We're all thinking this: Absolute vs Gauge pressure problem

[AntiKarateKid]

In the Berkely Review book, a question in the fluids section reads:

A 1 m^3 block of aluminum sank to the bottom of a depth of 750m. By how much did the block shrink?

My ONLY problem with this is that they used gauge pressure instead of absolute pressure in the solution. Why is that and how would we know when to use which?

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[Ssina]

because the atmospheric pressure is pressent before and after, the only thing that is changed is relative force on each side of the cube due to the liquid at that depth... and you know pressure is a force on each side of the cube (the area)... so the only extra thing now is the gauge pressure.

it's like squeezing the cube, in air, with an equal force on all sides of the box (pressure). Only thing added is the force per area.

(EDIT: I should add that actually in a liquid, the depth difference between the top of the box and the bottom creates the pressure difference and accounts for the Buoyancy, but that's another topic)

we have a bucket with a hole at the bottom: in order to stop the fluid flow, we have to apply a pressure that is equivalent to Ptotal (gauge+Patmosphere).... but once you let go and the fluid is running out, now the only pressure that is pushing the fluid out is Pgauge, since atmospheric pressure at top of the liquid or at the hole is the same... hence why velocity= sqrt 2gD
(Mr. Bernoulli's: 1/2(ru)v^2=(ru)gD ..... P1 and P2 are same Patm so cancel out and you can solve for v)