The Scientific
Method
An Overview of the Scientific Method
Observations
• As scientists observe nature and study what other scientists
have written, they develop questions and ideas of their own.
• As Jan Baptiste Van/(Von) Helmont (1579-1644) observed
the growing of plants asked himself where did all the extra
body (weight) of the plant come from?
The Question/Problem
• Van Helmont (a Belgian) wondered: When plants grow,
where do they get their extra body material (weight) from?
The water they use? The soil they are in?
The Hypothesis
• A hypothesis is a proposed explanation for something. It is
often expressed in an if… then… form which helps to
suggest experiments that could be done.
• Van Helmont’s hypothesis could have been that a plant gets
its body weight mostly from the soil or mostly from the water
or mostly from water and soil or something else. The
hypothesis if.. then… form would be:
– If a plant is grown for some time, then its weight gain will come
mostly from the soil it is in.
Or,
– If a plant is grown for some time, then its weight gain will come
mostly from the water it is given.
– Etc.
The Hypothesis
• Van Helmont’s hypothesis was likely (He never actually
reported a hypothesis since Science methods had not fully
developed yet): If a plant is grown for some time, then its
weight gain will come from the soil. When a hypothesis is
expressed as an if… then…, it becomes a prediction and
suggests experiments that can be done to test the
prediction.
Planning/Designing an Experiment
• Variables are aspects/elements of an experiment that are able to
change. (ex: Plant height, plant weight, soil nutrients, sunlight, amount of
water given, etc)
– Independent variable
– a variable that is
changed or manipulated
by an experimenter.
(ex: change in sunlight)
– Dependent variable –
a variable that changes
or is expected to
change when an
experimenter changes
some other variable.
(ex: change in plant
growth)
– Light (left) vs no light
(on right)
Designing an Experiment (cont.)
• In an experiment, two or more situations are set up. Everything in the two
situations is kept the same except for one variable which is changed, the
independent variable. Only one variable is changed so that if the dependent
variable changes (plant height in this experiment) a person knows what
caused the change. The situation with a changed variable is called the
experimental group and the original situation it is compared to is called the
control group.
– In the experiment shown here,
two bean plants were grown
under identical conditions except
that one bean plant was given
light and the other was given no
light. The experimental group
had no light while the normal
group compared to (control
group) grew with light.
Designing an Experiment (cont.)
• Control and Experimental Groups
–
–
–
–
In the experiment shown, which group do you think is the control group?
Which groups are experimental groups?
What do you think is the independent variable?
What could be the dependent variable(s)?
Designing an Experiment (cont.)
• To do a good experiment, it must be repeated many times to be sure of
the results. This can be done by using many trials or by using many
individuals at once to see if changing the independent variable has an
effect on the dependent variable.
• If a certain action is seen in a
given situation, the situation
must be repeated many times
to see if the action is random
or has a cause
Van Helmont’s Experimental Design
• A willow tree sapling was to be weighed. Some dry soil was
to be weighed. The soil would then be dampened and the
seedling placed with the soil in a tub. It would be allowed to
grow for 5 years, with water and sunshine. After 5 years, the
tree would be removed from the soil, its roots carefully
washed off and the washed off soil would be added to the tub
soil. The tree would be weighed again and as well as all the
remaining soil from the roots and the tub.
Weaknesses in Van Helmont’s Experiment?
• Van Helmont was living when the concept of experimentation
was still developing. What might be some weaknesses of
Van Helmont’s experiment? What hypothesis was his
experiment really testing well?
Experimental Results: Keeping Records and
Organizing Data (I)
• In experimenting, number measurements are called data.
When doing an experiment, beginning and final data as well
as calculations with these data must be carefully recorded
and reported. Typically experimental data are organized into
tables with titles.
• Example: The pressure (independent variable) on a gas in a
balloon is increased. The volume of the gas (dependent
variable) is watched as the pressure is increased.
Experimental Results: Keeping Records and
Organizing Data (II)
• Typically experimental data are organized into tables with
titles.
Experimental Results: Keeping Records and
Organizing Data (III)
• Occasionally data are graphed with the
independent variable placed on the x axis and the
dependent variable placed on the y axis.
Van Helmont’s Results
• Van Helmont recorded that his tree had gained 164 pounds in
weight over 5 years. He also recorded that the dry soil had lost
only 2 ounces of weight (1 pound is 16 ounces) or 1/8 of a
pound.
• Van Helmont did not publish a table of his results since
conventions of science had not been established yet.
Conclusion (Is the Hypothesis Verified?)
• Scientists look at the data from their experiments and assess
whether they support (prove) or refute (disprove) their
hypothesis.
• Given Van Helmont’s hypothesis, (If a plant is grown for
some time, then its weight gain will come from the soil.),
is it supported or refuted by the data?
