DENSITY, VISCOSITY, and
BUOYANCY
Adapted from TAKS Study Guide
IPC 7A
O.K., what is density?
Density is a measure of a substance’s mass per
unit of volume. A dense object has much more
mass in a given space than an object that isn’t
very dense.
IPC 7A
Density example #1:
Aluminum isn’t very dense. That’s why
empty aluminum soda cans don’t have
much mass. If soda cans were made out
of a dense metal, such as gold or lead,
they would be much heavier than
aluminum cans, even if they were the
same size.
IPC 7A
Density example #3:
Suppose you are going to buy a gold
necklace at a discount jeweler. The clerk
claims that the necklace is made of pure
gold. How could you tell if he is telling the
truth? Calculate the density of the
necklace and compare it to the density of
pure gold.
IPC 7A
How do I solve density problems?
You can calculate an object’s density by dividing its
mass by its volume.
mass
Density 
volume
Density has 2 different units: g/mL or g/cm3
g= grams (mass)
mL= milliliters (volume)
cm3= cubic centimeters (volume)
IPC 7A
Density calculation #1
A graduated cylinder containing 20 milliliters of
mineral oil has a mass of 98.2 grams. The mass
of the empty cylinder is 79.8 grams. What is the
density of the mineral oil?
Given:
a. Empty Cylinder Mass
79.8 g
b. Mass of cylinder of mineral oil 98.2 g
c. Mass of mineral oil (b-a)
18.4 g
Unknown:
Equation:
d. Volume of mineral oil
D=?
D = m/v
Substitute:
Solution:
D = 18.4 g/20mL
D = 0.92 g/mL
IPC 7A
20 mL
Viscosity
Viscosity is the resistance of a liquid to flow. Water
is less viscous than honey, for example,
because water flows more easily.
More viscous
Less viscous
IPC 7A
Viscosity
Two factors that help determine a liquid’s
viscosity are its cohesiveness and the size
of its molecules. The tendency of water
molecules to stick together—water’s
cohesiveness—makes water more viscous
than it would be if it were composed of
nonpolar molecules. One reason that
water is less viscous than honey, however,
is that the molecules in honey are much
larger than water molecules and flow past
one another less easily.
IPC 7A
And what about buoyancy?
What’s that?
When an object is submerged in water, the
water exerts a force on all sides of the
object. This force increases with depth, so
the force at the top of the object is lower
than the force at the bottom of the object.
This means that the overall direction of the
force is upward. This upward force is
called the buoyant force.
IPC 7A
The force at the bottom
of a submerged object is
greater than the force at
the top. The net force –
the buoyant force – is
upward.
IPC 7A
When wood floats, the
buoyant force on the wood
is greater than the force of
gravity on the wood.
When the buoyant force pushing up on the
object is greater than the force of gravity
pulling down on the object, the object rises
to the surface. If the buoyant force is less
than the force of gravity, the object sinks
to the bottom.
IPC 7A
IPC 7A
How do we know which is greater,
the force of gravity or the buoyant
force?
We can determine which force is greater by
comparing the density of the object to the
density of water. If the density of the object
is greater than the density of water, the
force of gravity on the object will be
greater than the buoyant force, and the
object will sink. If the density of the object
is less than the density of water, the force
of gravity on the object will be less than
the buoyant force, and the object will float.
IPC 7A
The greater force?
Buoyant Force or Force of Gravity
For example, vegetable oil with a density of
0.93 g/cm3 will float on water, which has a
density of 1.0 g/cm3.
0.93 g/cm3 < 1.0 g/cm3
IPC 7A
The greater force?
Buoyant Force or Force of Gravity
For example, wood with a density of 0.4
gram per cubic centimeter will float on
water, which has a density of 1.0 gram per
cubic centimeter.
0.4 g/cm3 < 1.0 g/cm3
IPC 7A