an open cydrical tank 350 mm in diameter and 1.8 m high is inserted into a body of water with open end down and floats with a 1300N block of concrete (SG=2.4) suspended at its lower end. Neglecting the weight of cylinder. Determine the bouyant force on the concrete and the depth of water that is in the water. What are the pressure inside the cylinder before and after inserting water?
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Answers & Comments
answer:
First, let us calculate the buoyant force. We know that the force is equal to the weight of water in the volume of the body. The volume of block is
V = \dfrac{P}{\rho g} = \dfrac{1300\,\mathrm{N}}{2400\,\mathrm{kg/m}^3\cdot 9.81\,\mathrm{N/kg}} = 0.055\,\mathrm{m}^3.V=
ρg
P
=
2400kg/m
3
⋅9.81N/kg
1300N
=0.055m
3
.
The buoyant force will be \rho_{\text{w}}gV = 541.7\,\mathrm{N}ρ
w
gV=541.7N .
The system floats, so there should another force that is directed upwards and is equal to 1300-541.7 = 758.3\,\mathrm{N}.1300−541.7=758.3N. This force is equal to the buoyant force that acts on the volume of air under the water line. Therefore, this volume is
V_1 = \dfrac{758.3\,\mathrm{N}}{1000\,\mathrm{kg/m}^3\cdot9.81\,\mathrm{N/kg}} = 0.077\,\mathrm{m}^3.V
1
=
1000kg/m
3
⋅9.81N/kg
758.3N
=0.077m
3
. The height of such a layer is h_1 = \dfrac{V_1}{\pi D^2/4} = 0.8h
1
=
πD
2
/4
V
1
=0.8 m.
The pressure of such a layer of water should be balanced by the pressure in air in the tank:
p_1 = \rho_{\text{w}} g h_1 = 7882\,\mathrm{Pa}.p
1
=ρ
w
gh
1
=7882Pa. We should take the atmospheric pressure p_0p
0
into account to get the total pressure 10882 10882 Pa.
According to the ideal gas law, p_0V_0 = p_1V_1, \;\; p_0\cdot\pi \dfrac{D^2}{4} h = p_1\cdot\pi \dfrac{D^2}{4} h_2p
0
V
0
=p
1
V
1
,p
0
⋅π
4
D
2
h=p
1
⋅π
4
D
2
h
2
, where h_2h
2
is the height of air column. h_2 = h \cdot{p_0}{p_1} = 1.67\,\mathrm{m}.h
2
=h⋅p
0
p
1
=1.67m. Therefore, the depth of water will be only 1.8-1.67=0.131.8−1.67=0.13 m
Explanation:
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