Van Helmont’s Description of his Experiment
• By this apparatus I have learned that all things
vegetable arise directly and in a material sense from
the element of water alone. I took an earthen pot
and in it placed 200 pounds of earth which had been
dried out in an oven. This I moistened with rain
water, and in it planted a shoot of willow which
weighed five pounds. When five years had passed
the tree which grew from it weighed 169 pounds and
about three ounces. The earthen pot was wetted
whenever it was necessary with rain or distilled
water only. It was very large, and was sunk in the
ground, and had a tin plated iron lid with many
holes punched in it, which covered the edge of the
pot to keep air-borne dust from mixing with the earth.
I did not keep track of the weight of the leaves which
fell in each of the four autumns. Finally, I dried out
the earth in the pot once more, and found the same
200 pounds, less about 2 ounces. Thus, 164 pounds
of wood, bark, and roots had arisen from water
alone."
Conclusions in the Scientific Method
A New Hypothesis for Van Helmont?
• Since Van Helmont’s willow experiment disproved that a plants
new growth comes mostly from the soil, a new hypothesis could
be stated:
– If a plant is grown for some time, then its weight gain will
come mostly from the water it is given.
• Van Helmont actually made no more hypotheses related to
plant growth nor did he do any more experiments in this
regards. He believed rather than proved that water was the
source of the weight. He did experiment with many gases and
actually was the first person to call them gases.
Questions about Van Helmont’s Experiment
•
1. What was van Helmont's reason for conducting the experiment? In other words,
what question did he set out to answer?
Questions about Van Helmont’s Experiment
•
2. What was van Helmont's hypothesis?
Questions about Van Helmont’s Experiment
•
3. List at least five variables that might affect this experiment. Which variable(s)
is(are) controlled? Which is the experimental variable(s)?
Questions about Van Helmont’s Experiment
•
4. Identify the experimental group and control group in this experiment.
Questions about Van Helmont’s Experiment
•
5. If van Helmont had tested his hypothesis correctly, his experiment would have
failed miserably (i.e., the plants would not grow). What experiment should he have
done? (Hint: rain water lacks essential minerals for growth)
Questions about Van Helmont’s Experiment
•
6. van Helmont did not describe his methods in sufficient detail. For example, how
did he actually remove the soil from the roots without destroying or loosing too
many? Can you think of any other methods that need to be clarified?
So, Where does a Plant get its new Weight from?
• Joseph Priestley, an English Chemist, in 1771 , put a sprig of mint into a
transparent closed space with a candle that burned out the air (oxygen was
not discovered yet) until it soon went out. After 27 days, he relit the
extinguished candle again and it burned perfectly well in the air that
previously would not support it. And how did Priestley light the candle if it
was placed in a closed space? He focused sun light beams with a lens onto
the candle wick (Priestley had no bright source of light, and had to rely on
the sun).
So, Where does a Plant get its new Weight from?
• In another celebrated Experiment from 1772, Priestley kept a
mouse in a jar of air until it collapsed. He found that a mouse
kept with a plant would survive. These kinds of observations led
Priestley to offer an interesting hypothesis that plants restore to
the air whatever breathing animals and burning candles remove
(oxygen gas).
So, Where does a Plant get its new Weight from?
• Priestley’s experiments led to the discovery of the process of
photosynthesis (and respiration) in which plants take in
carbon dioxide gas from the air which they combine with water
to make glucose (becomes new plant body) and oxygen.
• A plant gets its new weight (body material) from an air gas,
carbon dioxide, and water it absorbs mainly from the soil.
A Second Example of the Scientific Method
Galileo and the Pendulum
• Galileo was in a cathedral and observed a slowly moving
chandelier. The back and forth motion seemed very regular.
He wondered what patterns (laws) might govern its swinging.
Galileo and the Pendulum: The Question/Problem
• Was the distance it was from its
rest position important in
determining how fast it swung?
• Was the distance from the chandelier
to the pivot point (in the ceiling)
important in determining how fast it
swung? Was the weight of the
chandelier important in determining
how fast it swung?
Pendulum Terms
Pendulum Terms
• The period of a pendulum is the
time it takes for one complete
back and forth motion (one
cycle).
• The displacement (amplitude) of
a pendulum is the horizontal
distance from rest to the
maximum distance from rest. (In
the diagram, the displacement is
X)
• The length of the pendulum is
the distance from the pivot point
to the centre of the bob. (In the
diagram, the length is L)
Accurately Timing a Pendulum’s Period
• Even with a stopwatch, timing a shorter pendulum’s period is
inaccurate since human response times can add or subtract
fractions of seconds to the measured time period for one
forth and back swing.
