Originally Posted by sailingdog
an object has negative buoyancy when it weighs more than the water it displaces, and as such will sink.
Unfortunately this is not correct. It is a term used to describe a condition of weighing more than the water you displace; but there really is no such thing as being "negatively buoyant". It's an oxymoron. A rock is buoyant, just not buoyant enough to float. The buoyant force is ALWAYS in opposition to gravity and as such it is ALWAYS non-negative. People who dive have coined the phrase "negative buoyancy" but unfortunately it is an incorrect term and kids who are trying to learn what buoyancy is get it totally wrong because of this nomenclature (believe me, my GF is a science teacher). Don't teach this; your children will be doomed to answering incorrectly in their school studies and on standardized tests if you do.
Take this example:
A diver who is neutrally buoyant tries to retrieve a friend's weight belt from the bottom. The weight belt weighs 10 lbs. When he picks the belt up off of the bottom; the diver cannot swim to the surface because he now has a total weight that is more than his buoyant force. Did the diver LOSE any buoyancy?
The answer is:
No, he did not lose buoyancy. He gained mass and his buoyant force remained the same. His buoyancy never became "negative"; he was no-longer buoyant enough to be able to return to the surface. If the diver had an air bladder he would increase his buoyancy by adding air to the bladder; offsetting the additional weight.