Problem 3.
60 [Difficulty: 2]
lbf
Given: Plug is used to seal a conduit. γ = 62.4⋅
3
ft
Find: Magnitude, direction and location of the force of water on the plug.
Solution: We will apply the hydrostatics equations to this system.
Governing Equations: dp
=γ (Hydrostatic Pressure - y is positive downwards)
dh
FR = pc ⋅ A (Hydrostatic Force)
Ixx
y' = yc + (Location of line of action)
A ⋅ yc
Assumptions: (1) Static fluid
(2) Incompressible fluid
(3) Atmospheric pressure acts on the outside of the plug.
π 2
Integrating the hydrostatic pressure equation: p = γ⋅ h FR = pc⋅ A = γ⋅ hc⋅ ⋅ D
4
lbf π 2 4
FR = 62.4⋅ × 12⋅ ft × × ( 6⋅ ft) FR = 2.12 × 10 ⋅ lbf
3 4
ft
π 4
⋅D 2 2
π 4 64 D ( 6⋅ ft)
For a circular area: Ixx = ⋅ D Therefore: y' = yc + = yc + y' = 12⋅ ft +
64 π 2 16⋅ yc 16 × 12⋅ ft
⋅ D ⋅ yc
4
y' = 12.19⋅ ft
The force of water is to the right and
perpendicular to the plug.