Download Constant Forces

Lesson: Constant Forces (Testing Exp.)
1. NJ standards addressed in the lesson:
5.1.12.B.1: Design investigations, collect evidence, analyze data, and evaluate
evidence to determine measures of central tendencies, causal/correlational
relationships, and anomalous data.
Students will be conducting an application experiment to determine if a balloon filled with
air exerts a constant force. Students will have to design an experiment(aka build balloon
car and measure mass, distance traveled, time force is exerted, etc) to determine if force
exerted by balloon on cart is constant.
5.1.12.B.4: Develop quality controls to examine data sets and to examine evidence
as a means of generating and reviewing explanations
AND
5.1.12.D.2: Represent ideas using literal representations, such as graphs, tables,
journals, concept maps, and diagrams.
AND
8.1.12.A.1: Construct a spreadsheet, enter data, and use mathematical or logical
functions to manipulate data, generate charts and graphs, and interpret the
results.
Students will have to present data with experimental uncertainties and graphs to pack their
judgment.
5.1.12.C.2: Use data representations and new models to revise predictions and
explanations.
Students will be taking an idea and conducting an experiment to test its validity, or rather,
to attempt to disprove it.
5.2.12.E.1 & 5.2.12.E.4: Compare the calculated and measured speed, average
speed, and acceleration of an object in motion, and account for differences that
may exist between calculated and measured values. AND Measure and describe
the relationship between the force acting on an object and the resulting
acceleration.
Students will be looking at whether a balloon when filled with air and released exerts a
constant force by making calculations involving time, distance, acceleration, and average
velocity.
2. What students should know before they start the lesson:
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Newton’s Second law and how to calculate applied force from acceleration and mass.
Kinematics with constant forces
Newton’s third law
Friction Forces
3. Goals of the lesson
Content:
Goals
Standards Addressed
Properties of constant acceleration
5.2.12.E.1 & 5.2.12.E.4,
Multiple representations of objects in motion
5.1.12.D.2
Calculating average velocity
5.1.12.D.2, 8.1.12.A.1,
5.2.12.E.1 & 5.2.12.E.4
Procedural:
Goals
Standards Addressed
Conducting a Testing experiment
5.1.12.B.1,5.1.12.B.4,
5.1.12.C.2
Determining sources of uncertainty
5.1.12.B.1,5.1.12.B.4,
8.1.12.A.1
Analyzing assumptions
5.1.12.B.1
Revise explanations
5.1.12.C.2
Epistemological:
Goals
Standards Addressed
How do we know there is a constant force being applied?
5.1.12.B.1,5.1.12.B.4,
5.1.12.C.2
What is the difference between velocity and average velocity?
5.2.12.E.1 & 5.2.12.E.4
When can the kinematic equations be used?
5.2.12.E.1 & 5.2.12.E.4
What error is due to experimental error? Assumptions?
5.1.12.C.2
Metacognitive:
Goals
Standards Addressed
Why is it important to build an accurate device?
8.1.12.A.1
Why is it important to have accurate data?
8.1.12.A.1
How do I know the explanation is wrong?
5.1.12.C.2
4. Most important ideas
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Designing an apparatus which will accurately represent the data you are looking for or test
your explanation.
When an error in data is present, is it because an explanation which is being tested is incorrect
or is it because an assumption you made is incorrect and affecting data.
Using graphing programs to portray your data with the purpose of explaining your conclusion
or judgment of the statement being tested.
5. Student potential difficulties:

Knowing to determine average velocity rather than instantaneous velocity.
 The process in which they will be taking data will consist of two parts, the acceleration
phase, when the air is leaving the balloon, and the coasting phase, when the car is moving
by itself. In both parts, technically, the car is never moving at a constant velocity, though in
the coasting part, the velocity may only be changing slightly. So students need to realize
that because the velocity is changing over time, this results in the necessity to calculate
average velocity. This will be dealt with by making sure the students realize that one value
for time is not enough to determine velocity, but rather a time interval is necessary.

Understanding how to disprove the explanation.
 The explanation states that a balloon releasing air exerts a constant force on whatever
object it is interacting with. So when students test this, they will need to differentiate
between experimental errors and what is not working because the explanation is wrong.
