# Fluid Mechanics Crane Rope Tension

A piece of concrete has density of 2300 kg/m3 and is 400 x 600 x 3000mm thick. It is lowered into the sea by a crane for an underwater construction project. Determine the tension in the rope of the crane
(a) When the concrete is suspended in the air
(b) When the top is 1.5m above water,
(c) When the top is just below water, and
(d) When the top is 2m below water.
For each case please state the apparent weight of the concrete. The density of sea water is 1025 kg/m3. The apparent weight is the real weight minus upthrust due to buoyancy. Neglect the buoyancy of air and the weight of the ropes.

#### eiddy

• Jun 1st, 2017

a) The tension force equals the weight of the concrete = d*v*g (density, volume and acceleration of gravity respectively).

b) Approximately the same result.

c) Here there is an affective buoyancy force on the concrete by the water.
so F_tension=F_buoyancy-W_concrete
F_buoyancy=D_water*V_concrete*g

d) Here the tension force is less than (c) because of the pressure on the bottom of the concrete is larger than the top of concrete.  