Very good question. It makes sense that a wood
boat floats because we see wood logs and sticks
floating all the time. Our common sense is baffled
when we see a boat made out of steel or cement
floating because we don't typically think of these
items as being very buoyant.
It turns out this problem of floating
doesn't depend directly on the type of material
but rather how the material is used.
Floating objects are described by the
Archimedes found that in order for an object to
float, an object must be able to displace the
amount of water equal to its own weight. If
the object can't displace this amount of water, it
This theory of displacing water can be seen
when you get into a bath tub and the water level
rises. We know that a ship's shape is very
crucial to its ability to float. So now we have
said that floating not only depends on weight
but also on shape or size. Since the floating
depends on weight and size, we see that a ship's
density is a good way of determining if it will
float. Density is equal to the objects mass
divided by the objects volume (density =
Water has a very high density. A cube
of solid steel has even a higher density, hence
why it doesn't float.
Because of a ship's design it has a unique
density. A ship's density has to include all the
weight in the entire volume of the ship.
Therefore, this includes the center of the ship
which is mostly empty, or air. So a steel ship has
a smaller average density than water because of
the empty center of the ship. By being less
dense than water the ship floats.
Therefore, when determining if a ship is going
to float it is important to analyze the density of
the whole ship, not just the materials on the
exterior. I hope this helps.
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