Archimedes (c. 287-212 BC) was the first to formally declare that an object immersed in
a liquid is buoyed or thrust upwards by a force equal to the weight of the liquid
This principle is widely used to explain not only buoyancy and flotation, but it also
forms the basic principle upon which convection depends.
For instance, we observe that whenever portions of a fluid such as a gas or a liquid
become warmer than the surrounding fluid, convection occurs. This is because warmer
portions of the medium expand and displace the cooler (and therefore denser)
Let’s consider air. As predicted by Archimedes, the difference in the weight of the warm
air and cooler displaced air creates a net buoyant force which acts on the warmer region,
causing it to rise. This effect is generally known as convection.
Convection can be easily observed. In fact, we observe the effect so often that we
commonly use the expression “heat rises”. What we really mean is that warm air rises.
The clouds that form towering thunderstorms (cumulonimbus clouds) are formed entirely
by convection and the turbulent activity in a boiling kettle is a manifestation of
convection (enhanced by the action of the gas bubbles of steam).
We know from experience that convection works well on earth. In the winter Canadians
heat their homes with systems that depend upon convection to circulate either hot air or
hot water, although they are usually supplemented with fans or pumps to enhance the
Many household appliances depend upon convection to prevent them from overheating.
The active components of TVs and toasters are good examples of devices that would melt
or burn unless they were cooled by convection.
The question we will investigate asks how effective is convection in cooling instruments
and other devices used in the weightless environment of space? Of course the best way
to get a definitive answer to this question might be to perform a series of experiments in
space. The difficulty with this idea is that it is also a very expensive way to investigate
the question. We need another approach to simulating a weightless environment.
To investigate convection in a weightless environment.
The simplest way to simulate a weightless environment is to duplicate the phenomena in
exactly the same way that astronauts in orbit create a weightless environment, namely, in
Recall, that convection is a manifestation of buoyancy, and therefore to investigate
convection we will look closely at buoyancy in a free-fall (i.e. weightless) environment.
1 plastic peanut butter jar with lid (NEVER USE GLASS!)
1 plastic soda straw
construction paper and scissors
1 ping-pong ball
5-minute epoxy or similar glue
large cardboard box
large plastic garbage bags or plastic sheet
small drill with bits
some (2 or 3) discarded foam pillows
optional: digital camera with flash
1. Begin by gluing the ping-pong ball on to the end of the soda straw. Epoxy works well
but smells bad and should be used in a ventilated area or large open space.
Alternatively, a low temperature glue gun may be used. (Do not use a high
temperature glue gun).
2. Next, drill a small hole, large enough for the soda straw to pass through freely, in the
exact centre of the jar’s lid.
3. Finally pass the straw through the lid and glue a small paper flag to the top of the
soda straw as shown in the diagram.
1. Remove the lid and ping-pong assembly from
the jar and fill it with clean tap water.
2. Replace the lid and ping-pong assembly
securely over a sink or basin since the water
may overflow when the lid is screwed down.
3. Push the ping-pong ball to the bottom of the jar
by gently pushing down on the soda straw.
Release the straw quickly to verify that the ping-
pong ball “pops” smartly to the top of the jar. If
it does not, determine the reason and make any
4. Line the cardboard box with the pillows and
cover them with some plastic sheeting or plastic
garbage bags to prevent them from getting wet
in the event of a water spill.
5. Find a safe location from which the jar may be
dropped. Make sure that you cannot fall and also
that there is no danger of the bottle being
dropped on an observer.
Procedure (the Experiment):
1. Set up a location in which you can drop the jar into a cardboard box.
2. Depress the ping-pong ball to the bottom of the jar.
3. Align the jar so that it will drop into the centre of the box.
4. Simultaneously release the jar and the soda straw.
If you have access to a digital camera, try to capture a photograph of the bottle in mid-
Explanation and Discussion
The bottle can be dropped many times without damage provided that it is adequately
protected by padding in the landing box.
