# Upwards buoyancy pressure only acts on the surface of a structure that

1. Aug 21, 2009

### gloo

so with buoyancy and liquid pressure i am pretty sure i know the answer but i just want to confirm (please don't laugh if it's obvious)

- upwards buoyancy pressure only acts on the surface of a structure that is faced down
- the bottom of the structure that supports all the mass of the structure (assume a cube like structure for simplicity) is the only area that has to be taken into account when calculating pressure on the downside.
- if the pressure on the bottom of the cube surface is 2700kpa, it will find equilibrium and stop at 2700 kpa of the ocean (270 meters)

2. Aug 21, 2009

### Staff: Mentor

Re: bouyancy

No, it is the total resultant force.
...and supports the water above the object.
No, if the pressure is 2700 kpa, then it is at a depth where the pressure is 2700 kpa! It can't be anywhere else!

3. Aug 21, 2009

### gloo

Re: bouyancy

thanks Russ - i just want to clarify points 1 and 3

1. I guess i was making an assumptiom buoyancy meant upward for this point. I was trying to say that buoyant force pusing upward on the object only acts on the surface of the cube facing downward (partial submersion). The forces on the side will cancel out and have no net effect upward

3. in a partial submersion scenario, if the formula of pressure=force/area - and whatever the force of gravity on the cube and it's contents and divide by the area of the bottom of the cube, and it's answer is 2700kpa, then the bottom of the cube will sink until it reaches 270 meters (where the pressure is 2700kpa). There the pressure downward on the cube's bottom surface is equal to the upward pressure of the water at that depth, and that is where the cube will stop sinking (equilibrium?)

4. Aug 22, 2009

### Staff: Mentor

Re: bouyancy

Yes, in the case of a partially submerged cube with its sides kept perfectly vertical, only the water pressure on the bottom surface contributes to the upward buoyant force. (But don't forget air pressure pushing down on the top surface.)

5. Aug 22, 2009

### Staff: Mentor

Re: bouyancy

For #3, that's only true if the top surface is not submerged. If the top surface is under water, then the bouyant force is equal to the difference in pressure (times area). For a submerged object, bouyancy is not a function of depth.