Floating and Sinking
Floating and Sinking
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How is it possible that huge ships made
of steel can float easily in water?
Yet in a few hours the same ship can
become a sunken wreck like the Titanic.
Why does most of an iceberg lie hidden
beneath the surface of the water?
To answer these questions, you need to
find out what makes an object float and
what make an object sink.
Comparing Densities
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One reason objects float or sink is their
density.
An object that is more dense than the fluid in
which it is immersed sinks.
An object that is less dense than the fluid in
which it is immersed floats to the surface.
If the density of an object is equal to the
density of the fluid in which it is immersed,
the object neither rises nor sinks in the fluid;
it floats at a constant level.
Floating and Sinking
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Now you know why lead sinks: it is
several times denser than water.
Cork, which is less dense than water,
floats.
Is an ice cube more or less dense than
water?
Iceberg straight ahead!
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An ice cube floats in water because the
density of ice is less than the density of
water.
But it’s just a little less!
So most of a floating ice cube is below the
surface.
Since an iceberg is really a very large ice
cube, the part that you see above water is
only a small fraction of the entire iceberg.
This is one reason why icebergs are so
dangerous to ships.
Buoyant Force
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If you have ever picked up an object
under water, you know that it seems
lighter in water than in air.
Water exerts a force called buoyant
force that acts on a submerged object.
Buoyant force acts in the upward
direction, against the force of gravity, so
it makes an object feel lighter.
Weight vs. Buoyant Force
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There is always a
downward force on a
submerged object.
That force is the weight
of the object.
If the weight of the
object is greater than
the buoyant force, the
object will sink.
If the weight of the
object is less than the
buoyant force, the
object will begin to float
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What can you infer about
the weight and buoyant
force from the last picture?
Archimedes’ Principle
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You know that Archimedes discovered that a
submerged object displaces, or takes the
place of, a volume of fluid equal to its own
volume.
Using this idea, Archimedes came up with a
principle that relates the amount of fluid a
submerged object displaces to the buoyant
force on the object.
It states that the buoyant force on an object
is equal to the weight of the fluid displaced
by the object.
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Therefore, the more fluid an object displaces,
the more buoyant force it will have.
The more surface area an object occupies,
the more water is displaces.
This is why it is easier to float in water when
you are lying down on your back – you take
up more surface area, displacing more water,
therefore creating a greater buoyant force.
Floating by changing mass
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Changing the density of an object can make it
float or sink in a given fluid.
Even though you cannot change the density
of an individual substance, you can change
the overall density of things that are made of
several materials.
Submarines can change their overall mass,
and therefore their density to allow them to
float and sink.
They do this by filling or releasing water from
their flotation tanks.
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When water is pumped out of its flotation
tanks, the overall mass of the submarine
decreases.
Since the volume remains the same, its
density decreases when its mass decreases.
So the submarine will float to the surface.
What do you think a submarine does in order
to sink?
To dive, the submarine takes in water.
In this way, it increases its mass (and thus its
density) and sinks!
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Most ships are also designed to alter
their overall density.
The hull (bottom part) of a ship
contains a large volume of air.
This air reduces the ship’s overall mass,
and therefore its density, and helps it
to float.
Floating by changing volume
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Another way of changing density is to change
volume.
If the mass is kept the same, increasing the volume
of a substance, will decrease its density.
Increasing the surface area of a substance, will also
increase its buoyant force.
This is because the greater the volume of water
displaced, the greater the buoyant force (Archimedes
Principle).
The shape of a ship causes it to displace a greater
volume of water than a solid piece of steel of the
same mass because it has a greater surface area.
The more water it displaces, the easier it will float!
A ship stays afloat as long as the buoyant force is
greater than its weight.
The Titanic…
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