|How do boats float on water when they are so heavy?
|Question Date: 2014-12-12|
Awesome question! In fact, there's a cool
story behind this - the ancient Greek scientist
and mathematician Archimedes was thinking about a
question similar to yours while he was taking a
bath. He suddenly had a realization, and was so
excited that he went running naked through the
streets shouting "Eureka!"
This story is a little silly, and is probably
exaggerated, but the idea that Archimedes had was
correct. What was his idea? It had to do with
something called buoyancy, or what happens
when you put something in a fluid (like water).
When you put an objects underwater, some water
has to move out of the way to make room for it;
this is called displaced water. Archimedes'
principle says that when this happens, the object
gets pushed up with the same with the same weight
as the displaced water.
This is how boats float: when you put a boat in
water, it has to push some water out of the way.
But as the water is pushed out of the way, the
boat feels an upward force equal to the weight of
that water. If the boat pushes enough water out
of the way, the upward force manages to balance
out the force of gravity pulling the boat down,
and so the boat floats.
But! What happens if you start filling the
boat with water? Well, the boat becomes
heavier, so it needs to sink lower to push more
water out of the way. Eventually, the boat gets
so full of water that even if it sinks completely
it's unable to push enough water out of the way to
stay afloat, and it sinks. So it's important that
boats be full of air, not water! Either that, or
the boat needs to be made of something light, like
wood or plastic, that floats on water.
I hope that helps!
Boats float because although they are heavy,
they are still less heavy than the amount of water
needed to fill the boat would be. This is
something that is called density: the amount of
weight in the amount of space that it takes up. If
something is denser (higher density, more weight
for space) than water, it will sink; otherwise, it
The design of boats is what makes them float,
even though they are really heavy. As you might
know, most boats have a metal shell that is filled
with cargo and people, but also a lot of air. The
air is light compared to the water below, so the
total weight of the ship, including the air, is
less than the water below it. This makes the boat
float on top of the water!
A heavy rock sinks, but a boat, equally heavy,
doesn't! Neat! So what gives? The Biggest
factor is the contact area between the boat and
the water (the area where the boat and water
touch). The secret to floating boats is a
problem of spreading the weight over a large area
of water. If you think of a bunch of water
molecules trying to hold up a boat, they're
pushing against the boat to keep it on top of the
water. If the contact area between the water and
the boat is larger (a large area of the boat is
touching the water), more water molecules can push
against the boat, so they can keep it afloat. You
can think of it like lifting furniture. You might
not be able to lift it alone, but if several
people help out, each person lifts a part of it.
Boats tend to have big flat surfaces on the bottom
(the hull of the boat), which helps them float.
Although they're heavy, they spread their weight
across a large surface in the water.
There's a second, and more complicated, part of
floating that has to do with pressure. If
you've ever tried to swim to the bottom of the
deep end of a pool, you know that your ears get
sore unless you equalize the pressure in them.
This is because the pressure of a fluid (like
water) increases as you go further down. If
there's a big object in water, the bottom of the
object (in deeper water) tends to get pushed up by
the higher pressure, and this also helps it float.
When a boat is in the water, there are two
forces acting on it: 1) weight (pushing down)
and 2) buoyancy (pushing up). The boat floats
in the water because the two forces are balanced
(weight = buoyancy). The "weight" force
exists because of gravity acting on the boat. The
"buoyancy" force is a little more complicated- it
exists because the boat takes the place of some of
the water it is floating in. The weight of this
water is more than the weight of the boat (even if
the boat is made of steel!), so the boat floats.
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