(a). Consider the free-body diagram of the concrete block. The forces acting on the concrete block in air are its weight and the upward pull action (tension) by the rope. These two forces must balance each other, and thus the tension in the rope must be equal to the weight of the block:
FT=W=mg=ρVg=2300⋅0.4⋅0.4⋅3⋅9.81=10830.24N ≈10.8kN .
(b). When the block is immersed in water, there is the additional force of buoyancy acting upward.
FB=ρwatergVcube=1025⋅9.81⋅0.48=4826.5N≈4.8kN.
The force balance in this case gives
FT=W−FB=10.8−4.8=6kN.
Answer: 6kN.
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brief and clear!!
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