Why do Life Jackets Float?
|Emily asked, "Why do life jackets
float? They are so small compared to my size and they don't weigh much."
As you recall from reading How Boats Float, a body displaces water, which pushes back and creates buoyancy. The more water you displace, the greater the force pushing back.
Think about this: A cubic foot of water weighs about 62 lbs. A solid cubic foot of wood weighs about 50 pounds. The solid block of wood will float because it can displace enough water to equal its weight. Now try the same thing with a box made of wood that is hollow inside. Let's say it weighs about ten pounds. It will float much higher because it only has to displace 10 pounds of water to float.
When you are in the water, you feel much lighter. That's because your body has displaced water that is pushing back and creating buoyancy. A life jacket is filled with a very light material, usually foam, that can displace alot of water compared to its weight. A cubic foot of foam weighs about one pound. It will float on top of the water because it only has to displace one pound of water.
Because your body already has some buoyancy, a life jacket doesn't need to support all of your weight. It just needs to displace enough to keep your head above the water.
It is important to get a life jacket that fits you properly. It won't be much use if it's too big and you slip out of it. Also, choose a life jacket that's designed for the activity you are doing. Off-shore life jackets are designed to keep you floating with your head above the water even in rough sea conditions, while life jackets designed for water skiers help you float in calm water and can also protect you from impact injuries.
Life jackets also help your body retain heat if you should fall into cold water. And, as we learned earlier in Why Life Jackets are Orange, a bright colored life jacket makes you easier to see and to find.
Thanks for the great question, Emily!
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