Unit 2 Fluids
Buoyancy
and
Archimede’s Principle
Buoyancy and Buoyant
Forces
When you are swimming in
water, there are two forces that
work against each other and
affect the motion of your body.

the force of gravity is pulling
you down
 the water is also pushing you
up with a buoyant force
Bouyancy

Buoyant force, or buoyancy, is the upward
force on objects submerged in or floating on
fluids.
 A buoyant force pushes away from the
centre of Earth.
Bouyancy
Bouyancy

an object will float if its buoyant force,
when fully immersed, is greater than its
weight (gravitational force)
 it will sink if its weight is greater than the
buoyant force
 it will float when the buoyant force is equal
to its weight (or the force of gravity)
Archimede’s Principle
This principle
explains why
some objects
float in water
and others
sink.
Salt vs. Fresh water

Seawater (salt water) has a density of 1.03 g/mL
and fresh water has a density of 1.00 g/mL.

Therefore, one litre of salt water weighs more than
one litre of fresh water.

That is, salt water can support more weight per
volume than fresh water, so it is easier to float in
salt water.
Average Density
The average density of an object is the total mass of
all substances that make up the object divided by the
total volume.
 Average density results in objects that would
normally sink being able to float.

Examples of technologies that have been
developed because of our understanding of density
and buoyancy include:
1. Ships
Ships can be built of steel
because their hollow hull
ensures that the average
density of the ship is less
than that of water.
2. Personal Floatation devices
(Life jackets)

Personal flotation devices
(ex.Life jackets) are filled
with a substance of very
low density.

This way, a life jacket
lowers a person’s average
density, allowing the
person to float.
3. Submarines
o
By allowing water to flow
in or out, a submarine can
rise or sink in the water.
o
The submarine floats when
its weight is equal to the
buoyant force and it sinks
when its weight is greater
than the buoyant force.
4. Hot Air Balloons
When the air inside a hot-air
balloon is heated, the air
particles:
• gain energy and
• spread out (forcing some of the
particles out of the balloon)
The air inside the balloon becomes
less dense than the air surrounding
it, so it rises.
Average Density
So:
 an object will float if its average density is less
than the fluid in which it is immersed

an object will sink if its average density is denser
than the fluid in which it is immersed

when the object’s density is the same as the
medium, an object will neither sink nor float; it is
said to be neutrally buoyant.
Sink or Float?

wooden boats vs. a water logged stick

metal block vs. metal boats

a sealed, empty plastic bottle vs. a plastic
bottle full of water