• Density is a measure
of the amount of
matter in a particular
volume.
• It depends on two
things:
• So, using these two things…
and
We can calculate the density of an object!
Use this density triangle
to figure out the proper
equation.
An object placed in a fluid
experiences an upward force
called BUOYANT FORCE [Bf].
Pressure acting
at the sides are
equal.
Upward force is
greater because of
greater pressure =
Fluid pressure
increases with
depth.
Fluid Pressure Creates Buoyancy
• The deeper you go in the water,
the greater the fluid pressure.
• Therefore, there is greater
pressure on the bottom of an
object. This causes the upward
force on the object (buoyancy).
Archimedes, a Greek
Mathematician,
studied the force of
buoyancy. He came up
with this PRINCIPLE:
“The BUOYANT FORCE
(Bf) acting on an
object is equal to the
WEIGHT of the fluid
DISPLACED by the
object.”
Archimedes
supposedly run
out to the street
naked!
ILLUSTRATION
The weight of fluid
collected is equal
to the Bf acting on
the object
submerged.
• The WEIGHT
of the water
the man
displaces is
equal to the
SIZE of the
Bf acting on
him.
KINDS OF BUOYANCY
Positive
Buoyancy:
Neutral
Buoyancy:
Negative
Buoyancy:
How do we know if something
will sink or float?
Does SINKING or FLOATING depend
on the object’s weight?
NO. It depends on
the DENSITIES of
the object and the
fluid where it is
placed.
Floating: The Bottom Line
• A substance will float in any liquid if
it is less dense than the liquid itself!!
• Test 1) Which of the solids will
float in olive oil?
• Answer: pine and lithium
• Test 2) Which of the solids will sink
in mercury?
• Answer: osmium and gold
• Test 3) Which liquids will float in
water?
• Answer: oil and ethanol
Floating Applies to Gases Too!
• Test 4) Which of
these gases would
float in air?
• Answer: Helium and
Hydrogen
Density (g/mL)
•
•
•
•
•
Hydrogen 0.00009
Helium 0.00018
Air 0.00129
Oxygen 0.00143
Chlorine 0.00321