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In this picture the points on red have the same pressure because the weight of the fluid is the same above them. But according to pascal law the point on black must have the same pressure(both 3 points are on the same plan),but the weight of the fluid above the point on black isnt the same. So how do we explain this?

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    $\begingroup$ The top of the horizontal tube exerts a force on the liquid in the region of the black point which stops the fluid flowing out. $\endgroup$
    – Farcher
    Sep 17, 2018 at 17:13
  • $\begingroup$ So the pressure are equal on both points? That's right? $\endgroup$ Sep 17, 2018 at 17:17
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    $\begingroup$ Yes because if the pressures at the same horizontal level were not the same the fluid would move. $\endgroup$
    – Farcher
    Sep 17, 2018 at 17:19
  • $\begingroup$ But if we apply hydrostatic formula we willn't find the same pressure,because the height isnt the same on black point. Except we choose the surface in contact with air as (Z=0). $\endgroup$ Sep 17, 2018 at 19:46
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    $\begingroup$ Possible duplicate of Pressure at base of 3 different dam designs $\endgroup$ Sep 19, 2018 at 1:09

1 Answer 1

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Part of the weight of each fluid is supported by the base of each of the two upper tanks. So, the only part of the weight that really matters is that situated directly above each of the lower tubes (as if they were each extended vertically upward). The weights of the fluid on the two sides within these more restricted regions would be the same. If you calculate the pressure at the red points, based on the amounts of overlying fluid, the pressures will be the same.

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  • $\begingroup$ If i undersant what you said, the pressure on the two red points are the same, but does the pressure in the black point is the same as in the red points? $\endgroup$ Sep 17, 2018 at 19:43
  • $\begingroup$ Yes, the black point is in the same fluid and at the same depth as the two red points, so it is the same as at the red points. $\endgroup$ Sep 17, 2018 at 20:42
  • $\begingroup$ I'm confused, does the pressure depend on the depth of a point or the height of the colum above it? $\endgroup$ Sep 17, 2018 at 21:40
  • $\begingroup$ At hydrostatic equilibrium, the pressure varies with vertical position z according to the equation $dp/dz=-\rho g$, where $\rho$ is the density, and it is independent of horizontal position. $\endgroup$ Sep 17, 2018 at 23:13
  • $\begingroup$ If dP=-(rho)*g*dZ but dZ depend on frame of reference, so pressure depend on frame of reference. But is more logicaly to remplace dZ by H which is the heigjt of the liquid above which apply pressure? $\endgroup$ Sep 18, 2018 at 16:30

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