Conservation Laws Independent Lab

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1 PHYSICS LAB Conservation Laws Independent Lab Printed Names: Signatures: Date: Lab Section: Instructor: GRADE: PHYSICS DEPARTMENT JAMES MADISON UNIVERSITY Revision August

2 Laws of Conservation Independent Student Lab As you ve discussed in lecture, there are fundamental laws of conservation that underlie all physical phenomena. In this lab, you will examine the laws of conservation of momentum and/or conservation of energy (in particular, we will restrict ourselves to conservation of mechanical energy). Given the equipment you have used previously this semester, it will be up to you and your partner to devise experiments that will verify some aspect of these laws. OBJECTIVES?? To verify the law of conservation of momentum for both elastic and perfectly inelastic collisions.?? To verify the law of conservation of mechanical energy in the absence of nonconservative forces.?? To gain experience in designing and analyzing an experiment. Your lab instructor will clarify how to achieve these goals. Each group should have their experiment approved. MATERIALS Any materials you have used up to now in the lab this semester will be made available to you. PROCEDURE The following section must be read carefully. Some aspects of the report, which are described below, are mandatory. Lab Report The lab report should be written so that students enrolled in introductory physics courses can understand the lab. The report must be typed. Neat pencil sketches and diagrams will be accepted. Each student should submit a separately written report. Groups work together through the measurements and analysis but each student is responsible for summarizing the work in an separate individual report. Your report should demonstrate that you successfully completed all steps. Try to find the optimal method to inform the reader. Diagrams, plots, figures, sample calculations, tables, pictures and lists are often clearer than written explanations. Find the clearest most succinct method to make your point. Most importantly: You state a goal (Title page abstract). Measure all quantities necessary. Establish an uncertainty for all parts of the measurement. Calculate a result. (This may be a graph). 94

3 Deduce an overall uncertainty for this result. (This may be error bars on a graph. ) State that the result demonstrates what you stated as your goal or fails to demonstrate it based on the uncertainties. UNCERTAINTIES An important aspect of this experiment will be the estimation of uncertainty and the use of this uncertainty to draw your conclusions. Please reread the appendix on establishing uncertainty. Remember:?? Absolutely no error can be introduced into any discussion in this report unless some quantitative estimate can be made for its size and all estimates need to be justified.?? It is okay to have a measurement that disagrees with expectations as long as another experimenter using the same equipment would reach the same result. (No mistakes.)?? It is advisable to do a dry run and perform calculations immediately to see that things are going as planned.?? If your experiment is unsuccessful and there is no obvious flaw then you should receive a good grade on your report. Naturally the grader will try and find the source of the discrepancy. Your report will therefore require a very complete description and will be more difficult to write. TITLE PAGE You need to include a title page that contains a title, your name, the names of your lab partners, section number, your instructor s name and the purpose of the experiment. The title page must include a one or two sentence purpose statement or abstract. Think carefully about this sentence and write exactly what you are investigating. Your grade will primarily be based on how well you carry out the exploration described in this sentence. Be specific. INTRODUCTION Include a discussion of the physics and the formulas that you are using and studying. Be sure that you state what aspects of the physics are being investigated and how this physics will elucidate or clarify your purpose. You should also discuss relationships, which are important for the completion of the experiment but not relevant to your purpose. For example, if you were exploring harmonic motion, the spring constant might be unknown and might be needed. The physics that enables you to measure this constant must be discussed. (Maximum 1 page of text.) PROCEDURE Use a bullet or list style to write this section. There must be a diagram for all apparatus. This can be a simple block pencil sketch. If you carefully show important aspects of your setup you may be able to avoid extensive discussion in the report. The grader will know that there are pitfalls and challenges to a successful experiment. The grader expects to be convinced that the experiment was performed correctly. Spend time thinking about what things you need to know in order to 95

