Legend has it that around 200 B.C., the Greek philosopher Archimedes took the first step to determine why objects float. Archimedes noticed as he was getting into his bath that when he sat down, water flowed over the sides of the tub. His weight had moved, or "displaced," it. He concluded that water pushes upward with a force equal to the object's weight.
When an object is floating, part of it is under the water; how much of it is underwater depends its weight. For instance, if a boat weighs 1,000 pounds, it will sink into the water until it has displaced 1,000 pounds of water. The object will float unless it is too heavy to push away enough water equal to its own weight and still have part of itself above the water.
But weight is not the primary factor in determining whether something will float. After all, big ships are very heavy, yet they stay afloat. It all comes down to an object's density: Objects with lower density will float more easily than objects of higher density. The density of the liquid is also a factor.
A boat may weigh 1,000 pounds, but it is not solid steel throughout: Much of its interior is air. So the average density of a boat is very light compared to the average density of water. Filling the boat with heavy rocks will increase its density, and eventually the boat will sink when its density becomes greater than that of the water.
The American Geophysical Union contributed to the TV portion of this report.