PHYSICS 212 LABORATORY MANUAL CALVIN COLLEGE

Transcription

1 PHYSICS 212 LABORATORY MANUAL CALVIN COLLEGE 2003

2 Physics 212 Calvin College Variables and Fair Tests (adapted from Physics 113 lab manual) Suppose I wanted to determine whether being in darkness would affect the health of a plant. So I took two plants, placed one on the window sill where it could catch the bright afternoon sun, and I placed the other in the freezer where it was very dark. Comparing the two plants the next day, I discovered that they looked very different! But it would be foolish of me to think the differences between the plants were only due to light or dark! No. This wasn t a fair test of light vs. dark because other variables could have entered into play. A fair test can be defined as a comparison between (or among) two (or more) objects or systems such that the intended comparison is not influenced by unknown, uncontrolled or extraneous factors. Put in another way, you might say that a test is fair if it really tests what you intend to test. The concept of a fair test is very important in science. New information is learned in science as comparisons (tests) are made between different objects and systems. If the tests conducted are not fair, then the investigators will be led into drawing false conclusions. You can check to see whether or not a test is fair by considering the variables involved in the test. (The word variable comes from the word vary, meaning to change.) If the variables associated with a test are not the same for all of the items being tested, then your test may not be fair. In our above plant example I neglected the variable of temperature! As an example from physics, suppose you are given two different balls, and you wish to find out which of the balls is a better bouncer. You might test for bouncing ability by dropping the balls and watching to see which ball bounces higher. One variable associated with this test would be the height from which you drop the balls. Of course, in order to conduct a fair test, both balls should be dropped from the same height. If one ball is dropped from higher up than the other ball, then the test would not be fair, because a variable (the height at which you dropped the balls) was not the same for both balls. The change in this variable affected the outcome of the test. It is not hard to think of many other variables associated with the bouncing ball test besides the height at which you drop the balls. For example, the way you release the balls is a variable, and the type of surfaces the balls bounce off might also affect the outcome of the test. Even the temperature of the balls could be important. An important variable is defined to be any variable that affects the outcome of a test. Sometimes, certain variables are not important to the outcome of a test. For example, in the test described above, you might consider dropping the balls one at a time instead of 2

3 together. If you first dropped one ball and measured the height of its bounce, and then you dropped the second ball and measured the height of its bounce, you would not have treated the balls equally. (One ball got to go before the other.) However, the order in which you dropped the balls would not affect the height to which the balls bounced, and therefore the order in which you drop the balls is not an important variable. You do not have to worry about unimportant variables when you conduct tests. One of the main goals of any scientific investigation to make sure that all tests are fair. When you have successfully conducted fair tests, then you are said to have controlled the important variables. Put in another way, when you control variables, then all of the important variables in a test have been kept constant. Since testing and investigations are at the heart of science, learning to recognize and control important variables is crucial to doing good science. Summary: A variable refers to anything that you can change that might affect the outcome of a test. An important variable is anything that does affect the outcome of a test in a significant way. A variable that does not affect the outcome of a test in a significant way is called an unimportant (or insignificant) variable. A variable is controlled if it is made to be the same for all of the items being tested. You must control important variables in order to conduct fair tests. A good experimental procedure is to test variables one at a time, while holding all other variables unchanged. Exercise: A simple pendulum consists of a weight suspended from a string. The weight can swing freely from the end of the string. The period of a pendulum is the amount of time it takes the weight to travel back and forth one time if the pendulum is swinging. What are the important variables affecting the period of a pendulum? Before beginning measuring, list variables you think might be important, and variables you think won t be important. Might be important Don t expect to be important 3

4 Now think about ways that you can test 3 of the variables you think will be most important. Not only will you want to hold other variables constant, but you will want a method for measuring the period as accurately as you can. A few things to think about: If you use a stopwatch, does it matter who controls it? Are there advantages to taking turns running the stopwatch? Would using multiple stopwatches be useful? Is it better to measure the time for one swing, the time for multiple swings, or the number of swings in a given time interval? Can you identify a point in the swing that works best for starting the stopwatch? Are your measurements prone to errors from people s reaction times? Write a summary that describes your procedure and presents your results and conclusions. Where possible, include a graph for each variable tested. (Graph paper is available on request.) Details of the format for your summary can be found on the next page. 4

5 FORMAT FOR LABORATORY WRITEUPS PHYSICS 212 Please writeup your results using the following format. (If you work carefully, you may be able to accomplish most of the writeup while still in class. Neatness is important, but it is OK to cross things out. You may also start fresh pages whenever you are at a natural breaking point.) Title Physics 212 Name: Co-workers names: Date: Purpose: [Give one or two sentences indicating what it is you are trying to accomplish in the lab. E.g., The purpose of this lab is to identify variables that influence the period of a simple pendulum.] Procedure, measurements, and inferences: Start by giving a brief summary of what you did. Be sure you specify precisely what you measured and how you measured it. Diagrams can be very helpful ( a picture is worth a thousand words ). Be sure to record all of your measurements. (Record the actual measurements. Don t average them or manipulate them in any way before writing them down, or an error might sneak in!) Usually it works well to record data in tables. Be sure to include units. You can go back and forth between procedure and measurements. Example: Our pendulum consisted of blah blah blah We first investigated the importance of the VARIABLE A of the pendulum, holding the VARIABLE B and VARIABLE C constant at and respectively. Betty released the pendulum at our chosen amplitude, and John and Susan blah blah blah (Say precisely how you got your data!). We obtained the following results Time for 10 swings (s) Period (t/10) Our results seem very inconsistent, so we decided to do new trials, this time with John releasing the pendulum, blah blah blah. Time for 10 swings (s) Period (t/10) These results are more consistent. A graph is shown on the next page. We see that as VARIABLE A increases, the period steadily increases, showing that Variable A is important in determining the period of a pendulum. 5

6 We next investigated variable B, blah blah blah Conclusions: Here give a summary overview of the entire lab. Personal comments are welcome. Evaluation of lab: A few rules for graphing: 1. All graphs should be clearly labeled and should fill at least half a page. The axes should be labeled and the units given. (Your axes need not start at zero.) 2. Graphs should be close to the data being graphed whenever possible. 3. The usual convention in graphing is to place the independent variable (what we fix) on the horizontal axis and the dependent variable (what we then measure e.g., the period of a pendulum) on the vertical axis. 4. Data on your graph should be "connected" by a smooth curve. Smoothness is generally more important than actually having each point exactly on the curve. Don't just "connect the dots." 6