Physics Labs
I.
Inquiry/Modeling Based Labs
The national academy of sciences and other science organizations have noted that
good science education must involve the teaching of science in an “inquiry” or “modeling
instruction” manner in which students are forced to construct their own knowledge. This
recommendation has been based upon the results of years of science education research.
However, the teaching philosophy has been shared by great teacher including Plato for
thousands of years. Students who develop their own knowledge retain the knowledge
longer and have a deeper understanding that those students who are taught just by lecture.
In the case of science, it also teaches the student how science knowledge is developed
which is vital to their ability to make informed decisions in a technical world. In
Modeling Instruction or Inquiry Based Learning, the student goes through the process of
developing a hypothesis, conducting an experiment, and comparing the results of
experiment to the predictions of the model. The process is shown in schematic form
below:
Pose A Question
Improve Your
Theory/Model
Make Prediction
Using Model/Theory
Design An Experiment
No
Yes
Compare Results To
Predictions From
Model/Theory
Match?
Collect Data
Analyze Data
Most physics labs are Tarleton State University are taught using either Modeling
Instruction or Inquiry. Thus, you will usually not be given a set of laboratory procedures.
Thus, you may be required to develop your own experimental question as well as some or
all of your own laboratory procedures. You will also be required to collect and analyze
data as well to compare your experimental results to predictions by accepted physics
models/theories are to explain any discrepancies.
II.
Philosophy of Science
Physics is an experimental science. It doesn’t matter how you fell about the world
or a concept or how beautiful a math formula or model may appear. Experiments
determine scientific truth. If the model doesn’t fit the experimental data then it must be
discarded no matter how appealing it may be to the scientist on an aesthetic level. We
know that the planets revolve around the sun and not the planet earth because this is what
astronomical observations show. Does this mean that science is impersonal or without
prejudice? No!! Scientists are humans and the models which our minds are able to
conceive are always limited by our societal and intellectual abilities. We bring our
personal prejudices and technological limitations into the problem with us. However,
experimentation provides us with a way of categorizing nature that allows us to overcome
these limitations. Nature through experimentation and not the scientist always acts as the
final judge and jury. We demand that experiments be carefully conceived and
implemented such that any scientist in any country can reproduce the results.
Furthermore, we can test concepts such as the acceleration of a falling body through a
wide range of methods and with a wide range of equipment. We therefore expect that the
results of these experiments must not only be consistent with each type of experiment but
with any experiment measuring a particular concept. This is different than in other forms
of human endeavors. We don’t expect that everyone who scores a 100 on I.Q. test will
miss the same problems on an Algebra I exam.
In physics, we also demand that our models describing nature work 100% of the
time within our experimental uncertainties!! If they fail then we must discard our model
and develop a better model that does work. Physicists like all individuals are not always
happy with what nature appears to be telling us, but we accept the fact that experiments
don’t lie and that we unlike nature are plagued with prejudices and limited intellectual
ability. Sometimes the calculations for our models may be too difficult and we will
simplify our models by ignoring some less important effect like air resistance for a bolder
falling off a cliff. Some individuals trying to promote intelligent design and other forms
of pseudoscience like to claim that this means that physics is incorrect. This
misunderstanding is due to these individuals’ lack of understanding of science and what
real since involves. The fact that we approximate doesn’t mean that our physic model is
wrong any more than saying that multiplication is wrong when you approximate 4.0025 x
3.00086 as 12. The model is also not wrong when they use the model incorrectly any
more than saying that the process of multiplication is flawed because you incorrectly
wrote 4x3=13.
