Life Jackets: How Do They Keep Us Afloat?

A life jacket is like a magic suit of armor that turns intimidating bodies of water into fun, helpful friends. Learn how life jackets work with Groupon’s exploration of flotation.

Archimedes, a bathtub, and the crown of a suspicious king—those whose physics teachers had a fondness for the Greeks may vaguely recall that these have something to do with flotation. The first, supposedly first noticed by the Greek physicist splashing around in his bath, is that submerged bodies displace a volume of water equal to their own volume. The second is what’s appropriately known as Archimedes’s principle, and was purportedly used by the man to determine, by comparing densities, whether a goldsmith had substituted some silver into a king’s crown. This principle states that a body is buoyed up by a force equal to the weight of the fluid it displaces.

An example may help demonstrate how these two pieces of information work together to keep us from drowning. Imagine a very small, very heavy teddy bear that weighs 10 pounds. When placed in water, the bear displaces its own volume—what equals 64 cubic inches. That same amount of water, however, weighs only 2.3 pounds, so the water’s buoyant force is not enough to counteract the 10 pounds of force pressing down from the teddy bear’s weight, thus the bear sinks. Now, suppose we place the bear on a raft weighing 1 pound but which has a volume of 240 cubic inches. Two hundred forty cubic inches of water weighs about 8.7 pounds. If we add together 8.7 and 2.3 pounds, the combined weight of the water displaced by the bear and the raft, we get a total buoyant force of 11 pounds, precisely the weight needed to keep the bear and its raft afloat. Of course, the density of a fluid makes a difference: the teddy bear would not need as much help in a lake of molasses, since an equivalent volume of molasses would weigh more and thus exert more buoyant force than water.

Because the human body is itself mostly made up of water, the average person only needs about 7–12 pounds of additional buoyancy in order to float. Most life jackets have internal pouches filled with air when activated or are filled with plastic foam that does not absorb water, providing volume with very little additional weight—just like the teddy bear’s raft—and overcoming the difference in density between a person and the surrounding water.