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 CollectedPeriods of Pendulum with 20 cm Displacement

Length Bob # Displacement Trials s/10 swings s/1 swing

100 cm 1 20 cm 1

100 cm 1 20 cm 2

100 cm 1 20 cm 3

100 cm 1 20 cm 4

Organizing the Data CollectedPeriods of Pendulum with 40 cm Displacement

Length Bob # Displacement Trials s/10 swings s/1 swing

100 cm 1 40 cm 1

100 cm 1 40 cm 2

100 cm 1 40 cm 3

100 cm 1 40 cm 4

Organizing the Data CollectedPeriods of Pendulum with 60 cm Displacement

Length Bob # Displacement Trials s/10 swings s/1 swing

100 cm 1 60 cm 1

100 cm 1 60 cm 2

100 cm 1 60 cm 3

100 cm 1 60 cm 4

Graphing the Dependent Variable (Y) vs the Independent Variable (X)

A Pendulum's Period vs Displacements

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 10 20 30 40 50 60 70

displacement (cm)

per

iod (s)

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 Bob #Displaceme

nt Trialss/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 swing100 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 swing100 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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50 60 70

Displacement (cm)

Per

iod (s)

(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.