cos2θdθ
=784000(2cos3θ3+10(θ2+sin2θ4))]π0
=784000(2(−23)+10(π2+0))
=784000×14.3ˉ6
=11263467
Therefore, the total hydrostatic force on the gate is 11263467 N .
Chapter 6 Solutions
Single Variable Calculus: Concepts and Contexts, Enhanced Edition
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