# Origin of upthrust

1. Sep 16, 2015

### Janiceleong26

1. The problem statement, all variables and given/known data
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?
^ This statement does not states the origin of upthrust...Why? It seems right ! Feeling confused
2. Relevant equation
P=F/A
P=hρg

Last edited: Sep 16, 2015
2. Sep 16, 2015

### Staff: Mentor

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):

3. Sep 16, 2015

### Janiceleong26

But upthrust is a force, whereas pressure isn't ..

4. Sep 16, 2015

### SteamKing

Staff Emeritus
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.

5. Sep 16, 2015

### Staff: Mentor

But you use pressure (and area) to calculate the force, as you stated yourself in your first post.

6. Sep 16, 2015

### Janiceleong26

I see, thanks ! Got it
Thanks! I got it now