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SPH3U: Forces
Teacher Demo: Neutrally Buoyant Helium Balloon
Topics
Newton’s Laws
Bernoulli’s Principle
Buoyancy
Timing
preparation: less than 5 min
demonstration: 10-20 min.
Specific Expectations SPH3U
Introduction
Almost every child at one time or another has experienced the wonder, and later, a sense
of loss, from a helium balloon. The emotional attachment most students have to helium
balloons can be used to focus them on a variety of Physics concepts.
Adjust the balloon’s weight until it neither sinks nor rises and introduce it to one or more
classes (physics and/or science) during the day. There is much value to using the balloon
to focus discussion on scientific literacy and scientific investigation skills.
Encourage students to recall, use, and integrate a variety of concepts from previous
courses.
Use this demonstration as a reach ahead activity for topics yet to be discussed.
Materials
one helium balloon on a string, ribbon or stick
masking tape, paper clips or other masses which can easily be added or
removed to adjust the weight
scissors, to remove bits of tape or ribbon until the system is neutrally
buoyant
Safety Considerations
Check for latex sensitivity before selecting a balloon.
Pieces of rubber balloon can be a choking hazard. Pieces of burst
balloon or deflated balloons should be picked up and disposed of
properly.
Procedure
There are several demonstrations that can be completed:
Balanced forces when v = 0
a. Simply releasing the balloon usually elicits surprise, since the balloon neither
rises nor falls. A quick request for a Free Body Diagram elicits the question
“What is the upward force that balances the weight?”
b. Once the concept of buoyant force has been introduced, one can then talk about
the nature of this force. Archimedes Principle can be mentioned and the nature
of pressure in a fluid can be discussed.
c. The magnitude of the buoyant force caused by the displaced air can be
estimated by calculating the volume of the inflated balloon and multiplying this
value by the density of air ( ~1.2 kg/m3) to obtain the mass of the air displaced.
The weight of the air displaced can then be found using Fg = mg and this is
equivalent to the buoyant force, according to Archimedes Principle.
d. During the discussion, it will be noted that, due to the buoyancy of air, (1) the
spring scale alone is not always adequate when trying to determine the weight
of an object and (2) a balance is not always adequate when trying to determine
the mass of an object. (If these devices were used to measure the weight or
mass of the neutral balloon, the value would be zero!) Not many (including
teachers) realize that, when the mass of object is measured in Chemistry class
using a balance, the value obtained from reading the scale is smaller than it
should be by the mass of the air displaced by the object. The value obtained is
inaccurate, unless the measurement is performed in a vacuum.
2. Balanced forces when v ≠ 0
a. If there are drafts in the room, the neutrally buoyant balloon may move off at an
essentially constant speed. The same Free Body Diagram still applies.
i. If thermal convection currents cause the drafts, the discussion could be
about the rising warm air and sinking cool air, explained by kinetic
molecular theory and density.
ii. If the drafts are caused by the ventilation system in the room, then one
can simply sit back and watch the path the balloon follows because of
these air currents. In my room, there was an interesting vortex above the
main bench. The balloon seemed to orbit around a virtual attractive
centre.
b. Another way to get balanced forces with v ≠ 0 is to add a small weight (a bit of
tape) to the system. When released, the balloon will initially have a downward
unbalanced force that results in a downward acceleration. The acceleration,
however, is very short lived; the balloon quickly attains terminal velocity. The
frictional force due to air resistance quickly increases and, together with the
buoyant force, balances the force of gravity again. The magnitude of the
friction is of course equivalent to the weight of the bit of tape added. Using a
balance, the mass of the tape can be measured and its weight determined using
Fg = mg.
3. Bernoulli’s Principle
At some point during the demonstration, the balloon will drift away from the
desired location. To cause it to move back, rapidly wave the hand beside it in
the desired direction. The air rushing past the balloon will exert less pressure
on the balloon than the stationary air on the other side producing an
unbalanced force toward the waving hand.
4. Electrostatic Effects
The neutrally buoyant helium balloon can act as a large scale electroscope.
The balloon can be charged and another charged object can be brought close to
show its influence on the balloon’s motion.
5. Thermal expansion of a gas
The helium within the balloon can be heated by shining a heat lamp on it for a
certain time. The expansion of the gas causes more air to be displaced, thus
increasing the buoyant force. The balloon now rises due to the combined
buoyant effects of hot air and helium.
Disposal
Collect any pieces of balloon, tape, ribbon or plastic and dispose these in the garbage
basket.
What happens?
See above as part of Procedure.
How does it work?
The balloon and all its attachments are in a “sea of air” which has a pressure gradient due
to the gravitational field of Earth.
The top of the balloon experiences a slightly lower pressure than the bottom of the
balloon, so there is a net upward force called the buoyant force. The net force on the
balloon is the vector sum of the system’s force due to gravity and the buoyant force
(which is equivalent to the weight of the air displaced by the system).
By adding just the right amount of mass to the system, one can achieve a net force of zero
and the system will not accelerate when released. Once weight and buoyancy are
balanced, this large and very visible system becomes noticeably responsive to other
relatively small influences, such as:
a. Air currents, either thermal or mechanically driven, will move the balloon.
b. The pressure reduction, due to fluid motion from Bernoulli’s Principle, has a
noticeable effect.
c. Small electrostatic forces can also be used to move the balloon.
d. Often, the thermal gradient in the classroom will result in the balloon rising
only to a certain height (referred to as “the ceiling”) because the warm air is
slightly less dense.
Teaching Suggestions/Hint
1. Helium balloons are fun (maybe too much fun) and a little expensive, so I save
these for class demonstrations.
2. I usually keep the discussion light and qualitative. Many of the explanations
students use are primitive. It is tricky to ask them to integrate some of these new
concepts in their explanations of these observations without crushing their
enthusiasm.
3. There are several choices when selecting a balloon:
a. a simple uncoated rubber balloon will leak helium rapidly and will need
frequent adjustments to maintain neutral buoyancy. It will usually last in this
state less than 24 hours.
b. a rubber balloon which has been treated with a gel coating inside the balloon
will last for many days and needs much less frequent adjustment.
c. a Mylar balloon can last for months and will remain stable over a long period
once adjusted. This balloon is the best option when latex sensitivity is an issue.
Next Steps
1. The dynamics of the helium balloon used by Felix Baumgartner in his greaterthan-MACH 1 dive could be researched. Note: Felix jumped from a height where
the balloon stopped rising (it reached its ceiling and was neutrally buoyant).
2. Quantitative investigations can arise from the discussions. For example: The
relationship between air speed and wind resistance as outlined in __________.
Additional Resources
Facts about Felix Baumgartner’s balloon.
http://www.redbull.ca/cs/Satellite/en_CA/Article/Everything-you-need-to-know-aboutthe-Red-Bull-Stratos-balloon-021243269632712
Specific Expectations
SPH3U
A1.1 formulate relevant scientific questions about observed relationships, ideas,
problems, or issues, make informed predictions, and/or formulate educated
hypotheses to focus inquiries or research
C2.2 conduct an inquiry that applies Newton’s laws to analyse, in qualitative and
quantitative terms, the forces acting on an object, and use free-body diagrams to
determine the net force and the acceleration of the object
C2.3 conduct an inquiry into the relationship between the acceleration of an object and
its net force and mass, and analyse the resulting data
C3.3 state Newton’s laws, and apply them, in qualitative terms, to explain the effect of
forces acting on objects
C3.4 describe, in qualitative and quantitative terms, the relationships between mass,
gravitational field strength, and force of gravity
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