# Upthrust equation

1. Nov 28, 2015

### Olivia197

Please may somebody explain why this equation for the pressure at the bottom of a fully submerged object is wrong (assuming it is a cube). Thank you!

Pressure = ( density of the fluid x h[1] x g ) + ( density of the cube x (h[2] - h[1] ) x g)

2. Nov 28, 2015

### Mister T

What makes you think it's wrong?

3. Nov 28, 2015

### Staff: Mentor

Why do you expect it to be right?
How do you imagine the pressure distribution close to the edge of the object? Jumping?

Is this a static situation? If you include dynamics, things get more complicated.

4. Nov 28, 2015

### Olivia197

Well, in my textbook it says that the pressure is the density of the liquid x h [2] x g.

5. Nov 28, 2015

### Olivia197

Sorry I am not quite sure what you mean!

6. Nov 28, 2015

### Mister T

Then anything different from that would be wrong. It seems to me that you've answered your own question.

.

7. Nov 29, 2015

### Staff: Mentor

Well, reduce it to the first question: why do you expect your expression to be right?

8. Nov 30, 2015

### sophiecentaur

Could (s)he express, in words, what that equation is describing? That may help with the understanding.

9. Nov 30, 2015

### Staff: Mentor

That equation might work -- is the object neutrally, positively or negatively buoyant? Is it moving or constrained not to move?

10. Nov 30, 2015

### sophiecentaur

I see where you are going with this but the hydrostatic pressure is not really affected by the density of the object (or the object at all). The pressure at the bottom face is due to the column of fluid, rather than the object (unless there is some extra factor involved that we haven't been told about).

11. Nov 30, 2015

### Staff: Mentor

The answer to my question is "other factors" and will actually push the solution toward the normal hydrostatic pressure equation when the OP realizes the impact of the constraints the answer adds.