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# Pressure question watch

1. Why is –ρg(Z2-Z1) less than 0? I mean if Z2-Z1 < 0 (i.e. its negative) then surely –ρg(Z2-Z1) > 0? So that means that P2 is not always smaller than P1 (as the sheet suggests)

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2. (Original post by GPODT)
Why is –ρg(Z2-Z1) less than 0?[/IMG]

Instead of thinking about z2 - z1, just think about the difference in height, delta z. The pressure will always be higher at the bottom because there is a much larger mass of fluid resting on it. If pressure is force/area, and force is mass times gravity, the deeper you go the larger mass of fluid is resting above that point, hence pressure will be higher.
3. (Original post by Smack)

Instead of thinking about z2 - z1, just think about the difference in height, delta z. The pressure will always be higher at the bottom because there is a much larger mass of fluid resting on it. If pressure is force/area, and force is mass times gravity, the deeper you go the larger mass of fluid is resting above that point, hence pressure will be higher.
I understand that the pressure is higher at the bottom because of the larger mass resting on it but I just want to know why there appears to be a discrepancy in the maths. The sheet says that –ρg(Z2-Z1) is less than 0 but as I explained in my original post this isn't necessarily the case. It is possible that –ρg(Z2-Z1) > 0, in which case P2-P1 > 0 and therefore P2 > P1? I know that logically this doesn't make sense but I'm just a bit confused.
4. (Original post by GPODT)
I understand that the pressure is higher at the bottom because of the larger mass resting on it but I just want to know why there appears to be a discrepancy in the maths. The sheet says that –ρg(Z2-Z1) is less than 0 but as I explained in my original post this isn't necessarily the case. It is possible that –ρg(Z2-Z1) > 0, in which case P2-P1 > 0 and therefore P2 > P1? I know that logically this doesn't make sense but I'm just a bit confused.
Z2 - Z1 is defined as always being positive. It is the "difference in height" and is used to calculate the weight of water above. This "weight" cannot be negative.
It's how the proof is set up. Those are the conditions.

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