4 complete the experiment and describe how you resolved these questions. Do not assume that any instrument is calibrated correctly. Mention any calibration procedures. Describe briefly how you verified that your measurements were being done correctly. Can you find something that you can measure where the answer is known? If your procedure becomes complicated summarize the important steps in a table or summary list. (Maximum 2 pages of text.) DATA A small sample of the data should be provided. At least one disk with the complete data should be available should the instructor request to examine the data. (No more than one page.) ANALYSIS (You may merge the analysis and results together into one section.) A description of the analysis should be given. Only those aspects of the analysis that relate to your conclusion should be discussed. Sample calculations should be included. Plots of analyzed data are crucial in describing your analysis. (No more than two written pages, no limit on the number of plots but do not include plots that are not discussed.) RESULTS A summary of the important results as ascertained from the analysis should be given.. A discussion of the overall impact of the uncertainties in the measurement and in the analysis should be included as they pertain to your final result. Be sure your final result is given with the correct number of significant figures In this section you discuss in detail the facts that you will use in your conclusion. (No more than two pages.) CONCLUSION A brief summary that informs the reader what was being investigated and how well the experiment succeeded in performing this investigation. Be sure to convince the reader that you succeeded or that any discrepancies were not due to an oversights or mistakes made in performing the experiment. You may remind the reader of important points made in previous sections but do not lose the focus. This section should be short and to the point with no questions left in the reader s mind about what you accomplished. (No more than one page.) Your lab instructor can be consulted for help on all aspects of the lab. This includes discussing the initial plan, helping with the equipment and advising on the write-up. However, students are expected to have developed some laboratory skills through their earlier work during the semester. GRADING POLICY You must devise and fully carry out an experiment that has been approved by your instructor. Your instructor will clarify the details of how the work will be graded. GENERAL ADVICE When writing remember that what is crystal clear to you at the conclusion of an experiment is not clear to your reader. In general a good student or an instructor from any of the labs sections should be able to read and understand your report. A classmate would therefore make a good proofreader. Be sure that you draw attention to all of the important points and link all of the pieces of the lab. 96

5 Here are some general comments.?? Don't get things wrong. For example, I=mL 2 with I as moment of inertia, L as length, and m as a mass is not a formula for the period of a pendulum.?? Read your textbook. Don't claim to have verified an incorrect relationship.?? Be careful with straight lines. Don't include them where they don't belong.?? If you include tables, graphs, calculations, etc. then you need to comment on why they are included. Explain what conclusions should be drawn from them.?? Plots and diagrams are very effective ways to communicate information but if you expect your plot to illustrate a certain relationship don t assume that the reader will make this connection. Tell the reader what conclusion he/she should draw from the plot.?? Don't include all the raw data.?? Don't make complicated statements that you don't understand.?? Avoid hand-waving arguments. Be sure you establish a valid comment and explain how it can be verified.?? Measure the appropriate data range. This may take considerable thought, design and care. The pendulum lab provides some good examples for discussion.?? Abstract: The student could reference the textbook result T? 2? l g and then discuss this important relationship. The period of the pendulum shown above is an approximation that is valid for small amplitudes. This result shows a dependence of the period on length only. The period in this approximation does not depend on either mass or amplitude. We will examine how well this relationship holds for all three variables. We will examine at what level the period can be considered to be independent of mass and amplitude by using a simple linear relationship between period and amplitude and period and mass. A straight line will be fit to the data (T vs A,T vs m). A non-zero slope will indicate a dependence and the magnitude of the slope will give an indication of how large an this effect is. The reasons for the breakdown of the approximation (deviations from the above equation) will not be treated only measured and reported.?? The characteristics of a pendulum are mass of bob m, length of the pendulum l, and amplitude of the swing A. These should be described. 97

6 ?? Because the derivation of the motion is complicated the student might choose to show the forces that determine this motion in a free body diagram.?? An effective way to explore the relationship is to use Excel to plot the equation and then add measured values with errors on the same graph. You can plot models by connecting the model points but not including the data point markers. The model then appears as a smooth line. The measured in contrast can have large data markers and error bars.?? A good researcher would have found that the motion of the pendulum is usually described in terms of angle. However it is probably better to measure the position in terms of height or horizontal distance from the equilibrium position. A diagram relating the usual variable angle to the measured value height and the algebraic relationship should be included in the report. The position of the pendulum was determined by the location of the photogate. We measured the distance of the photogate (using the center of the photodetector) from the equilibrium location of pendulum with a ruler. (Good idea to show a simple diagram.) x pendulum photogate 98

Teaching a Laboratory Section

Chapter 3 Teaching a Laboratory Section Page I. Cooperative Problem Solving Labs in Operation 57 II. Grading the Labs 75 III. Overview of Teaching a Lab Session 79 IV. Outline for Teaching a Lab Session