Buoyancy is the upward force that keeps things afloat. When placed in water, an object will float if its buoyancy is greater than its weight. And it will sink if its weight is greater than its buoyancy. Learn more about the physics of buoyancy and density at the following sites.
"People have been aware of objects floating on water (or sinking) since before recorded history. But it was not until Archimedes of Syracuse came along, that the theory of flotation and the buoyancy principle were defined." Archimedes was a mathematician born in 287 BCE, in the city of Syracuse on the island of Sicily According to this University of Utah site, Archimedes is best remembered for a discovery involving the crown of King Hiero II. Learn why Archimedes shouted "Eureka!" and how he proved that the king's crown maker had defrauded him.
For high-school and college physics students, HyperPhysics is an illustrated, hyperlinked, mind map of hundreds of physics topics. The material can be scrolled through from top to bottom or you can jump around, following the links in each short article. Topics related to buoyancy include density, mass, weight, fluid pressure, submerged volume, buoyant force, Archimedes Principle, and Pascal's Principle.
Helicopters and airplanes depend on thrust and forward speed to fly. Hot air balloons and dirigibles rely on buoyancy (or differences in air density) for lift. This NASA page for high school students is part of a larger site on aeronautics. It offers a short article on buoyancy, and concludes with five exercises to test your understanding of the Archimedes Principle. At the bottom of the page you'll find links to more aeronautics topics.
"When you place a block of wood in a pail of water, the block displaces some of the water, and the water level goes up. If you could weigh the water that the wood displaces, you would find that its weight equals the weight of the wood." After you've digested the basics of buoyancy, you'll be ready for the three Buoyancy Brainteasers. This PBS site is my pick of the day because of its clear explanations, simple graphics, and challenging brainteasers!
Although it may not be immediately apparent (at least it wasn't to me) the green boxes represent multiple-choice answers to the questions posed by each of the headlines. Clicking will reveal both the correct answer, and links to more information. Best click is the lab experiment to determine the density of water and ice. You'll find it at the bottom of the page.