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Old Aug 13th 2014, 11:20 AM   #1
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Pressure at a certain depth is the same on all sides of the object?

Assuming that the object's surface is at the same depth, for example an object that is a line ----, why is there an equal pressure on the underside of the line?

I know that at the top of the line, the perpendicular force is due to the weight of the mass of the liquid.

Is the perpendicular force on the underside due to the weight of the liquid, which causes a normal force? (assuming the liquid is incompressible)
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Old Aug 13th 2014, 11:45 AM   #2
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Consider that for any drop of liquid the pressure must be the same on all sides, or the imbalance would cause the water to flow. So at a certain depth h the drop must experience pressure from above that balances pressure from below, and pressure from all sides that exactly balance as well. In the case of a drop that is pressed against the underside of a plate (or the line in your example), it must "feel" pressure from the plate above exactly equal to the pressure of the water below. That pressure is the same as if the plate wasn't there.
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Old Aug 13th 2014, 03:50 PM   #3
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Is the upward balancing pressure on the object due to the normal force of the incompressible liquid, where the liquid is the surface?
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Old Aug 14th 2014, 05:18 AM   #4
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Try this thought experiment,

Imagine a fish tank half full of ball bearings,
Push the ball bearings to one side and they will tend to roll back to create a fairly level surface.
In otherwords it is behaving rather like a liquid.

Place a piece of card on top of the ball bearings, then add some more bearings on top.
This could be considered as a (simple) scale model of the problem you originally posted.

The card is being pressed down from above, but can't move because the balls below are pressing back.
The balls below are pressing back because they can't move, because of their neighbouring balls.

It is similar in the liquid, the individual molecules push and pull against each other to maintain an even spacing,
If a gap opens up anywhere, the pull on that side won't match the push from the other, and the molecule will move to even everything up
.
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Old Aug 14th 2014, 03:24 PM   #5
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hmm this concept was similar to the problem I was having for a different post. This totally helped clarify! Thank you ChipB and MBW.
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Old Aug 14th 2014, 07:37 PM   #6
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Thanks ChipB and MBW!
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