A submerged wreck, mass 104 kg and mean density 8 x 103 kg m-3, is lifted out of the water by a crane with a steel cable 10 m long, cross-sectional area 5 cm2 and Young’s modulus 5 x 1010 N m-2. Determine the change in the extension of the cable as the wreck is lifted clear of the water. /b] This is from the British physics olympiad, and they also have solutions up on their site. Their method of solving this problem was straightforward, just using Young´s modulus = stress/strain, where stress = force/area normal to the force, and strain = extension/original length. This all seems logical, but what confused me is that they calculated the upthrust, and that was their force. Isn't this incorrect? I would guess that you'd have to use the weight - upthrust as your force, but apparently I'm wrong. Why?