physics notes

The American Heritage Dictionary defines buoyancy as "the tendency or capacity to remain afloat in a liquid, or rise in air or gas.". Buoyancy is the capacity to remain afloat in a liquid, likely water. What causes buoyancy? When an object, like a boat, displaces more water, by weight, than the boat weighs, it will not float, but sink to the bottom. That is unless the boat is fitted with buoyancy compensators, like air-filled tanks, styrofoam blocks attached to the sides of the boat, or any other material that is lighter than the weight of water, as in less than one kilogram per cubic meter of space taken up by the object (or less than 62.4 pounds per cubic foot of space taken).

First discovered by the philosopher Archimedes, the principle of buoyancy states that "the buoyant force acting on an object placed in a fluid is equal to the weight of the fluid displaced by the object". Imagine a block of wood that has a volume of one cubic foot. A cubic foot of water weighs 62.4 pounds. Therefore, water pushes upward on the wooden block with a force of 62.4 pounds per cubic meter of mass. As long as an object weighs less than 62.4 pounds per cubic foot of water that it displaces, it will stay afloat in the water, and this is called buoyancy.

So, what causes buoyancy? The fact that a boat, or any other object, weighs less than the amount of water that it displaces, will make the boat stay afloat. This can be observed in a bathtub with any solid object. Fill the tub halfway full with water. Mark the spot on the tub that is the top of the waterline. Now, add the object to the water. Mark where the waterline mark is now. Remove water in a container until the water is back to the initial top waterline. This is the amount of water that the object has displaced. If the object weighs less than this amount of water, the object will float.

But, why do steel liners float, while lighter weighing rocks will sink? It is also to do with the shape of the object, the amount of gases that are lighter than water inside that liner (inside her ballast tanks, etc.), and the speed at which the boat travels. A faster moving boat requires less buoyancy than a still boat, but since boats do eventually come to a rest, they are all built with the principle of weighing less than the amount of water that they displace.

As for people swimming, there are other factors at work. If the person has a high fat-content, then they would be more likely to float easily, since fat weighs less than muscle. If that person has their lungs filled with air, they will float better than with the air expelled from their lungs. And, a body in motion takes less buoyancy to stay afloat. All of which helps the ocean liners in rough seas, as they take on ballast water to become heavier, and thus float lower in the water, not affected as much by the motion of the top layers of the water (in this instance, the waves).

All of this comes from a King who thought that his crown was not made of pure gold. (It wasn't, and the maker of the crown was killed for it after Archimedes came up with his theorem while taking a bath). All in the name of science!