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Consider two different shaped containers having same area: one is cylinder, the other is like an inverted pyramid (roughly). Both have the same level of water, the weight of the inverted pyramidal container will therefore be greater than that of the cylinder. But I know from Pascal's law that the pressure should be same in both containers. If the base areas are the same then the force (weight as measured by a scale) should also be the same. Why are the weights different?

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  • $\begingroup$ See, for example, the questions in Halliday & Resnick (at least, this question was in the Eggplant edition) $\endgroup$ Apr 15, 2016 at 15:45

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The flaw in your reasoning is that in the inverted cylinder, the hydrostatic pressure is also applied to the "diagonal" walls of the container. This causes a net downward force that, if left unbalanced, would cause a downward acceleration. However, the force is transmitted down through the (rigid) walls of the container to the base, and from there to the scale, which registers a higher weight.

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At first I assume that you are weighing them separately .

If my assumption is correct, then

According to pascals law pressure is same on both base of both containers but as you weigh them on a machine in case of cylinder no component of force on walls due to pressure by fluid add to weight but in case of inverted pyramid the component of force due to pressure at side walls too contribute to weght so since the volume is different and in second case force due pressure component also add so weight is more. Hope you understand.

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