# What causes buoyancy?

1. Dec 18, 2004

### chevap

Hello.

I have a question or two about buoyancy, in water specifically. What causes buoyancy, if there is something more than the difference of pressures above and beneath an object?
What would happen if I put a very thin object in water, so that the difference in pressures would be extremely small?
And what would happen if I put an object (cone, for example) at the very bottom of a water filled glass, would it come back up (supposing it wouldn't sink if I just put it in the glass), and why? Which forces would affect the object?

English is not my native language, so I can't express myself the way I want to, but oh well.

2. Dec 18, 2004

### Staff: Mentor

That's all there is to it: Buoyancy is the net force on an object due to the pressure of the fluid surrounding it. There's a net upward force because pressure varies with depth.
It doesn't matter. The buoyant force would still equal the weight of the displaced fluid. (That's Archimedes' principle.) Of course, a thin object will displace less fluid than a fat object (of the same length).
If the density of the object is less than that of water, then the buoyant force will be greater than the weight of the object: thus the object will come back up. The forces acting on the object are: its weight (acting down) and the buoyant force of the water on it (acting up).

And welcome to PF!

3. Dec 18, 2004

### chevap

Hmm, I can understand all that, but there's one problem that keeps bothering me.

If we have a perfectly shaped cone at the bottom of a glass, with no water beneath it, which pressure will lift it up (assuming its density is less than the density of water)?

4. Dec 18, 2004

### Staff: Mentor

If you have a perfect seal between the cone and the bottom of the glass, then the water pressure will exert a net downward pressure on the cone. There will be no "buoyant" force and the cone will not rise up.

5. Dec 18, 2004

### chevap

Okay, thanks a lot!

6. Dec 19, 2004

### Clausius2

And what happens if you put a steel needle onto the water?

7. Dec 19, 2004

### Staff: Mentor

If you put the needle (gently) onto the water (instead of into the water) then other factors come into play, such as surface tension.

8. Dec 19, 2004

### Clausius2

Right :!!)

And Arquimedes Principle is not satisfied. That astonished me some time ago.