C4.2 Buoyancy
Physical Science
C4.2 Buoyancy
• Supplies: Pencil and Science Journal
• Standards:
– 8c) buoyant force on an object in a fluid is an upward force
equal to the weight of the fluid the object has displaced
– 8d) students can predict whether and object will float or
sink
– 9f) Apply simple mathematical relationships to determine
a missing quantity in a mathematical expression
C4.2 Buoyancy
• You will be taking notes using Cornell Notes format or
Charting Notes format.
Warm Up
1. Find the density of an object if its mass is
143 g and its volume is 13.8 mL.
2. Calculate the mass of matter with a density
of 1.2 g/cm3 and a volume equal to 76 cm3
C4.2 Buoyancy
• Definitions
• Weight: the downward force of gravity acting
on a mass
• Buoyancy: the measurement of the upward
force a fluid exerts on an object that is
submerged.
• Archimedes’ Principle: states that the buoyant
force is equal o the weight of the fluid
displaced by an object.
Buoyancy
• Mass vs weight
Weight and buoyancy
• A force is a push or a
pull
• Buoyant force is the
upward push a fluid
exerts on an object that
is submerged.
• Buoyancy is the
measure of a buoyant
force
Your Own Words
• What is the difference between buoyant force
and buoyancy?
Weight and buoyancy
• Depends on how much of the object is underwater
• As you push a beach ball down into the water, the
buoyant force gets stronger
Imagine this
You are in a swimming pool, with your hands on
the edge at the deep end. What happens as
you fill your lungs with air? What happens
when you exhale the air?
Write down your answer. Then compare the
answer with your neighbor.
Archimedes’ Principle
• 3rd century, Greek
mathematician
• Buoyant force is equal
to the weight of the
fluid displaced by an
object
Sinking and floating
• If the buoyant force is
greater than an objects
weight, it floats. Positive
buoyancy
• If the buoyant force is less
than an objects weight, it
sinks. Negative buoyancy.
• If the object stays where
you place it in the fluid,
Neutral buoyancy.
What floats
• List 3 things that float in a pool.
• List 3 things that sink to the bottom of a pool.
Density and buoyancy
• If you know an objects
density, you know if it
will sink or float.
• Water has a density of
1.00 g/cm3
• Consider average
density
What is a Fluid
• Buoyancy doesn’t just
apply to water.
• A fluid is a liquid or a
gas.
• Heavier gases sink
toward the ground
Why
• Why does a hot air balloon float?
• What makes it stop going up?