• A better method is to time 10 forth and back swings and then
to divide this number by 10 to get the time for 1 forth and
back swing.
Question: What is the Effect of a Pendulum’s
Displacement on its Period?
• Hypothesis: If the displacement of a pendulum
is increased, then the period of the pendulum will
be decreased.
• Experimental Design: The control group will
be the periods of a pendulum given a
displacement of 20 cm, repeated 4 times,
discarding the time that is farthest from the other
times. The experimental group will be the
periods of a pendulum given a displacement of
40 cm, repeated 4 times, discarding the time
farthest off from the rest. A second experimental
group will be the periods of a pendulum with a 60
cm displacement, as measured above. The bob
weight and pendulum length will be the same in
all trials so that the only variable being changed
is displacement.
Organizing the Data Collected
Periods of Pendulum with 20 cm Displacement
Length
Bob #
Displacement
Trials
100 cm
1
20 cm
1
100 cm
1
20 cm
2
100 cm
1
20 cm
3
100 cm
1
20 cm
4
s/10 swings
s/1 swing
Organizing the Data Collected
Periods of Pendulum with 40 cm Displacement
Length
Bob #
Displacement
Trials
100 cm
1
40 cm
1
100 cm
1
40 cm
2
100 cm
1
40 cm
3
100 cm
1
40 cm
4
s/10 swings
s/1 swing
Organizing the Data Collected
Periods of Pendulum with 60 cm Displacement
Length
Bob #
Displacement
Trials
100 cm
1
60 cm
1
100 cm
1
60 cm
2
100 cm
1
60 cm
3
100 cm
1
60 cm
4
s/10 swings
s/1 swing
Graphing the Dependent Variable (Y) vs the
Independent Variable (X)
A Pendulum's Period vs Displacements
2.6
2.4
2.2
2
1.8
period (s)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
displacement (cm)
50
60
70
Conclusion: How does a Pendulum’s
Displacement Affect its Period?
• A pendulum’s period is unaffected by its displacement.
• The hypothesis was proven false by the experimental data
(within the range of possible experimental error)
Reporting Your Results
• Experimental results
are always shared
between scientists.
• Experiments are written
up as Scientific papers
and published in a
specific format. Full
versions of the
experiments are
published as well as
smaller versions called
abstracts.
Reporting Experimental Results Completes the
Scientific Method
Reporting Experimental Results in High School
• High School students often are asked to write up lab reports
of experiments they have done. These lab reports may be
simply answers to questions, brief descriptions or formal lab
reports.
• It is important for HS students to learn how to write a formal
lab report with sections similar to Research papers. Further
education in the Sciences will likely require both informal and
formal lab reports.
Formal Lab Reports (High School)
• Unless indicated otherwise, every student is individually
responsible to write up their own separate lab report. Every
part of the lab report must be the student’s own work.
(copying results in a zero for persons copying and for person
who lent their work to be copied.) However, students may
copy titles and may copy data tables. All other lab parts must
be original.
DCS Formal Lab Report Format
•
•
•
•
•
•
•
•
•
•
•
•
•
Note: All type for all parts should be set at size 12. The entire lab is written from an
impersonal point of view.
After the title page, parts are NOT on separate pages.
Parts of a Formal Lab Report include:
Title Page:
John Brown
(The student’s name in placed in the upper right hand corner).
The effect that changing a pendulum’s displacement has on its period
(Title in centre, bold and underlined. The title is very concise and specific, explaining
exactly what the lab experiment was attempting to do. The title is written in the
present tense.)
Bill Parker
Anne Gilmore
( The Lab partner(s) who worked with the person writing the report is(are) placed in
the lower right hand corner)
September 23, 2009
(The date the lab report is due is placed in the lower right hand corner under the lab
partner(s) name(s))
Formal Lab Report Format (Cont.)
• Purpose:
• The purpose of this lab was to investigate if
different displacements of a pendulum’s bob
from rest would have any effect on the period
of the pendulum’s swing. It was desired to see
if there were any patterns of change in the
pendulum’s period as its displacement was
changed. The hypothesis formed for this
experiment was that if the displacement of the
pendulum was increased, then the pendulum’s
period would decrease.
• (This section gives the reasons why the lab
was done, the questions that you hoped to find
answers for, the hypothesis/hypotheses that
guided the experiment. Note that this section
is written in the past tense and in an
impersonal style – avoiding personal
references – like I…, we…, Mike … etc. This
section is identified in bold and underlined with
a colon.)
Formal Lab Report Format (Cont.)
•
•
•
•
•
•
Procedure:
A displacement of a pendulum 40 cm long was varied as follows. It was given a
displacement of 20 cm and timed for 10 swings. The period or time for 1 swing was
determined by dividing the time by 10. This was repeated 4 times and the three
closet times were selected and recorded. An average of these three times was
determined to make a graph. The procedure above was repeated exactly with a new
displacement of 40 cm. Finally a displacement of 60 cm was used with the same
procedure. Throughout this lab, all variables were controlled except displacement
and the effect of changing displacement on the period was monitored.
