PHYSICS LAB: CONSTANT MOTION Introduction Experimentation is fundamental to physics (and all science, for that matter) because it allows us to prove or disprove our hypotheses about how the physical world works. Though you won t likely be discovering any new laws of physics in this class, lab activities do allow you to actually see how the concepts you study work. Sometimes, however, the true meaning of what is happening in an experiment isn t readily apparent it s the data that make things clear. Graphing is one of the most effective ways to interpret and display data, and you will use this valuable tool throughout the course. In the first part of this lab, you will set up an experiment and collect data (it s not as simple as you might think!). Later, you will explore what your data means by graphing it. When finished with this lab you should be able to: Describe how the choice of coordinate system affects the values of x, t, x, & t. Derive values for x, t, & v from tables of x and t values. Demonstrate that you can correctly read x, t, x, t and v from an x vs. t graph. Describe (in words) the motion of an object given its x vs. t graph. Include information about position, displacement, times, and speed. Procedure: You will need the following materials to complete this activity: Buggy (1): There are red and blue buggies, either color may be used. Meterstick or Stop Watches: 3 or a combination of lab times to take 3 different times. Make sure to use the metric units. We will measure in cm for this lab. Be sure to use timers that all give times to the same number of digits. 1. The buggies will be run along the floor. Clear a path that is 3-4 meters long. Start the buggy and notice how far you can run it before it begins to drift off a straight path. Make sure that your buggy path will run fairly straight for about 3 meters.
Lab: Constant Motion Page 2 2. Mark the floor (use the colorful lab tape) at 4 locations between, and including 0 and 300 cm. Place a tape measure along the floor to establish the position of each piece of tape. Starting Position = 0 cm @ t = 0 Position = 300 cm @ t =? Buggy Path 1 (Motion 1): Buggy starts at x = 0 cm at t = 0 sec and goes until it has passed position x = 300 cm. 3. Assign timers to 3 different tape positions beyond x = 0, making the last one x = 300 cm. 4. Station an observer with a stopwatch at each of the 4 tape positions (except the 0 cm position). Turn on the buggy and set it down 20-30 cm behind the starting position. Each observer should start their timer when the buggy crosses the starting position mark and stop their timer when the buggy crosses each assigned position. Record the times. Repeat this step three times so that you have three times at each assigned position to average. Coord. System x = 0 cm Starting Position = 50 cm @ t = 0 Buggy Path 2 (Motion 2): Buggy starts at x = 50 cm at t = 0 sec and goes until it has passed position x = 350 cm. 5. The experiment will now be repeated. Select 4 taped locations with one being the 50 cm location and one the 350 cm location. Start all timers when the buggy passes the 50 cm location and stop each timer when the buggy passes each of the other tape locations.
Lab: Constant Motion Page 3 6. Repeat your measurements step three times so that you have three times at each assigned position to average. Buggy Path 3 (Motion 3): Buggy starts at x = 300 cm at t = 0 sec and goes until it has passed position x = 0 cm. Coord. System x = 0 cm, t=? Starting Position = 300 cm @ t = 0 7. The experiment will now be repeated with the buggy running the path in the reverse direction. The coordinate system is still in the same place. Collecting Data Uncertainty: Before taking down your equipment, you will need to do a mini-experiment to find the value of the uncertainty for position and time for this lab. : Use the multiple reading method (Type A in the uncertainty directions from Lab 1) to find the time uncertainty for this lab. Run the buggy along the floor from one mark to another while timing it. If all team members time, you will have several times. Repeat this until you have at least 10 times for the same event. Why 10? Remember that generally, you must keep 90% or more of your values. Then use the technique from the uncertainty handout to determine an uncertainty for time. Be sure to follow the rounding guidelines for uncertainty. Position: Use the single reading/analog method for position uncertainty in this lab. (Review the Type B: analog directions from Lab 1).
Lab: Constant Motion Page 4 Working Data Tables Sample Table for Buggy Path 1. Create similar ones for Buggy Paths 2 and 3. Buggy Path 1 Tip: don t forget to record your starting position! Zero is an important number also! Position (cm) Avg. Formal Data Table: (This one goes in your formal report) The position and time data was the result of direct measurement. We call this measured data. Even though the average time was not directly measured, it is a best value description of the time for each position. For this reason, the average value is usually listed in your formal data table as measured data. The uncertainty should also be given. You may state the uncertainty anyway that you consider clear. One method is to list it as part of the column heading. Create a formal data table that contains only the average values for each run (and units and uncertainty). Analysis: The analysis technique featured in this lab is the graph. You will demonstrate two skills related to using graphs in lab. The first is drawing a proper graph. You should review the Data and Graphing Guidelines document as you work with your graph for this lab. The lab involved three different motions. In order to compare these motions, they need to be placed on one graph. Plot a position vs. time instant graph (on engineering graph paper) using data for each buggy motion from the FORMAL data tables. This will be a multiple line graph so you will need to include a legend (colored pencils are recommended). Draw a best-fit line for each run. If your data does not look linear, ask for help with the best-fit line. Find the slope of the best-fit line for each run. (Include the units!) Write the equations for each line. Explain. (Hint: Remember y=mx+b. Both the slope (m) and the intercept (b) values must have units. Also the equation variables should match the variables in the graph axes labels.)
Lab: Constant Motion Page 5 Sample Test Questions for Study: Approximately 20% of exam questions will relate to labs. You should use the following questions as you prepare for exams and quizzes. Most of these questions are in reference to the analysis section. You should also be prepared to answer questions about the procedure, data collection, tables, graph, uncertainty, and any calculations. 1. What do the units tell you about the meaning of the slope of the line on a position -vs- time graph? 2. What is the meaning of the vertical axis intercept (the b in the standard algebra notation)? 3. How is average velocity calculated for the buggy motion? 4. Plug in values from buggy motion 2 to explain why average velocity. x t will not yield the correct 5. Do the lines have different slopes? What does this tell you about the motion? 6. Do the lines have different intercepts? What does this tell you about the motion? 7. How would you read the following information off of the graph you made for this lab? a. The position of the buggy at 0.75 sec. b. The displacement of the buggy between its position when t=0.75 sec, and its position when t = 1.0 sec c. The time instant when the buggy reaches the position x = 118 cm. d. The time interval between when the buggy is at x=118 cm, and x=150 cm. e. The initial position of the buggy.