PHYS 221 Laboratory #1. 1 Interpreting Real Measurements

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1 PHYS 221 Laboratory #1 Modeling Human Size and an Introduction to LabPro 1 Interpreting Real Measurements 1.1 Preliminary In this introductory laboratory, you will be taking data as a group, plotting the data, and making some inferences from the plot. Experimenters must be aware of errors which may occur during measurements. Briefly, these are Random errors. These occur due to unpredictable variations in measurements. Often they are introduced by human judgement, which may vary from person to person. In other experiments these uncertainties may enter due to electrical noise, mechanical vibration, or related phenomena. Systematic errors. These are due to the experimental method or apparatus itself. The way to take care of these errors is to design the experiment properly and use good laboratory technique. There is a difference between accuracy and precision. An instrument may have good precision, but poor accuracy. Conversely, instruments may have good accuracy but relatively poor precision. This may be illustrated by an analogy to shooting a rifle or an arrow at a target: Good precision Poor accuracy Good accuracy Poor precision 1.2 Lab Report Format It is extremely important that you understand the need for, and format of, a good report. Scientific work of any sort is useless unless its results can 1

2 be communicated to others. Over the years a particular format, or general outline, has evolved for the preparation of scientific reports. Your reports should have a... Title Page. The title page should have the title of the lab, a 2-5 sentence abstract of the report, your name, the name and of the course (e.g. Phys 201 General Physics), my name (Dr. Tovar), the name of your lab partners, and the due date. the Sections that follow the Laboratory assignment Discussion - Was what you did a good way of doing it? Discuss possible sources of error. How might one improve the lab given more time? Conclusions - 1 paragraph. References Finally, there is a note about form and style. Philosophically, science should be independent of scientists, their times, and places of work. That is the actual discoverer of a principle is less important than the principle itself. If, for example, Watson and Crick had not developed the structural model of DNA, then surely someone else would have done so. In conformance with this philosophy scientific writing should be impersonal. Thus, reports should be writteninthethirdperson, ratherthanthefirst. Itisalsotraditionaltowrite scientific papers in the passive, rather that the active, voice. For example, it is customary to avoid statements like, I heated the protein solution to 100C and formation of a precipitate was noted. 1.3 The Measurements We are going to measure the height and the length of the forearm for every person in the class. We ll then plot this data and see what we can infer from it. Divide yourselves into groups of 4 or 5. Each person should measure the two quantities for someone else, and someone in the group should record this data in a table. Do all measurements in cm. We will then summarize all the data on the blackboard or overhead. and everyone will make a copy of this table in his/her report. 2

3 1.4 Analysis The first thing to do is to plot the data, with height on the vertical axis and forearm length on the horizontal axis. Steps in good graphing technique are to Choose good scales, so the entire area of the graph will be used. Special note: usually in this class, I ask that the origin be included in the graph: both scales should start at zero. However, in this special case it does not work well to do that. Therefore, neither scale should start at zero. Label the axes clearly with what you are measuring and the units. Plot the points, circling each point to make it unambiguous that it is a datum point. Label the graph somewhere in the body, describing what the graph is showing. (It does not need to be lengthy, e.g. Height vs. Forearm Length.) You may have a spreadsheet make the graph for you, if you wish. We will be doing this type of thing later in the course. Draw a good straight line through the data and determine an equation for the line. For example, we could use the equation h = Al+B where h is height, l is forearm length, and A and B are two constants we have to find. Answer these questions: What is the physical meaning of the constant B? If you measured someone s forearm length to be 48 cm, what would you predict his/her height to be? Would you expect this prediction to be accurate within, say, 1 cm most of the time? Or would it be within 10 cm most of the time? Just estimate we don t need precise numbers here. Does your equation predict how high someone would be if his forearm length is 10 cm? Does this make sense? 3

4 1.5 Conclusions part 1 Overall, would you say that forearm length is a good predictor of height? Discuss the precision and accuracy of the experiment. 2 Introduction to LabPro 2.1 Purpose To learn how to interpret graphs of distance and velocity as functions of time. To do this we will use some microcomputer-based tools. 2.2 Getting started The Logger Pro icon should be on the desktop. Double-click the icon to launch Logger Pro. Now use the File menu to Open a file. You may have to navigate through the following folders in the menu box: Logger Pro Experiments Physics with Computers Exp 01 Graph Matching. From this folder, open up the Distance Graph setup. If the program asks you about saving files, click on the Don t Save button. Set up the motion detector so you can move in front of it, and turn the computer monitor so you can see the screen at the same time. What happens when you move toward the detector? Far away from it? What, then, does the vertical scale on the graph represent? Move slowly toward the detector, then quickly. How does the graph change as you change from slow motion to a faster speed? Properties of the motion detector. It detects the first object in front of it. This includes your arms if you swing them as you walk, so keep your arms in back of your torso. It will not correctly measure anything closer than about 50 cm. Distance is short for Distance from the motion detector the motion detector is at the origin on the vertical axis. 4

5 2.3 Activity 1 Open the Distance Match 1b file. Your object is to match the red line (which is drawn as the data is taken) to the green line on the graph. Everyone in your group will do this,several times. When you get one you like, print it out to paste into your lab book. 2.4 Activity 2 Open the Distance Match 1c file, and repeat. 2.5 Activity 3 Open the Velocity Match 1d file. Everyone in your group will now duplicate the velocity vs. time graph shown by the green line. What happens to the red line when you stop in front of the detector? This is different from the behavior of the distance graphs. 2.6 Activity 4 Open the Velocity Match 1e file and repeat. 2.7 Exercises/Questions 1. For your report, sketch what a Distance vs. time graph would look like for a person walking slowing away from the detector for 3 seconds, then moving quickly toward the detector for 1.0 second. Repeat this for a Velocity vs. time graph. 2. For the distance match in LabPro Activity 1, draw the corresponding velocity vs. time graph (be sure to label axes). 3. For the distance match in LabPro Activity 2, draw the corresponding velocity vs. time graph (be sure to label axes). 4. For the distance match in LabPro Activity 3, draw the corresponding distance vs. time graph (be sure to label axes). 5. For the distance match in LabPro Activity 4, draw the corresponding distance vs. time graph (be sure to label axes). 5