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Floating and Sinking
Think about this…
 How is possible for a huge ocean liner
like the Titanic to float?
 How is possible in a few hours for it to
become a sunken wreck?

Buoyancy
If you have ever picked up an object in
water, you know that it seems lighter in
water than in air.
 Water exerts a force called the buoyant
force that acts on a submerged object.
 The buoyant force acts in the upward
direction, against the force of gravity, so
it makes an object feel lighter.

Take a look…

Turn to page 91 and look at Figure 13.
 As you can see in Figure 13, a fluid exerts
pressure on all surfaces of the submerged
object.
 Since the pressure in a fluid increases with
depth, the upward pressure on the bottom of
the object is greater than the downward
pressure on the top.
 The result is a net force acting upward on the
submerged object. This is the buoyant force.
Figure 14
A submerged object displaces, or takes
the place of, a volume of fluid equal to
its own volume.
 Look at Figure 14. An object that floats
on the surface of a fluid, however
displaces a smaller volume.
 It displaces a volume of fluid equal to
the portion of the object that is
submerged.

Archimedes’ principle
Archimedes’ principle relates the
amount of fluid a submerged object
displaces to the buoyant force on the
object. This relationship is named for its
discoverer, the ancient Greek
mathematician Archimedes.
 Archimedes principle states that the
buoyant force on an object is equal to
the weight of the fluid displaced by the
object.

Floating and Sinking

Remember that 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 net
force on a submerged object will be downward and
the object will sink.
 If the weight of the object is less than the buoyant
force, the object will begin to sink. It will only sink
deep enough to displace the volume of fluid with a
weight equal to its own.
 At that level, it will stop sinking deeper, and will float.
 If the weight of the object is exactly equal to the
buoyant force, the two forces are balanced.
Density
Exactly why do some objects float and
other sink?
 By comparing density of an object to the
density of a fluid, you can decide if it will
float.
 What is density? The density of a
substance is its mass per unit volume.
 Density =
Mass
Volume

Examples
The density of lead = 11.3g = 11.3g/cm
1cm
The density of cork is .25 g/cm
You could say that lead is more dense
than cork.
The density of water is 1.0 g/cm so it is
less dense than lead but more dense
than cork.

Figure 15

By comparing densities, you can explain the
behavior of the objects shown in Figure 15.
 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.
 And 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.
Instead it floats at a constant level.
Figure 16
Figure 16 shows several substances
and their densities. Notice that liquids
can float on top of other liquids. You
may have seen that salad oil floats on
top of vinegar.
 Objects with the greatest densities are
near the bottom of the cylinder.

Gasses
Don’t forget that air is also a fluid.
Objects float in air if their densities are less
than the density of air. A helium balloon rises
because helium is less dense than air.
 An ordinary balloon filled with air, however is
more dense than the surrounding air because
it is under pressure. So the balloon falls to
the ground once you let go of it.
 Changing the density of an object can make it
float or sink in a given fluid.


Submarines
The density of a submarine, for
example, is decreased when water is
pumped out of its flotation tanks. The
overall mass of the submarine
decreases. So the submarine will float
to the surface.
 To dive, the submarine takes in water.
In this way it increases its mass and
density and sinks.

Buoyancy and Density

Another way to change density is to change
volume.
 In figure 17, the amount of steel present in
the three objects is the same. Yet two of the
figures float, and one sinks. Solid steel sinks
rapidly in water, and so will the hull of a ship
that is full of water.
 Usually however, the hull of a ship contains a
large volume of air. This air reduces the
ship’s overall density and so allows it to float.