Any ideas?

Question

A seagoing crane lifts a sealed safe from a sunken ship. The average density of the safe is 5.3 grams/cc and its mass is 120. kg. What is the tension in the lifting cable when the safe is still submerged?

Answer 1

What's the volume of the safe? Convert the safe density to kg/m^3:
(5.3 g/cc) * (1 kg/1000 g) * (10^6 cc/m^3) = 5300 kg/m^3

120 kg * 1 m^3/5300 kg = .0226 m^3

So the safe is displacing that much water. What's the weight of that much water? Density of water is 1000 kg/m^3.

.0226 m^3 * 1000 kg/m^3 = 22.6 kg

So the safe appears to have a mass of 120 - 22.6 kg. So its apparent weight is W = m*g = 97.4 kg * 9.8 m/s^2 = 955 N.
And that's the tension in the cable.