How does a life preserver work?
A life preserver works on a rule called "buoyancy". This rule says that
when an object is placed in a liquid, for example water, it is held up by
an amount that is equal to the weight of the liquid (water) that the
object replaces when it is put in the liquid (water). So if a piece of
aluminum foil is crumpled up into a ball and dipped in water, it sinks.
But if the same piece of aluminum foil is formed into the shape of a
"boat", it floats -- even though the weight of the piece of aluminum foil
is the same in both cases. A life preserver does a similar thing. The life
preserver is made like a balloon, filled with air, or of a light material
like foam or cork. The person and the life preserver together replace a
certain amount of water when placed in the water. The person plus life
preserver will sink into the water until the weight of the water they
replace equals the weight of the person plus life preserver together.
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Update: June 2012