Consider a unit volume element of fluid consisting two horizontal circular areas S1 and s2, connected by vertical lines as boundries.
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Now for derivation of pressure change with depth we need to make an fbd for the above described unit volume element.
For this my book takes weight downward , F1 on the bottom surface S1 upwards and F2 on the top surface S2 downwards.
My question is won't there be a force downwards on S1 and upwards on S2, which will have an effect of cancelling out F1 and F2 respectively.
I have reasoned this out from the fact that pressure is same in all directions in a fluid. Please help me clear this doubt.
Thanking you in advance.