# Origin of upthrust

## Homework Statement

When an object is submerged or floats in a fluid, the..
Pressure on bottom surface > Pressure on top surface
So the resultant force is acting upwards.

Why the difference in pressure? Must both, the top and bottom surfaces, have the same area of contact with the fluid? Since P=F/A ?

A sealed cylindrical steel can is submerged in water. What is the origin of upthrust that acts on the can?
The weight of displaced water acts upwards on the can.
^ This statement does not states the origin of upthrust...Why? It seems right ! Feeling confused
2. Relevant equation
P=F/A
P=hρg

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Doc Al
Mentor
Why the difference in pressure?
Pressure depends on depth below the surface--the deeper you go, the greater the pressure.

As expressed by this equation (h is the distance below the surface):
P=hρg

Pressure depends on depth below the surface--the deeper you go, the greater the pressure.

As expressed by this equation (h is the distance below the surface):
But upthrust is a force, whereas pressure isn't ..

SteamKing
Staff Emeritus
Homework Helper
But upthrust is a force, whereas pressure isn't ..
Yes, but a pressure applied over an area results in creation of a force, F = P * A.

What you seem to be missing is information about Pascal's Law, which Doc Al provided in mathematical form in Post #2, and also Archimedes' principle and the concept of buoyancy, which is another name for this upthrust. Doc Al
Mentor
But upthrust is a force, whereas pressure isn't ..
But you use pressure (and area) to calculate the force, as you stated yourself in your first post.

But you use pressure (and area) to calculate the force, as you stated yourself in your first post.
I see, thanks ! Got it
Yes, but a pressure applied over an area results in creation of a force, F = P * A.

What you seem to be missing is information about Pascal's Law, which Doc Al provided in mathematical form in Post #2, and also Archimedes' principle and the concept of buoyancy, which is another name for this upthrust. Thanks! I got it now