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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. Ancient anecdotes aside, there are two key principles at work here. 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 properly known as Archimedes’ 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 the water, it displaces its own volume of 64 cubic inches. That amount of water, however, weighs only 2.3 pounds, so its buoyant force is not enough to counteract the 10 pounds of force pressing down from the teddy bear’s own weight. Now, suppose we place the bear on a raft weighing one pound and having a volume of 240 cubic inches. 240 cubic inches of water weighs about 8.7 pounds, so we now have a total buoyant force of 11 pounds—precisely the combined weight of the bear and the raft, which, accordingly, will now float happily along. (Of course, the density of the 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 are filled with plastic foam that does not absorb water, or have internal pouches that are filled with air when activated—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.