Density and Buoyancy
Thank you to Robin Paul for sharing many of these slides!
Density: Density of a fluid is its mass per unit of volume. It’s the
amount of mass (stuff) in a given space.
Density = mass d = m
The units for density are grams/ml or grams/cm3
Comparing Densities: Each substance has a particular density that is
characteristic of that substance.
•The density of water is 1gram per cubic centimeter (1g/cm3
•A substance with a density greater than 1g/cm3
will sink in water.
•A substance with a density less than 1g/cm3
will float in water.
•A substance with a density equal to 1g/cm3
will float at a
constant depth in water.
Buoyancy: This is the ability to float. Fluids exert an
upward force, called the buoyant force, on submerged
At rest, the gravitational force and
the buoyant force are equal and
opposite. The net force on the surfer
plus the board is zero.
•The buoyant force acts in a direction opposite to gravity
(weight) and thus makes an object feel lighter.
•Fluid exerts pressure on all surfaces of a submerged object.
Since pressure increases with depth, however, there is greater
pressure on the bottom of an object than at the top. (The pressures
on each side cancel out.) The greater bottom pressure creates a
net upward force called the buoyant force.
Archimedes’ Principle: the buoyant force acting on an object
that is submerged is equal to the weight of the volume of fluid
the object displaces.
•When an object is placed in a fluid, it takes up space and takes the
place of some of the particles of the fluid.
•The submerged object displaces (takes the place of) a volume of
liquid equal to its own volume.
•If you measure the weight of the displaced liquid it will be equal to
the buoyant force acting on the object.
“just the tip of the iceberg”
• This iceberg weighs
100,000 pounds. The
weight of the water
displaced by the
submerged section of
the iceberg weighs
100,000 pounds. The
iceberg will sink until
100,000 pounds of
water has been
•Submarines: By changing the level of water in its flotation
tanks, the sub changes its weight. When its weight is greater than
the buoyant force it sinks. It rises when its weight is equal to the
•Balloons: A balloon filled with air is denser than the surrounding
air because it is under pressure. Because it is denser it sinks. If
you heat the air in the balloon, it becomes less dense and rises.
•Ships: Since the buoyant force is equal to the amount of fluid
displaced, a large object, like a ship, will have a greater buoyant
force than a smaller object of the same weight. If you had a block
of steel the same weight as a ships hull, the ship would displace
more fluid and float while the block would sink.