Mass and Density
Mass is how much matter is present in an object, while density is a measure of how tightly packed that mass in a given space. For example, lead weighs more than the same volume of tin because it is denser.
Basics of Buoyancy
When something is less dense than the liquid it is resting in, it will float on the surface of that liquid. Oil floats on the surface of water because it is less dense, while wood floats because the wood fibers and resins trap air inside them, lowering the wood's average density.
Life jackets float because they are made out of materials that are buoyant, but it is not enough for them to simply float. They must be designed so they have a lot of lift, so that being worn by a person does not lower the overall buoyancy of the wearer and the life jacket to the point where both sink.
Foam-based life jackets float because their plastic foam entraps micro-bubbles of air, lowering its average density in much the same way as wood's structure does for it.
An inflatable life jacket is filled with either regular air or carbon dioxide, using a small cartridge of compressed gas that can be activated in an emergency. Either air or carbon dioxide is substantially less dense than water, so it has a lot of lift.
Pros and Cons
Inflatable life jackets provide a lot of lift and are easy to stow, but are not very durable. Foam core life jackets, on the other hand, are very durable, but bulky.
Article Written By Edwin Thomas
Edwin Thomas has been writing since 1997. His work has appeared in various online publications, including The Black Table, Proboxing-Fans and others. A travel blogger, editor and writer, Thomas has traveled from Argentina to Vietnam in pursuit of stories. He holds a Master of Arts in international affairs from American University.