PROPERTIES OF MATTER 12.3
Chapter Twelve: Properties of
Matter
12.1 Properties of Solids
12.2 Properties of Fluids
12.3 Buoyancy
Chapter 12.3 Learning Goals
Define buoyancy.
Explain the relationship between
density and buoyancy.
Discuss applications of
Archimedes’ principle.
Investigation 12B
Buoyancy of Fluids
Key Question:
Can you make a clay boat?
12.3 Buoyancy is a force
Buoyancy is a measure of the
upward force a fluid exerts on an
object that is submerged.
The water in the pool
exerts an upward
force that acts in a
direction opposite to
the boy’s weight.
12.3 Volume and buoyancy
The strength of the buoyant force on an
object in water depends on the volume of
the object that is underwater.
As you keep pushing downward on the ball, the
buoyant force gets stronger and stronger. Which
ball has more volume underwater?
12.3 Weight and buoyancy
Weight is a force, like any
other pushing or pulling
force, and is caused by
Earth’s gravity.
It is easy to confuse mass
and weight, but they are not
the same.
Weight is the downward
force of gravity acting on
mass.
What is the rock’s
weight?
What is the rock’s
mass?
12.3 Weight and buoyancy
In the third century BC, a
Greek mathematician named
Archimedes realized that
buoyant force is equal to the
weight of fluid displaced by
an object.
A simple experiment can be
done to measure the buoyant
force on a rock with a spring
scale when it is immersed in
water.
12.3 Weight and buoyancy
In air the buoyant
force on the rock is
29.4 N.
When the rock was
submerged, the scale
read 19.6 N.
The difference is a
force of 9.8 N, exactly
the amount of force
the displaced water
exerts.
12.3 Weight and buoyancy
These blocks are the same total volume.
Which block has more buoyant force acting on it?
Which block weighs more in air?
12.3 Weight and buoyancy
Buoyancy
explains why
some objects sink
and others float.
Whether an object
sinks or floats
depends on how
the buoyant force
compares with
the weight.
12.3 Density and buoyancy
If you know an object’s density you
can quickly predict whether it will
sink or float.
Which ball will sink in water?
Which ball will float in water?
12.3 Density and buoyancy
Average density helps determine
whether objects sink or float.
 An object with an average density
GREATER than the density of water will
sink.
 An object with an average density LESS
than the density of water will float.
12.3 Density and buoyancy
What can you say about the
average density of these blocks?
12.3 Density and buoyancy
When
they the
are steel
completely
underwater,
However,
ball has
more
both
balls
have
buoyant
weight
since
it the
has same
a higher
density.
force because they displace the same
volume of water.
12.3 Boats and average density
Use your understanding of average
density to explain how steel boats
can be made to float.
12.3 Boats and average density
If
A you
full ship
havehas
seen
more
a loaded
masscargo
than an
ship,
empty
you
might
ship. have noticed that it sat lower in the
water than an unloaded ship nearby.
This means a full ship must displace more
water (sink deeper) to make the buoyant
force large enough to balance the ship’s
weight.
Investigation 12C
Density of Fluids
Key Question:
What is the maximum load a boat can hold before
sinking?
How is the maximum load affected by the density
of the water in which the boat floats?
The Hull
There are many different
types of boats, but all
have one thing in
common—the hull.
The hull is the main body
of the boat. It displaces
the water that provides
the upward buoyant
force. It also provides
stability.