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I cannot understand the concept of fluid pressure.

Consider a liquid molecule at some point inside a liquid vessel. All the other molecules which are in contact with this molecule will exert normal force upon this molecule. So, this liquid molecule experiences zero net force. If there is no force on the liquid molecule, there will be no pressure because

pressure=magnitude of net force on the area/ area of the surface

If the net force is zero, how will a pressure be exerted at all? Then how does the concept of fluid pressure comes into picture?

Qmechanic
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Arishta
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1 Answers1

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Consider a vessel filled with liquid of density d Lets take two points A and B such that A is x distance below the surface and B is x+h below the surface.

cylinder

Now consider a cylinder of fluid with ends A and B and area of crossection A'.

This cylinder is in equilibrium hence the forces must balance out hence

$ A'(Pb - Pa) = mg = A'hdg $ $ Pb = Pa + hdg $

That is the pressure difference between the points.

The example that you gave was basically a cylinder of A' = 0 and h = 0 hence the pressure difference was also zero.

You can clearly see that its the force of $mg$ downwards that actually leads to a pressure difference and thus pressure is simply a consequence of gravity on a fluid.

Rishabh
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