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Scientific Models What is a scientific model? The scientific process making observations, identifying patterns in data, and developing and testing explanations for those patterns is what scientists do as they develop explanations for natural phenomena. Phenomena (just one is a phenomenon) are things that occur in the world that we observe and try to explain. Such explanations are called scientific models. p.1 Scientists use drawings, graphs, equations, 3-D structures, or words to communicate their models (which are ideas and NOT physical objects) to others. For example, you may have seen mobiles or drawings of our solar system that show the planets orbiting the sun. The diagrams are teaching tools, but the models are the actual ideas about how the solar system operates and is set up. Similarly, a scientist might use a three-dimensional ball and stick representation to help her communicate her ideas about atoms to her peers, etc. You may have already heard about many models in your middle school science classes. For example, Galileo developed a model to explain the patterns in the movements of objects in the sky, including the rising and setting of both the sun and moon. Recently scientists have eliminated Pluto as a planet based on new data that no longer fits the planetary model. There are countless other important models already accepted by scientists and many others that are currently being developed. You can expect your experience here at MG to involve thinking as scientists yourselves; collecting data, discussing ideas with peers, and evaluating models. How do you decide whether a model is acceptable? When a community of scientists gets together they may have more than one model to explain the same phenomenon. Their work then involves discussing the data, testing the models, and coming up with ways to assess each model. When assessing how acceptable a model is, scientists use three general criteria: 1. Can the model explain all the observations and data? Sometimes there is data that does not fit with the patterns scientists observe. It is important that scientists collect a lot of data because then they can see patterns in the data and ignore small parts that don t fit. 2. Can the model predict future outcomes for the phenomenon? For example, the phases of the moon have a 28-day cycle and if this model is acceptable you should be able to predict what the moon will look like any day of the cycle. Being able to correctly predict experimental outcomes is a powerful way of testing some models. 3. Is the model consistent with other ideas we have about how the world works? For example, some people have an idea that babies come from the stork. Anyone that has seen a pregnant woman would have data that conflicted with this model, making it unacceptable. Throughout history some models have not been consistent with how we think the world works and it turns out these models are some of the most acceptable in later years.
p.2 Acceptability is based on a model s ability to do the three things outlined above: explain, predict, and be consistent with other ideas. More than one model may offer an acceptable explanation for the same phenomenon. Sometimes it is okay, even good, to have more than one model that explains the same phenomenon. For example, physicists think about light being wavelike or particle-like and each model of light s accounts for this phenomenon differently. Do models ever change? Absolutely!!! Historically, many scientific models have changed a great deal in light of new data and new ideas. For example, at one time scientists thought that human sperm contained a tiny, pre-formed person (a homunculus ) and that the human egg was mainly a source of food for the developing organism. When more advanced microscopes made possible closer observations of sperm and egg, this model was discarded. The current model in which both egg and sperm contribute DNA to the future organism is better able to explain the fact that organisms inherit characteristics of both parents as well as explain observations of cellular mitosis. What is the point of this reading? If you have skipped the rest of the reading and are trying to cheat by reading only this section, STOP IT! Go back, read and highlight! If you already read the previous paragraphs, here s the summary: Models are that scientists use to explain patterns they observe in the world Models are considered acceptable or not based on how well they can phenomena and future data and how consistent they are with. Models are constantly being used to ask more questions about the world; and when new data are gathered, models are or discarded altogether. Scientific knowledge is not static, but is always changing.
Reviewing Models p.3 1. Given the data below, is the drawing that represents this group s black box acceptable? Why or why not? DATA Water in (ml) Water Out (ml) 400 0 400 400 400 600 400 400 400 0 400 1000 400 0 400 400 2. Look at the drawing on the right. Is this an acceptable representation of a model for that same black box data? Why or why not?
3. The third thing that is used to judge the acceptability of a model is whether or not a model can do what? p.4 4. Summarize the three criteria (from Questions 1-3) that are used to judge a scientific model. a. b. c.
