July 28, 2003
A: FROM MENTOR KATHERINE DRENNON IN KY

Courtnie, to understand why things float in water, you need to understand density. An object's density is how much mass it contains per unit of volume. For instance oil is less dense than water, which is why it floats on top. More dense objects will always sink to the bottom. That is why if you put a rock in a glass of water it will sink. Some substances, like pumus (a kind of volcanic rock), are really porous, so it may be big, but it has a lot of air space in it, which decreases it's density and lets it float in water. We humans also float in water. This is partly because our bodies are ~75% water, and partly because we breathe air (all gases are less dense than water).
Density is a cool concept. Did you know that when stars go nova or supernova, they collapse into a dwarf star. Dwarf stars are so dense that a teaspoon of their matter would weigh more than the whole earth! How's that for dense!
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A: FROM MENTOR MOLLY WILLIAMS IN MI
There are two things that determine whether or not something floats: (1) the weight of the object itself, and (2) the bouyant force that the surrounding water exerts on the object. That bouyant force is equal to the weight of the water that is displaced when the object is put into the water. So something like wood or styrofoam or ice, that has a lower density than water, floats with part of its volume above the water. The bigger the chunk of material, the more water it displaces, so it doesn't matter how big or heavy the item is; it still floats. Ships that are made of steel, which is more dense than water, might be expected to sink, but they displace more water than just the volume of the steel itself. They also displace the air volume that's contained inside the shell. So the average density of a ship, including the steel, open space, and other cargo and passengers, is still less than water, so it floats * at least until it springs a leak! Once the inside of the ship fills with water instead of air, then its average density becomes more than water, and it sinks.
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A: FROM MENTOR KRIS MOODY IN NH
: The reason some heavy objects can float is because of displacement. If
you search for information on displacement, you'll find a lot of
information, especially in the area of boat design. There is even a
particular kind of boat hull that is called a "displacement hull." The
basic explanation of displacement is that an object placed in a liquid,
like a boat in the water, displaces that liquid, meaning it pushes it
away. If the object weighs less than the weight of the liquid that it
pushed away, then it will float. If the object weighs more than the
displaced liquid, it will sink. So when a displacement hull is designed
for a boat, the designer has to figure out the size of the hull needed for
the volume of water that must be displaced in order to allow that boat to
float.