ok so my friend told me that in a container, the pressure exerted by the walls on the liquid in the container act in the upward direction.Is he correct ? so what I am imagining is a cylindrical container kept on the ground. according to me the pressure by the wall of the container should act perpendicular to the surface of the wall.Am i going wrong somewhere ? Any help would be appreciated.
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Pressure is a scalar and does not have a direction. This is discussed in some detail in the answers to Define Pressure at A point. Why is it a Scalar?, though this might be a bit technical.
When you measure a pressure you are actually measuring the force applied to a surface. For some small bit of surface $\delta {\bf A}$, the force produced on that surface due to a pressure $P$ is:
$$ \delta {\bf F} = P \delta {\bf A} $$
and the direction of the force is normal to the surface. So it's the orientation of the surface that determines the direction of the force.
We use small surface elements $\delta {\bf A}$ so that we can apply the formula to curved surfaces. Your particular case is a lot simpler, and you are basically correct. However note that when you say:
the pressure by the wall of the container should act perpendicular to the surface of the wall
Note that this should be:
the force by the wall of the container should act perpendicular to the surface of the wall
John Rennie
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Pressure at a point in a static fluid is independent of direction.
The force exerted by the walls on the liquid will be pointing inwards. Imagine if there is a hole in the container and water is liquid out, it is easy to see that you have to apply a force inwards in order to prevent the liquid from leaking. Since the force is proportional to the area of the hole, you'd want a dimensionally equivalent form of pressure in order to eliminate the dependence on area. And that is stress. (Note that stress is not a scalar nor a vector - it is a tensor)
t.c
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