The Scientific Method p.5 Most scientists follow a general outline for figuring out how the world around us works. Some scientists do not always follow the outline precisely; they might add steps, repeat some steps, or skip other steps altogether. Nevertheless, it is this organized set of investigative procedures, called the scientific method, that allows us to explore our world in the most accurate way that we know of. You and you group members will read p.7-10 in the physical science textbook. You will each be responsible for summarizing one part of the scientific method below. State the Problem Gather Information Form a Hypothesis Test the Hypothesis Analyze Data Draw Conclusions Hypothesis NOT Supported Hypothesis Supported Model Criteria: 1) 2) 3)
Experimental Variables p.6 Let s say that you are going to do an experiment to determine which of three fertilizers will help plants to grow the biggest. Before you would begin a test or investigation, you would need to think of all of the factors (besides the fertilizer) that might affect the outcomes of the experiment. These may include: plant type, amount of water, amount of sunlight, soil type, and temperature. These factors are called variables. A variable is something that can be measured or manipulated in an experiment. There are two types of variables that exist in most experiments: Independent and dependent variables. Independent variables are factors that can be controlled or manipulated by the researcher. Dependent variables are factors that are measurable and change because of the independent variable. (The dependent variable depends on the independent variable.) Let s pick out the dependent and independent variables for this experiment! Independent Variable: Dependent Variable: Use your knowledge of dependent and independent variables identify each in the following situations: 1. We want to find out if the amount of time that students study is related to their quiz scores. Independent Variable: Dependent Variable:
p.7 2. A teacher was curious if a money reward would influence performance on a test taken at the end of the course. Half the students were offered $5 for receiving an A on the test, the other half were not offered money. Independent Variable: Dependent Variable: 3. A researcher hypothesizes that blondes really do have more fun. To test this hypothesis, she interviews a natural brunette who has recently become a blonde to determine if there is any change in the amount of fun she has. Independent Variable: Dependent Variable: 4. Angelo wants to find out if the color of his mom's birdhouse is related to the number of birds that fly in and out of it. Independent Variable: Dependent Variable: 5. A doctor is testing whether a new medication, Root-tastic will cause people to drink more root beer soda. To test this, she gives 100 people Root-tastic for one month and 100 people a placebo drug. Independent Variable: Dependent Variable: 6. Carmen wants to know if the type of food she gives her dog Fido for breakfast affects how many times a day Fido barks. Independent Variable: Dependent Variable: 7. Lola wants to know if the amount of stuffing inside of a pillow affects how much sleep people get. Independent Variable: Dependent Variable:
What are the Components of a Graph? p.8 Define the following: Data Dependent Variable Independent Variable Key Title X- axis Y-axis Now show where they would be found on the graph below:
Practice Reading Graphs p.9 Graphs are an important tool scientists use to share their information. In order to be a successful scientist you will need to be able to read graphs to understand other s data as well as make graphs to share your data. Below are some graphs that illustrate the results of an experiment about fertilizer and plant growth. Figure 1. Fertilizer effects on plant height The following graph was produced at the end of week three: Figure 2.
Figure 1 1. What is the dependent variable and which axis is it on? 2. What is the independent variable and which axis is it on? 3. Add a label to the X-axis. 4. What does the key tell you? p.10 Figure 2 5. What is the dependent variable and which axis is it on? 6. What is the independent variable and which axis is it on? 7. Add a Title to the figure. Both Figures 8. Based on the graphs above, what would you recommend to a person who wanted very tall plants? 9. How do plant height and mass relate to each other?
Practice Making and Interpreting Graphs p.11 You are probably aware that hurricane season has begun in the Gulf Coast. You probably have heard of a couple of hurricane names by now. Every year hurricanes strike the coasts all over the world, inflicting great damage and multimillion dollar losses. Scientists are still trying to create a hurricane model (an idea that explains how hurricanes work) that would help us predict their behavior and perhaps minimize their damage. In order to create a hurricane model, we need to look at some data. Here is some data from Hurricane Rita, a very damaging hurricane that struck the Gulf Coast on September 2005. Table 1 Wind speed and atmospheric pressure for Hurricane Rita for September 17 to 26, 2005 time millibars Wind speed (m/s) (mb) 17 3 1017.2 17.5 2.7 1016.8 18 5 1015.8 18.5 6 1015.3 19 9 1015 19.5 7 1015.7 20 6 1015 20.5 7.5 1015.3 21 13 1014.8 21.5 11.7 1015 22 8.7 1015.6 22.5 15 1015.3 23 9 1013 23.5 13 1010 24 18 1004 24.5 31 1001 25 21 1006.5 25.5 17 1009.2 26 17.5 1010.5 Time is given in days on September, wind speed in meters per second, and pressure in millibars (it is important to not forget the units!!). You will need to graph the data in figure 1. In order to do that, you will need to figure out the following information: Out of the three variables presented, 1. Which are the dependent variables? 2. Which is the independent variable? Now produce two graphs of the data on the following page. Be sure to label the axis and give a title to each graph.
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p.13 You now need to interpret the graphs. An interesting question is to find out if there is any relationship between wind speed and atmospheric pressure. 1. What relationship do you observe between wind speed and atmospheric pressure? 2. What could you do with the data to see if there is any relationship? It turns out that the winds depend on the atmospheric pressure. Now that you know this, make a graph that shows the relationship between pressure and wind speed in the following figure.
p.14 3. Now that you have this plot, what relationship do you observe between wind speed and atmospheric pressure? You attempted to find the relationship between pressure and wind by two methods, by comparing two graphs, and by creating a graph with both variables. 4. Explain which method you think is more useful. Be sure to mention what you liked about each method and what you did not like.
Review Scientific Models, Scientific Method, Variables, and Graphing 1) Define scientific model. p.15 2) What are the three criteria that are used to judge a scientific model? a. b. c. 3) Give an example of a scientific model that has changed over time. Explain why the model has changed. 4) What is the investigative procedure that most scientists use to explore how the world works? 5) Explain the steps in the scientific method. 6) Define Hypothesis: 7) What is the difference between a dependent and independent variable?
p.16 8) Dr. M.S. Fradin and Dr. J.F. Day tested 16 types of insect repellents on 5 men and 10 women volunteers. The volunteers put their arms into a test cage that contained 10 hungry female mosquitoes and the researchers recorded the time until the first bite. The scientists hypothesize that the most effective insect repellents will be those that contain the chemical called DEET. (Modified from http://faculty.washington.edu/chudler/deet.html) a. What is the independent variable in this scenario? i. What axis of a graph should the independent variable be written on? b. What is the dependent variable? i. What axis should the dependent variable be on? c. Use the graph below to answer the following questions: i. Add a title: ii. Write in the correct label for the X-axis. iii. Would you reject or accept the scientists hypothesis? Why?