Question 1

Telephone Pole Partially
Submerged in a Lake


Problem Diagram


A wooden telephone pole paritally falls into a lake as shown. What is the density of the pole? Assume the density of the water is 1.94 slug/ft3.

  Solution 1


The pole will come to a static position where the sum of the moments about the point O is equal to zero.

     ΣMo = 0

Therefore, the buoyancy force multiplied by its lever arm must be equal and opposite the gravitational force times its lever arm.

     ΣMo = 0 = (WpoleLpole - FBLB)cosθ


Lpole = L/2 (gravity acts uniformly along the entire length), and

(buoyancy force acts at the centroid of the submerged part of the pole). Note, θ is not needed since it factors out of the equation. Summing moments at O gives,

     WpoleLpole = FBLB


     Wpole = ρpole V g = ρpole (π r2 L) g

     FB = ρwater V g = ρwater (π r2 L/3) g


      ρpole (π r2 L) g (L/2) = ρwater (π r2 L/3) g (5L/6)

Cancelling terms,

     ρpole (1/2) =(1.94 slug/ft3) (5/18)

The density of the pole is

     ρpole = 1.078 slug/ft3

    Question 2

Septic Tank


Septic tanks are used by homes in rural areas not serviced by sanitary sewers. Assume the tank is 6 ft x 6 ft x 10 ft (long) with 4 in. concrete walls, and assume 1 ft of soil over the tank. At what groundwater depth, d, if any, will the tank float? Any recommendations to keep it from floating? Assume γconcrete = 150 pcf, γsoil = 120 pcf, and γwater = 62.4 pcf

  Solution 2


Inside dimensions: h, w, l
Outside dimensions: h + 2t, w + 2t, l + 2t
where t is the wall thickness

     Vc = (h + 2t)(w + 2t)(l + 2t) - hwl
     A = (w + 2t)(l + 2t)

Since there is 1 ft of soil over the top of tank,

     z = (1 ft) + (h +2t) - d

where d is the water table.

     γconcrete = 150 pcf
     γsoil = 120 pcf
     γwater = 62.4 pcf

To float, the buoyancy force acting on the bottom of the tank must be greater than the weight of the tank and the soil on top.


septic tank: h = 6 ft, w = 6 ft, l = 10 ft, t = 4 in.

     Vc = 474.1 - 360 = 114.1 ft3
     A = 71.11 ft2
     z = 7.67 - d


If the groundwater table depth is less than 1.89 ft, the septic tank will float.

Recommendation to keep it from floating: use 6 in. walls instead of 4 in.

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