A rectangular granite block that archaeologists wish to recover from a shipwreck is fully submerged in seawater of density 1.03×103kgm−3. The block’s dimensions are 1.2m×0.80m×0.60m. Granite has a density of 2.70×103kgm−3. A lifting cable attached to the block pulls it upward at constant (very small) speed once it is clear of the seafloor.
Assume g=9.80ms−2.
(a) Calculate the magnitude of the upward force exerted on the block by the surrounding seawater.
(b) Determine the block’s weight in air.
(c) Using your results from parts (a) and (b), calculate the tension in the lifting cable while the block is being raised through the seawater at constant speed.
(d) The recovery vessel also carries a freshwater tank of density 1.00×103kgm−3 for rinsing artifacts. If the same block were lifted in the freshwater tank instead of seawater, determine the new cable tension and explain qualitatively why it differs from the value found in part (c).