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Home | Contents | 1-3 Science | 4.1 Experiments | 4.2 Units | 4.3 Measurement | 4.4 Safety | 5 Data | 6-7 Reports | 8 Appendix

VCS Science Handbook Sections 4.0-4.1

4.0.0 Experimentation

  • Scientists do experiments to test ideas about the world. Before science emerged about 400 years ago, philosophers made statements about the world based on logic and their notions of how a perfect world should work. When science was developed, people were realizing the importance of doing experiments to test whether their ideas about the world matched reality. For instance, it was common “knowledge” for thousands of years that heavier objects fall faster than light ones, but this knowledge was based on what seemed reasonable, not on actual observations of falling objects. Galileo Galilei disproved this notion in the early 1600’s by careful experimentation (but not by dropping weights from the Leaning Tower of Pisa as stories often tell).

  • In the following three sections, you will learn the basics of designing an experiment, how to express measurements in SI units, how to use a variety of scientific instruments to make measurements and observations, and how to be safe in the lab.

  • 4.1.0 Designing an experiment
    • 4.1.1 The conditions of an experiment must be carefully controlled to produce meaningful data. Therefore, scientists take care to design experiments in such a way that they will find answers to their questions. The numbered items in this section will explain how to design a good experiment. Following each explanation will be part of an example involving a pickup truck. Since most experiments begin with a problem to be solved, we will start there.

      • Suppose someone notices that air causes friction against moving objects. This observation could lead to the hypothesis that a pickup truck experiences more friction with the air when the tailgate is up than when it is down. The person decides to test the idea, but how can friction be measured? It is not easy to measure directly, but the researcher reasons that if the truck experiences less friction, then the vehicle will not have to work as hard to move a certain distance, thus using less fuel. The rate of fuel consumption is usually measured in terms of gas mileage, the number of miles traveled on one gallon of fuel.

    • 4.1.2 There are possibly many factors, or variables, that can affect the outcome of an experiment. Scientists try to determine the effect of only one variable at a time; if they tested more than one at a time, they would not know which variable was responsible for changes in the experiment. Therefore, scientists identify as many variables as possible and keep all but the one being tested constant throughout the experiment.

      • Besides air friction with the tailgate, there are many other factors that could affect the gas mileage of a truck. Since the tailgate position is the only variable being tested in this experiment, all other variables that could affect the fuel usage must be kept constant. Otherwise, the driver will not know for sure whether some factor other than the tailgate was responsible for changing the rate of fuel consumed.

      • Variables that would need to be kept constant (called constants) include the following: the type, age, mileage, and condition of the truck (the same truck should be used for all tests), the weight and speed of the truck; the distance driven, the highway traveled, the direction of travel, and proximity to other vehicles (following closely can decrease drag); and weather conditions like the time of day and wind direction.

    • 4.1.3 The variable that is tested in the experiment is called the independent variable, or manipulated variable, and it is purposefully changed in the experiment. The dependent variable, or responding variable, responds to a change in the independent variable.

      • In the tailgate experiment, the independent variable is the position of the tailgate, and the responding variable is the amount of fuel consumed. This is because it is assumed (or hypothesized) that changing the position of the tailgate will affect the gas mileage. In other words, the rate of fuel consumption depends upon the position of the tailgate. The rate also depends on the other variables (like distance driven), but those are held constant throughout the experiment.

    • 4.1.4 To establish some basis of comparison in the experiment, scientists often make a control group. The control group receives no special treatment, whereas the experimental groups are groups in which the independent variable is changed. Both the control group and the experimental groups have the same constants. The combination of having a control group and controlled variables (constants) makes the experiment a controlled experiment.

      • For the driver of the truck to determine whether having the tailgate down makes for better gas mileage, he will have to compare it to a truck with the tailgate up. Therefore, the truck that is driven under normal circumstances (with the tailgate up) is considered the control truck, while the truck driven with the tailgate down is the experimental truck. If the independent variable is changed (like having the tailgate at different angles), then there would be more than one experimental truck for this experiment.

    • 4.1.5 Once scientists have a good idea of what they want to measure and what variables they want to test, they devise a way to make the actual measurements and decide on the conditions of the experiment. Questions asked at this stage might include “Which instrument will I use to make measurements?” “How much precision does my instrument need?” How often will I need to make measurements during the experiment?” and “How long will the experiment run?” Armed with the answers to these questions, the experimenter will be ready to start.

      • For the truck driver-scientist to test his hypothesis, he needs to measure the amount of fuel consumed over some distance. There are many ways this could be done. For instance, the researcher could use the truck’s built-in fuel gauge to measure the amount of fuel consumed or find out how much gas is needed to fill the tank at the end of the drive by going to a gas station. However, the first method is fairly inaccurate, and the second requires that the tank be exactly full when starting the experiment. Therefore, the driver decides to put a gauge directly into the fuel line leading from the gas tank to the engine that can measure the flow of fuel accurately. The researcher also figures that driving one mile will not be enough to observe a difference in gas mileage, so he decides that he will drive a distance of 25 miles for all trials.

    • 4.1.6 The next step is to carry out the experiment. Data acquired from the example experiment will be discussed in a later section, “Gathering Data.”