Even without a camera students can see that the ping-pong ball remains at the bottom of
the jar during the fall. It is a curious effect, but the explanation is simple. In free-fall
objects have no apparent weight; therefore the weight of the water displaced by the ping-
pong ball is zero. According to Archimedes, if the displaced weight of the water is zero,
the buoyant force is zero. The ping-pong ball won’t float.
This effect has a significant and serious consequence for astronauts in space. Because of
the effect of weightlessness, there are no buoyant forces and hence no auto-convection
inside orbiting spacecraft.
Convection, which is important for cooling and heating on Earth, is absent in spaceflight.
Both human and robotic missions in space must have either forced air flow for cooling or
have alternative methods of dissipating heat.
Grades 11/12: Force, Motion and Work
apply Newton’s laws of motion to explain inertia, the relationship between force,
mass, and acceleration, and the interaction of forces between two objects;
analyse and describe examples where technologies were developed based on
carry out procedures controlling the major variables and adapting or extending
procedures where required;
select and use appropriate numeric, symbolic, graphical, and linguistic modes of
representation to communicate ideas, plans, and results;
work cooperatively with team members to develop and carry out a plan, and
troubleshoot problems as they arise.
R Level 1 Level 2 Level 3 Level 4
significantly below approaches the standard the standard exceeds the standard
(below 50%) (50-59%) (60-69%) (70-79%) (80-100%)
Understanding of Basic Concepts
produces insufficient demonstrates limited demonstrates some demonstrates general demonstrates
evidence to understanding of understanding of understanding of thorough
demonstrate learning Archimedes' Archimedes' Archimedes' understanding of
principle principle principle Archimedes'
significant by providing partial by providing partial by providing by providing
misconceptions explanations of the explanations of the complete complete
physical laws that physical laws that explanations of the explanations of the
requires additional cause the ping-pong cause the ping-pong physical laws that physical laws that
learning activities ball to remain at the ball to remain at the cause the ping-pong cause the ping-pong
and remediation bottom of the jar bottom of the jar ball to remain at the ball to remain at the
during the fall during the fall bottom of the jar bottom of the jar
during the fall during the fall
with significant with minor with no significant with no
misconceptions misconceptions misconceptions misconceptions
Application of Critical and Creative Thinking Skills and/or Processes
produces insufficient applies appropriate applies appropriate applies appropriate applies appropriate
evidence to skills/strategies to skills/strategies to skills/strategies to skills/strategies to
demonstrate learning investigate, observe, investigate, observe, investigate, observe, investigate, observe,
and describe and describe and describe and describe
requires additional buoyancy of a ping- buoyancy of a ping- buoyancy of a ping- buoyancy of a ping-
learning activities pong ball in a free- pong ball in a free- pong ball in a free- pong ball in a free-
and remediation fall (i.e. weightless) fall (i.e. weightless) fall (i.e. weightless) fall (i.e. weightless)
environment with environment with environment with environment with a
limited effectiveness some effectiveness considerable high degree of
Communication of Required Knowledge
produces insufficient communicates communicates with communicates with communicates
evidence to unclearly or some clarity and general clarity and clearly and precisely
demonstrate learning imprecisely precision precision
rarely using sometimes using usually using always using
requires additional appropriate scientific appropriate scientific appropriate scientific appropriate scientific
learning activities conventions, conventions, conventions, conventions,
and remediation vocabulary, and vocabulary, and vocabulary, and vocabulary, and
terminology terminology terminology terminology
Application of Required Knowledge
produces insufficient makes very simple makes simple makes connections makes complex
evidence to connections between connections between of some complexity connections between
demonstrate learning concepts related to concepts related to between concepts concepts related to
physical forces and physical forces and related to physical physical forces and
demonstrates their implications for their implications for forces and their their implications for
significant both human and both human and implications for both both human and
misconceptions robotic missions in robotic missions in human and robotic robotic missions in
space space missions in space space