# What causes pressure in a fluid from beneath?

Homework Statement:
So my understanding is that for an object submerged in a fluid, the pressure from above is because of the weight of the fluid directly above it. However, the pressure at the bottom of the object is greater because of the hydrostatic equation. What causes this pressure since it isn't weight clearly?
Relevant Equations:
And to check my understanding, this pressure times area of the object equals the object's weight times the weight of all the fluid vertically above the object right?
S

• Delta2

## Answers and Replies

Homework Statement:: So my understanding is that for an object submerged in a fluid, the pressure from above is because of the weight of the fluid directly above it. However, the pressure at the bottom of the object is greater because of the hydrostatic equation. What causes this pressure since it isn't weight clearly?
Relevant Equations:: And to check my understanding, this pressure times area of the object equals the object's weight times the weight of all the fluid vertically above the object right?

S
the pressure is a macroscopic property that is caused due to collisions with the molecules of the fluid and since the object is completely immersed in the fluid the pressure acts from all directions on the object (even from the bottom).
And to check my understanding, this pressure times area of the object equals the object's weight times the weight of all the fluid vertically above the object right?
I think maybe you are referring to the area of the base of the object? instead of "area of the object".

haruspex
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my understanding is that for an object submerged in a fluid, the pressure from above is because of the weight of the fluid directly above it.
No, it is because of the ambient pressure within the fluid at that depth. In an L-shaped vessel, the pressure all along the bottom is the same, and it depends on the vertical distance from there to the exposed surface.
The same applies at all points on the surface of the submerged object, so the pressure is higher on the lower parts than on the higher parts. One could in principle obtain the net buoyancy force by integrating over the surface of the object, but generally it is much simpler to apply Archimedes' principle.

And to check my understanding, this pressure times area of the object equals the object's weight times the weight of all the fluid vertically above the object right?
No, that's quite wrong.
The area has to be considered as made up of area elements, each of which is a vector. The magnitude of the vector is the magnitude of the area, and the direction is orthogonal to the element, into the object.
The net force is then ##\int P.\vec{dA}## taken over the area in contact with the fluid.
Multiplying two weights wouldn’t make much sense, and as noted above, it is not to do with fluid directly above the object.

• Hamiltonian299792458
Chestermiller
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Homework Statement:: So my understanding is that for an object submerged in a fluid, the pressure from above is because of the weight of the fluid directly above it. However, the pressure at the bottom of the object is greater because of the hydrostatic equation. What causes this pressure since it isn't weight clearly?
Relevant Equations:: And to check my understanding, this pressure times area of the object equals the object's weight times the weight of all the fluid vertically above the object right?

S
Are you familiar with Pascal's law. It says that, at a given location in a fluid, pressure acts equally in all directions. This is called isotropy of pressure.

jbriggs444
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One might think of Newton's third law. The upward force from the fluid below is equal and opposite to the downward push from the stuff above.