(This section gives a detailed recipe so that another student could perform this lab
experiment, given this procedure. This section again is delineated in bold and
underlined with a colon. It is written impersonally in the past tense)
General Observations:
As the experiment progressed, it was noticed that the pendulum tended to veer
slightly off course, the bob spinning somewhat. The period of the pendulum seemed
to be changing very little, if at all, but the speed of the pendulum seemed to increase
as the pendulum’s displacement increased.
(This section gives general observations – things seen, heard, smelled or felt through
the senses, that were related to the experiment. It is written impersonally in the past
tense.)
Formal Lab Report Format (Cont.)
• Data and Calculations:
Periods of Pendulum with 20 cm Displacement
Length
Displacemen
Bob #
t
Trials
s/10
swings
s/1 swing
100 cm
1
20 cm
1
9.85
0.99
100 cm
1
20 cm
2
9.82
0.98
100 cm
1
20 cm
3
9.81
0.98
100 cm
1
20 cm
4
9.81
0.98
average:
0.98
Formal Lab Report Format (Cont.)
Periods of Pendulum with 40 cm Displacement
Length
Bob #
Displacement
Trials s/10 swings s/1 swing
100 cm
1
40 cm
1
9.84
0.98
100 cm
1
40 cm
2
9.82
0.98
100 cm
1
40 cm
3
9.89
0.99
100 cm
1
40 cm
4
9.81
0.98
average:
0.98
Periods of Pendulum with 60 cm Displacement
Length
Bob #
Displacement
Trials s/10 swings s/1 swing
100 cm
1
60 cm
1
9.8
0.98
100 cm
1
60 cm
2
9.82
0.98
100 cm
1
60 cm
3
9.79
0.98
100 cm
1
60 cm
4
9.76
0.98
average:
0.98
Formal Lab Report Format (Cont.)
The Effect of Changing Displacement on a Pendulum's
Period
Displacement
Period
20
0.98
40
0.98
60
0.98
In the charts above, the period or s/1 swing was obtained by dividing the
s/10 swings by 10. The average of each table was obtained by adding up
the three closest periods and dividing by three.
(Data are number measurements taken in lab. Calculations should be
explained so that a person could get the same values indicated in your
tables. The data tables were made up in EXCEL on a spreadsheet. They
were highlighted, copied and pasted into WORD. Many labs do not
collect number data and therefore for those labs, this section would be
omitted.)
Formal Lab Report Format (Cont.)
• Graphs:
A Pendulum's Periods vs Displacements
1.6
Period (s)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
Displacement (cm)
(The graph is made with EXCEL, using the graphing wizard. The graph is
highlighted, copied and pasted into WORD. Many labs do not have a
graph section. When present, the graphs section is in bold, underlined with
a colon.)
Assigned Questions:
(This section contains answers to textbook or handout questions that were
given with the lab. This section is omitted when no questions are
assigned.)
Formal Lab Report Format (Cont.)
Conclusions:
The data show that when a pendulum’s displacement is changed, the period
remains the same. The data show that a pendulum’s displacement has no
effect on its period. For displacements of 20, 40 and 60 cm, the periods are
always .98s . So the time it takes for a pendulum to make one swing is the
same no matter how far it is pulled away from its rest position. The data for
this experiment refute the original hypothesis which suggested that the period
should be smaller as the displacement increased. There were some errors in
this lab such as the response time for a person to start and stop the stopwatch,
the tendency for the pendulum to veer off course, and some spinning of the
pendulum bob which may have affected its period. By timing 10 swings and
then dividing this time by 10, it was possible to compensate for the error of
response time.
(The conclusion tells what you have learned, what problems you had and if the
results confirmed your hypothesis or not. The basic conclusion is written
impersonally in the present tense. References to the experiment are in the
impersonal past tense. The conclusion is based on the data of the experiment.
Experimental errors are noted. This section is also where a percent error
calculation could be placed. If it is known that the lab results are similar to or
different from what Scientists typically find, this is noted also. This section is
also delineated in bold, underlined with a colon.)
Assignment: Design and Perform Experiments to
Determine How Pendulum Length and Bob Weight
Affect a Pendulum’s Period.
• Organize data in charts as shown before and complete a
graph for period vs pendulum length as well as period vs bob
weight. Then write up a lab report for these experiments
using the proper lab format (proper sections in proper
sequence) in microsoft WORD format.
Pendulums can be used for timing (clocks).
A Foucault Pendulum Shows the earth’s rotation.