Students will achieve this by assessing their assumptions and very carefully accounting for
uncertainties. In the end the uncertainties are what will determine if the data is incorrect
or if the explanation is.

How to build a device that will best help with the experiment.
 Students will be required to build an apparatus to salve a problem. This situation will call
for the simplest possible solution, as the more parts there are, the more uncertainties
there are and therefore the harder it will be to determine make a judgment about outcome
and whether it supports or disproves the explanation.
6. Equipment needed:
Student Use
 Push pop pistons
 Manila folders
 Tape
 Straws
 Rubber bands
 Balloons
 Meter sticks
 Stop watches
Teacher use
 Blackboard (if necessary)
 Lab Handout
7. Lesson description:
Balloon Forces!
Lab Goals:
 Construct a testing experiment to test a hypothesis
 Collect and analyze graphical data carefully
 Use graphical data to make a judgment about hypothesis
 Analyze assumptions and determine sources of experimental uncertainty
Available Equipment
 Push pop pistons
 Manila folders
 Tape
 Straws
 Rubber bands
 Balloons
 Meter sticks
 Stop watches
Problem: You and your friend Michael are getting ready for his birthday party by blowing up
balloons. Michael blows up a balloon and lets it go. You watch the balloon fly around the room.
Once you are done laughing, Michel tells you “Did you see how the balloon constantly got faster till
it ran out of air? I bet a balloon could be used to push something with a constant force, maybe even
be used as an alternative energy source (wind propulsion, not balloons).”
Your goal is to construct a testing experiment with the available materials to determine if your
friend Michael is correct.
Grading Rubric:
Scientific
Ability
Is able to
design a
reliable
experiment
that tests the
hypothesis
Is able to make
a reasonable
prediction
based on a
hypothesis
Is able to
decide whether
the prediction
and the
outcome
agree/disagree
Is able to make
a reasonable
judgment
about the
hypothesis
Missing(0)
Inadequate(1)
The
experiment
does not test
the hypothesis.
The experiment tests the
hypothesis, but due to the
nature of the design it is
likely the data will lead
to an incorrect judgment.
No attempt to
make a
prediction is
made.
A prediction is made that
is distinct from the
hypothesis but is not
based on it.
No mention of
whether the
prediction and
outcome
agree/disagree.
A decision about the
agreement/disagreement
is made but is not
consistent with the
outcome of the
experiment.
A judgment is made but
is not consistent with the
outcome of the
experiment.
Is able to
revise the
hypothesis
when
necessary
A revision is
necessary but
none is made.
No judgment is
made about the
hypothesis.
A revision is made but
the new hypothesis is not
consistent with the
results of the experiment.
Needs Some
Improvement(2)
The experiment tests the
hypothesis, but due to the
nature of the design there
is a moderate chance the
data will lead to an
inconclusive judgment.
A prediction is made that
follows from the
hypothesis but does not
incorporate assumptions
A reasonable decision
about the
agreement/disagreement
is made but experimental
uncertainty is not taken
into account.
A judgment is made and
is consistent with the
outcome of the
experiment but
assumptions are not taken
into account.
A revision is made and is
consistent with the results
of the experiment but
other relevant evidence is
not taken into account.
Adequate(3)
The experiment tests the
hypothesis and has a
high likelihood of
producing data that will
lead to a conclusive
judgment.
A correct prediction is
made that follows from
the hypothesis and
incorporates
assumptions.
A reasonable decision
about the
agreement/disagreement
is made and experimental
uncertainty is taken into
account.
A reasonable judgment is
made and assumptions
are taken into account.
A revision is made and is
consistent with all
relevant evidence.
Teacher notes:
Students will be expected to build some sort of apparatus with a balloon that will exert a force on it to
determine if the force that the balloon exerts on the object is constant. This will be achieved by the
students building their apparatus, most likely a car or simply a flat piece of the board, time how long it
takes the balloon to expel all its air, and how fast it is travelling after it is done accelerating. If the
force exerted by the balloon, and therefore the acceleration experience by the object, is constant, then
the typical kinematic equations can be used to predict the final velocity of the object. If the kinematic
equations do not give correct results, then the acceleration cannot be constant and therefore Michael’s
hypothesis is disproven.
This process will involve many assumptions including:
 Negligible wind resistance-since we are dealing with light materials, wind resistance may be
an issue.
