What is buoyancy?

[scilearner]

Hello everyone,

 

I'm bit confused about buoyancy. First of all I want to know if it is newton's third law, filling a vaccum or none of above. For example these are my 3 diffferent understandings of what buoyancy could be. Which one is right

 

I fall on water and diplace water molecules, do the water molecules give a reaction force back on me calling buoyancy. If that is the case why is it not Newton's third law.

 

when I displace water molecules, water molecules from underneath come to fill the vaccum and push me up(especially when you displace air isn't this what happens). Is this buoyacny?

 

Thanks

[Mr Skeptic]

You displace a fluid, and the fluid pushes up on you by an amount equal to the weight of the fluid you displaced (which may or may not be more than your own weight). Why? Consider that the fluid at deeper depths is at higher pressure, so that it pushes that part of your body more than the fluid at a lesser depth.

[scilearner]

You displace a fluid, and the fluid pushes up on you by an amount equal to the weight of the fluid you displaced (which may or may not be more than your own weight). Why? Consider that the fluid at deeper depths is at higher pressure, so that it pushes that part of your body more than the fluid at a lesser depth.

 

Thanks Skeptic Ok so if I get a water bottle, are the molecules at the bottom travelling towards the top because of the pressure difference? So then if I submerge an object with large volume, it will displace water to a greater depth, hence it has to overcome a higher pressure difference to sink. Is that what happens.

 

EDIT: Now I'm getting confused. If there is water in cup. At the bottom the pressure is higher and at the top pressure is lower, wouldn't this create a buyoncy force and raise the water molecules at the bottom to the top. Why doesn't this occur? When you displace water molecules do they follow this gradient to push back on the object?

Edited April 24, 2010 by scilearner