I dont understood the formula P=Pa+pgh and its derivation

Consider a bottle of radius ‘r’ containing water upto height ‘h’.

The volume of water contained is, V = πr2h

The mass of water is = ρV = ρr2h

The weight of water is = ρπr2hg

So, pressure on the base of the bottle only due to water is = ρπr2hg/ πr2

Now, over the water the air is also exerting pressure. This is atmospheric pressure. This atmospheric pressure also adds up to give us the net pressure on the base which is,

P = Pa + ρgh

Where, Pa is the atmospheric pressure.

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 me tooo

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