4. Investigation 4B
Sink or float
• Steel is more dense than water so why do
steel boats float?
5. 4.2 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 weight
of this rock?
What is the rock’s
mass?
6. 4.2 Weight and buoyancy
• 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.
7. 4.2 Weight 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?
8. 4.2 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.
9.
10. 4.2 Weight and buoyancy
• What is the buoyant
force on a rock with a
volume of 1,000 cm3?
• In air, the scale shows
the rock’s weight as
29.4 newtons.
• When the rock is
completely submerged,
the scale reads 19.6
newtons.
• The difference is a
force of 9.8 newtons.
11. 4.2 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. 4.2 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.
13. 4.2 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?
14. 4.2 Density and buoyancy
• Average density determines 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.
What can you say about
the average density of
these blocks?
15.
16. 4.2 Boats and average density
• Use your understanding of average
density to explain how steel boats can be
made to float.
17. 4.2 Boats and average density
• If you have seen a loaded cargo ship, you might
have noticed that it sat lower in the water than
an unloaded ship nearby.
• A full ship has more mass than an empty ship.
• This means a full ship must displace more water
(sink deeper) to make the buoyant force large
enough to balance the ship’s weight.
18. Physics Connection
Airships in the Sky
• Anything that flies is an aircraft.
• The hot air balloon was the first aircraft invented.
• A blimp is a type of airship, like a hot air balloon.
• Read more about balloons and blimps!