 Negligible friction and good traction- For those who build small vehicles, they will need to
assume that the rotating wheels are not being effected by friction and that there is enough
traction between the wheels and the ground.
 The direction of the air being expelled is straight- Although this can be accounted for using a
straw, those who choose not to use a straw will have to account for the fact that the balloon’s
mouthpiece is not rigid and will flop about in different directions.
 The object is traveling in a fairly straight line.
Because of these assumptions, students will have to account for considerable experimental
uncertainty including:
 Accurate timing of the length of the acceleration phase.
 Measuring the distance the object travels
 Coefficient of friction (If friction is being taken into account)
 Reaction times
Random uncertainty in the timing can be accounted for using multiple trials. The rest will involve
minimizing uncertainty by taking larger measurements as well as repeating trials to get a more
accurate uncertainty.
Finally, the procedure should follow something similar to blowing up the balloon to different
volumes, releasing the balloon, timing how long it accelerates, then how far it coasts over a given time
interval in order to calculate final average velocity. Then the mass of the object can be multiplied to
give the force exerted by the balloon on the object. The balloon should be increased to bigger and
bigger volumes, increasing the amount of time of Fballoon on obj. Each time, students should try to predict
what the final velocity will be and therefore what the acceleration and then the force should be using
kinematic equations. With all uncertainties accounted for, if the kinematic equation predicts the final
velocity within error, then the force exerted by the balloon on the object is constant. If not, then it is
not constant.
8. Time Table
Clock reading
during the lesson
0 - 5 min
“Title of the
activity”
Homework quiz,
receive feedback
Students Doing
Teacher Doing
Writing
5 – 40 min
“Balloon forces” Lab
and handout
Doing lab
40-45 min
Cleanup and wrap up Cleaning materials,
finishing data
collection.
Checking up
equipment for the
first activity
Helping where
necessary, keeping
materials clean and
organized.
Cleaning materials,
giving out
homework.
9. Formative Assessments:
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10.
Content goals will be assessed by students’ ability to relate constant force to constant
acceleration and to be able to show this using multiple representations such as graphs and
motion diagrams.
Procedural goals are assessed in the scoring rubric and the ability for students to meet the
expectations of the rubric.
Epistemological and metacognitive goals will be assessed based on the students reasoning
behind correctness of the stated hypothesis. We know there is a constant force being
exerted when the acceleration is constant. We can only use kinematic equations when the
acceleration, and therefore force, is constant (this will come up again in force e We know
there is a constant force being exerted when the acceleration is constant. We can only use
kinematic equations when the acceleration, and therefore force, is constant (this will come
up again in force exerted by springs since this force is not constant). The students will be
required to use average velocity in the calculations, so a necessity of differentiating between
avg. velocity and instantaneous is present. Finally, when making a judgment about their
results and the hypothesis, students will be required to remark on whether their results do
not match predictions because of uncertainties or because the hypothesis is incorrect.
Modification for different learners:
By nature of the course, different learners will automatically be accounted for. Students will be
working in groups, so the activity is already a cooperative learning activity. The activity could
utilize technology in the form of graphing or mathematical programs for learners who prefer the
organization of a computerized write-up. Bilingual or ELL students should have no dificulty as they
not only have peer instruction, but all concepts used in the lab have been previously addressed and
students are constructing new knowledge together. Since the teacher is not introducing new terms
or ideas, there is no risk of misunderstanding.
11.
Homework:
A. Lab write-up:
o Procedure with diagrams
o Assumptions
o What is the hypothesis to be tested
o Prediction using “If-and-then” format (ex: If I let the balloon go and wind resistance
in negligible, there is little friction between parts…etc. then the acceleration should
be…”)
o Mathematical procedure with uncertainties (random and instrumental)
o Judgment about results and how they compare to hypothesis
o Revision of hypothesis if necessary
B. A few years later you meet Michael at a party. He tells you he came up with a brand new
source of energy for cars: wind energy! He thinks that placing big fans on the backs of cars will
allow them to move at similar speeds while using much less energy. Do you agree with
Michael?
Do some research and make some assumptions about the kind of forces that would be
required to accelerate a car to normal speeds (60 mph) and how hard wind can blow.
(Don’t forget to think about the assumptions you are making!)