|Why do wood, cork, and ice always float?
|Question Date: 2002-03-20|
The simple answer to your question would be
buoyancy. Anything that floats on water
(I'm assuming you mean water) has more buoyancy
than water, by definition. A lot of different
things affect buoyancy, but one of these things is
air content. Wood and cork float for the same
reason that life jackets and Styrofoam float:
these materials have a lot of air in them, which
makes them extremely buoyant.
If you've ever looked at cork under the
microscope, you've probably noticed it has a lot
of holes. These holes trap air. It's the same
thing with wood and foam. You can think of these
materials as made up of thousands of tiny air
bubbles. Why ice floats is a little more
tricky. This is basically the same reason why
An ancient Greek mathematician named
Archimedes first came up with this principle while
sitting in the bath. He noticed that as he lowered
himself into the bath, the water level rose.
Basically, his body was displacing water. If the
object displacing the water (human, boat, ice
cube) weighs less than the water it displaces, the
object will float. You probably know that ice
expands when it freezes. Compared to water, then,
ice has more volume for the same mass, and so
weighs less than the water it displaces. This is
why ice floats. If water did not expand when it
froze, then ice would not float.
If you've ever been swimming in the ocean,
especially a really salty ocean like the
Mediterranean, you've probably noticed that you
float easier than in a pool. Can you think
why? Also, why do you think a feather
They are less dense than water. Pure
water has a density of 1 gram / cubic centimeter,
or 1 gram / milliliter. (1 milliliter = 1 cubic
centimeter). When water freezes, it expands into a
crystal lattice and its density is less than that
of liquid water. Cork and wood, too, are less
dense than water - although not all woods. Here is
an interesting fact for you: did you know that
the planet Saturn is less dense than water? It
is made of different ices, and its overall density
is less than liquid water. So, imagine if you
could make a bathtub big enough for the entire
planet, it would float!
Wood, cork, and ice float in water because
they are less dense than water. What does
that mean? Well, you've probably heard the old
trick question, "What weighs more: a pound of
lead or a pound of feathers?" A pound is a
pound, so of course they both weigh equal amounts.
But if I asked, "Which takes up more
space?" You'd tell me that the feathers do.
Scientists would say that feathers are less dense
than lead, because they take up more space for the
same weight. Now think about dropping a rock in a
glass of water full to the very top. Some of the
water will spill over, because the rock is now
taking up space at the bottom of the glass that
used to have water in it. However, if you drop an
ice cube in the water it will float. It floats
because it weighs less than amount of water it
would have to push out of the glass if it sank.
Wood, cork, and ice are all less dense than water,
and they float; rocks are more dense, so they
sink. Thanks for the question.
To explain this fact, I would ask you to
observe carefully what happens when you put any
object in water. The object starts to sink a
little. Then two things can happen. The object
stops sinking, it floats or the object sinks
completely. What is happening? As you
introduce the object into the liquid, some liquid
is "displaced", you need to make room for the
solid object. The liquid will push the object away
with a force equal to the weight of the displaced
volume of liquid. This is a law of physics and was
discovered by Archemedes in Ancient Greece. Now
whether the object sinks or floats depends on how
much it weights, compared to the weight of water
it displaces. If by displacing some volume of
water the object receives a push that equals its
own weight, it will float, like cork, ice or some
types of wood in water. Other objects even by
displacing their whole volume of water, do not get
enough push and therefore, they sink.
These substances float (in water) because
they are less dense than water. This means
that a cubic meter of water has more mass (and
thus weighs more on Earth) than a cubic meter of
cork, wood, or ice. Think about swimming in a
pool. You feel lighter in the pool than when
walking around on the ground. The is because when
you jumped into the pool you displaced a volume of
water that is equal to your volume. So you feel a
"buoyancy force" pushing you upwards that is equal
to the weight of water you displace. You feel
lighter because your "effective weight" in the
pool is your actual weight minus the buoyancy
force you experience. If you are less dense than
water, and thus weigh less that than the water
your body displaces, then you will float. If you
are completely submerged, the buoyancy force will
be larger than your weight and, if you don't work
to keep yourself submerged, you will rise until
you are only partially under water such that the
weight of the water you displace is equal to your
weight. Why do you think it is easier to float
if you take a big breath?
Think about what happens when something sinks
in a glass of water. When it does, the water
level rises because the object pushed all the
water away. All the water that got pushed out of
the way had to go up because it couldn't go
anywhere else and had to work against gravity to
All the water that gets pushed up has a weight
and the object that you dropped has a weight also.
The question you have to ask is: is the object
that you are putting into the water heavier or
lighter than the amount of water that would get
pushed out of the way and up if it sank? If
it is heavier, then the weight of the object will
cause it to sink. If it is lighter, then the
weight of the water will keep the object floating.
That is what is called buoyancy.
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