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Question 2.11: A block of concrete weighs 100 lbf in air and “weighs” only ...

A block of concrete weighs 100 lbf in air and “weighs” only 60 lbf when immersed in fresh water (62.4 lbf/ft^3). What is the average specific weight of the block?

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A free-body diagram of the submerged block (see Fig. E2.11) shows a balance between the apparent weight, the buoyant force, and the actual weight:
\sum{F_{z}}=0=60+ F_{B} -100
or F_{B} = 40 lbf = (62.4 lbf/ft^3)(block  volume, ft^3)
Solving gives the volume of the block as 40/62.4 = 0.641 ft^3. Therefore the specific weight of the block is
\gamma_{block}=\frac{100lbf}{0.641 ft^3}=156 lbf/ft^3

2.11

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