In addition to requiring their models to work 100% of the time, physicists also
require that their models have additional properties. First a model is not useful unless it
predicts the outcomes of new experiments which haven’t yet been performed (i.e. leads to
the development of new knowledge) and not just the outcomes of experiments that we
have already performed. Isaac Newton’s Universal Law of Gravity and Classical
Mechanics were developed in the 1600’s. They predict that it is possible for an object to
leave the earth and go into orbit as well as the speeds required to achieve this effect. They
predicted that a series of bright comet sittings over hundreds of years were actually a
single comet that would return periodically along with its exact time of reappearance
after Newton’s death. They predicted the acceleration of a falling body on the moon as
well as the mechanics required to get to the moon. All of these predictions were verified
long after Newton’s death including falling bodies on the moon in 1970. Finally,
physicists believe in a principle called “occum’s razor.” This principle says that nature
tends to simplicity. Of all the possible models that make the same predictions, a model
with the fewest assumptions is the best. We seek the fewest models and concepts which
explain the whole universe rather than a separate model for each any every observation or
field of study. Since the laws of physics explain the motion of atoms, energy transfer,
motion, etc., these same laws must still be true when doing problems involving
chemistry, biology, engineering, etc. or we must throw out the laws and replace them
with correct ones that work for every problem. Most pseudo science attempt to develop
special theories for each event that they wish to describe and make no new predictions. In
order to be a scientific a theory must be testable. You must be able to set up an
experiment in which there is a single outcome which matches the model’s predictions
while all other outcomes prove that the model is false. If you can’t think of an outcome
that disproves the theory or model then it isn’t science. It is some other form of human
endeavor (history, philosophy, religion, etc) which may be useful and valid but which is
not science.
III.
Experiments
According to classical physics, performing an experiment is an act of finding truth. How
does a scientist go about designing an experiment?
Most people think that scientists just happen to see something unusual and record their
observations. Some people may actually expand the definition to include the development
of a model to explain the observation. This does happen in some cases especially when
nothing about a subject is know, but it is the exception more than the norm especially in
physics. Early man observing the stars noticed that some stars appeared to follow a
“wandering pattern” and gave them the name planets which means “wanderer.” Today’s
astronomers are not just randomly looking at the heavens. They must have a definite plan
for their experiments before making observations as a modern astronomical telescope
costs tens of millions of dollars to build and millions to operate.
Usually, a physicist will consider the consequences of a particular model/law. For
instance, what does the universal law of gravity predict for the acceleration of a falling
body on a planet if the mass of the planet was doubled while the planet’s radius was kept
constant? The physicist then goes about setting up an experiment to test the predictions of
the model. If the results of the experiment confirm the predictions of the model, then the
model may be right. However, if the results of the experiment contradict the predictions
of the model then the model must be discarded and an “improved model” developed!!
This whole process is extremely important. Just presenting results or observations
without comparing them to theory/hypothesis or working to develop an improved theory
isn’t really doing science. Nobel Laureate Richard Feynman put it this way: “To just
know the name of an object is to know nothing!!”
An experimental physicist is by nature a skeptical. They demand proof of everything!!
He/She is skeptical that the equipment is working and checks it carefully both before and
after the experiment. They look for the presence of unexpected influences on their
experiment. They are skeptical of the validity of data and they are skeptical of all models.
In short, experimental science searches to prove the limits and fallibility of models
because this is how science advances. Thus, I will grade your labs and reports based upon
the proof and analysis that you demonstrate and not just on doing the lab.
IV. Doing an Experiment
A.
Outside Lab
So how do you design your own experiments?
1.
You need to develop a question that you want to answer and which tests a
model/hypothesis.
2.
Use the physics theory/models that are in your textbook to see if you can predict
what the answer to your question should be. This dictates how you should design
your experiment. If you skip this step then you can be sure that your experiment
will be poor. For instance, what physical quantity or quantities are you going to
measure and what quantities will you vary? What physical quantities must be kept
constant? These questions must be answered in order to determine both what
equipment that you will need and your experimental procedure. You should write
your question along with your model predictions and reasoning in your notebook.
3.
You need to consider the equipment that is available to you. I have a large amount
of physics equipment but I don’t have Black Holes or nuclear reactors in the store
room. In Inquiry based Science, they call this determining if you question is
“investigable.” If you don’t have the equipment then you will have to find a
different question.
