Expert: Expert - 5/7/2008

Question
QUESTION: I know that a body wholly or partially submersed in water experiences an upward force equal to the weight of water displaced but as a mature person learning to swim I am fed up with people spouting what I believe to be a myth that a body is MORE buoyant in deeper water. Surely buoyancy is not variable with depth?

ANSWER: Please forgive my delay in responding -- it's the only way I can think of to ensure I do not assist with academic work, of which homework is just one part.

Anyway, you understand the basic idea of buoyancy: that an object "in water experiences an upward force equal to the weight of water displaced." Now, let's say the density of water (ie, weight per volume) increases but the volume of the object stays the same. That would mean the VOLUME of water displaced would be the same, but the WEIGHT of the displaced water would be more. Thus, the buoyant force would increase.

It should be no surprise that the density of water DOES increase as one goes deeper into the water. Not terrible fast (water is called an "incompressible" fluid), but it does occur. Thus, as you go deeper, your volume (ie, the amount of the water you displace) stays the same -- but the weight of the water you displace goes up. Thus, the buoyant force of the denser water goes up. Thus, a body IS more buoyant as you go deeper into the water.



---------- FOLLOW-UP ----------

QUESTION: That is very interesting. So that I am sure I understand it clearly, the buoyancy at the surface will not increase no matter how much the depth under increases but will increase if the body goes (considerably) lower in the water.
As you will appreciate from my original question, the argument is about a body at the surface of the water learning to swim.

Answer
> the buoyancy at the surface will not increase no matter how much the depth under increases
> but will increase if the body goes (considerably) lower in the water

Your understanding is correct. I've not heard any statement that a person swimming on the surface would be more buoyant if the depth of the water underneath increased, and cannot see any reason why that would be the case. It is POSSIBLE that someone heard, "Buoyancy increases as you go into deeper water," and mis-interpreted that as, "Buoyancy increases as you stay at the surface over deeper water." Then the latter gets repeated until it takes on the level of "truthiness." I am not aware of any reason the latter statement would be correct, but that doesn't mean it isn't true.

If I can be referred to a URL explicitly stating the statement we seem to agree is false, then (perhaps) I can do a better analysis of it.