# What is the direction of the buoyancy force?

1. Jan 20, 2007

### harvhk852

What is the direction of the buoyancy force?
Why the buoyancy force acts in this direction??

Thanks for any help

2. Jan 24, 2007

### CPP

i believe the buoyant force acts upward throught the COG of the displaced volume.

3. Jan 25, 2007

### Staff: Mentor

CPP is correct (welcome to PF, both of you!). As for why...um...it seems so straightforward it is tough for me to answer.

Consider a container full of water - its weight acts at the center of mass. Now empty the container, seal it, and submerge it in a larger container of water. You can almost consider buoyancy a "hole" in the water, or even kinda like a negative weight.

4. Jan 25, 2007

### grant9076

Another way to look at the same concept is to consider a given volume of fluid containing a given amount and type of objects as a closed system with a center of gravity. The most stable state for such a system is with the center of gravity at the lowest possible point (while still remaining a closed system). This only happens when the denser objects sink to the bottom and the less dense objects (less dense than the fluid) rise to the top.

5. Jan 25, 2007

### Gokul43201

Staff Emeritus
This looks like homework. Is it?

Look up the standard textbook derivation for the buoyant force. It clearly shows how the direction of the buoyant force follows from the variation of pressure in the fluid.

6. Jan 25, 2007

### Ki Man

I doubt it's his undergrad thesis statment, so most likely

7. Jan 27, 2007

### venik

water is heavier than air, and i think it might have something to do with building pressure as you get deeper in water

8. Jan 27, 2007

### Staff: Mentor

Since water is largely incompressible, no, buoyancy of a given volume varies very little with depth.