4.
Once you have a question which is investigable, you need to obtain the required
laboratory equipment and develop your experimental procedure. You should write
your procedure in your laboratory notebook neatly so that anyone looking at your
notebook can reproduce your experiment. What data should I collect? Should I
present the data in a graph or a table? The answer depends on what will provide
the best evidence for answering your question. Sometimes a graph can
conclusively prove a point while other cases may require a scientist to use
statistics or other data analysis techniques. These questions should be answered
before you go to the lab and not in the lab!!
B.
Inside Lab
Finally, you are ready to go to the lab and collect data. If you have done the
design process correctly, you should be able to quickly collect the required data to
answer your question. You total attention should now be on possible problems
with your equipment or other factors you have forgotten to include. Sometimes
experiments don’t go as expected due to experimental difficulties. You may have
to go back and rethink your experimental procedure or design. This doesn’t mean
that the lab is a failure provided your analysis of the experiment is thorough. You
will have to identify the exact cause of the problem and explain your reasoning as
well as providing a plan for fixing the experimental problems. This automatically
determines your next lab experiment!! You should note that “human error,”
“experimental error,” “bad equipment,” and other vague statements are copouts
and not acceptable.
C.
Analysis
The final and most important part of a lab is the analysis of the experiment. In the
real world, data is very expensive to obtain. Thus, you must get all the
information that is contained in your data set without making untrue statements.
Scientists who make false statements about data are ruined professionally and can
suffer both financial and criminal penalties. Your analysis tells me much about
both your understanding and the time spent on the other parts of the lab. If you
have skipped the design process then your analysis will be weak and the data
collected will either not pertain directly to the question or answer the question
conclusively. In this section, you should tie your results back to the predictions of
the model/theory. If there is a discrepancy within statistical limits, you must
explain the source of the discrepancy. Just stating that the theory is wrong is
insufficient. Did you forget to account for some experimental parameter? Perhaps
the model only pertains to a particle of no size (point particle) while you used a
cart which has physical size? How would accounting for the cart’s size change the
predictions of the model?
D.
Publishing Your Results (Lab Reports)
I can grade your lab work in several ways including grading lab reports, lab
quizzes, and through formal typed lab reports. At various times though the
semester, I may require your group to provide a typed lab report of an experiment
in a scientific journal format. I will provide you a sample lab report which you
can use as a guide to style. You can also look at real scientific papers by going to
the Library’s online database and using Science Direct to pull up copies of articles
published in Nuclear Instruments and Methods B or other physics journals. For
each report, I only need a single copy for the whole group and not copies from
each individual. How you choose to break down the work of writing the lab report
is up to your group.
Scientists are graded through the publication of their research in peer review
journals. The scientist writes a paper following a very specific format which
efficiently tells the reader what the experiment was about, the theory or model
presently used to predict the results, how the experiment was performed, the
experimental results, and the implications of these results. They don’t include
every piece of data that they may have put in their lab note book. This paper is the
synthesis of their experiment and not just a pile of numbers and graphs. These
papers can usually only be 2-3 pages in length and will be reviewed by leading
experts in the field to see if the work is original and to see if there are any
discrepancies with previous experimental work. The reviewer may ask the
scientist for further evidence to support a claim in the paper or to respond to
questions. A disagreement between the scientists work and previous published
work will not prevent the new work from being published, but the discrepancy
must be addressed. Perhaps, the past work was flawed by failing to consider some
aspect of the problem or due to experimental error. Once the scientists work has
been published, scientists around the world will both read and try to repeat the
experiment. If other scientists can not reproduce your results following your
procedures, your work will be discredited. If it is believed that you have lied or
falsified your results, you will lose your job, and possibly face financial and
criminal penalties if the work was financially supported by an outside entity like
the State of Texas or the Federal Government.