Name____________________________________
PHYSICS 124 LAB 1: INTRODUCTION TO LINEAR MOTION
Goal:
Your mission is to learn how to use a computer to observe motion in one dimension with a motion detector and to start learning how to analyze one dimensional motion graphically.
Setup:
In this lab exercise you will use a device called an “Ultrasonic Motion Detector” to observe motion in one dimension. It’s a small box that is driven by a special apparatus and a computer. This motion detector does exactly what its name implies: it detects motion. It is not necessary for you to thoroughly understand how this device works for you to complete this lab, but for curious minds here is a brief description: the motion detector is capable of indicating the distance to an object, at which it is pointed, by sending and receiving sound waves which interact with the object. Because the sound waves travel at high speeds, the motion detector can tell how far away the object is, even as the object moves. This device will be plugged into a computer and used in a series of instructive exercises to get you used to thinking about motion in a critical manner. Your station should already be set up (computer, Lab Pro, and Motion Detector). Log on using your ECOM account. Wait for the computer to start. Then double click the Logger Pro icon on the “desktop” of the computer.
You now need to open an “experiment file”, so from the drop-down menu use the mouse to select File > Open > _Physics with Vernier > 01a Graph Matching.cmbl Once you click on that experiment file, a single position vs . time graph should appear. If you have failed to produce the position vs . time window by this point, yell for help from your instructor.
You should notice that the window is in the form of a graph with two axes. The horizontal axis is for time in seconds, and the vertical axis is for distance from the motion detector in meters. The motion detector will tell you the distance to an object at which it is pointing by plotting points in rapid succession. The time between successive points is so small that it will look like a line. Set the motion detector flat on your lab table so that it is pointed toward the ceiling. Since the time axis is set from zero to ten seconds, by clicking the
COLLECT button, you will be able to monitor the position of the ceiling (actually just the distance from the detector) for a period of exactly ten seconds. Click the COLLECT button now.
The resulting graph should be a horizontal line, but may have some waviness. If you click between the numbers on the vertical scale, you can choose “autoscale” to let the computer pick the range of the vertical scale. Then the line may look something similar to (but not exactly like!) the figure below. The computer can automatically choose a scale (the upper and lower bounds) for the vertical distance axis. You will want to return to the manual mode later.
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click this number to change the upper bound of the distance scale click this number to change the lower bound of the distance scale click this number to change the time duration for observing the position of an object
Fig.
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For the graph displayed in Figure 3, the motion detector has determined that the ceiling is about
1.453 meters away from the motion detector; but, the “jaggedy” line tells you that the motion detector is only good at indicating a position within roughly 4 thousandths of a meter (4 thousandths of a meter (0.004 m) is the same as 0.4 cm). Think about this and make sure you understand what you are observing before proceeding. It is entirely acceptable and good for you to discuss this with your lab partners, with other groups, and/or with your lab instructor.
Now, you should manually rescale your plot by clicking on the lowest number on the distance axis, as indicated in Fig. 3. You can then enter another number for the lower bound for the distance. For this plot, enter “0” as the new lower bound, and then click anywhere else on the graph for the change to take effect. Repeat the process by clicking on the new highest number on the distance axis, and enter “3” as the new upper bound. You should now see a relatively straight horizontal line indicating a constant, well-defined position for the ceiling. In other words, during the ten seconds over which you observed the ceiling, its position didn’t change! (Again, if things didn’t happen as described above, yell for help!) You can also change the length of the observation time by clicking on the highest number on the time axis, or by using the menus: “Experiment” … “Sampling” … “Experiment length” … then enter 10 seconds.
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Before you perform the actual procedure for today’s lab exercise it will be beneficial for your group to take a moment to play with the motion detector in the following way: have your lab partner stand so that the round part of the motion detector is pointing toward him or her. Now by walking toward (or away from) the detector, try to determine the minimum (maximum) distance for which the motion detector can determine a position. Determining the maximum distance may be particularly tricky, and you will likely notice that the detector sometimes “loses sight” of the object. This is another indication that the instrument is limited in its ability to detect motion. It will not indicate the distance properly for distances smaller than about 0.5 m.
Procedure :
Using yourself or your lab partner(s) as the "object", (or you might use a flat object like a book or notepad) perform the following set of activities using the motion detector and the Lab
Pro system. Make sketches of the resulting graphs as they appear on the computer screen, and thoroughly answer any questions in the space provided.
1. Constant position . Perform the "motion" a. that produces constant "50 cm" position. b. that produces constant "150 cm" position. a & b
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0 2 8 10 4 6 time (seconds) c. Describe the “motion” (or perhaps it is “no motion”) that was performed to obtain the graph for part a and for part b. (Sketch the results for both a & b on the graph above.) d. Are these motions related? How are they similar?
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2. Changing position . Perform the "motion" a. that produces an increasing position. b. that produces a decreasing position. (Plot these two on the same graph below: a & b.) c. that produces an increasing, then decreasing position. a & b c
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8 d. Describe the motion that you performed to obtain the graph in each part: part a: part b: part c: e. Are these motions related? In what way? f. A displacement is the difference between the final and initial positions during a time interval. What would you have to do to get a displacement of 2.5 meters over the 10 second time interval? g. If you start at a position of 1.0 m, and then increase your position by 2 m, what must you then do to get a displacement of 0 meters?
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3. Constant velocity . Perform the prescribed motion in each part. a. Walk b. Walk away away
from the detector
from the detector slowly and steadily. quickly and steadily. c. Walk toward the detector slowly and steadily. d. Walk toward the detector quickly and steadily. a & b
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2 4 6 time (seconds)
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6 time (seconds) e. Describe the difference between the graphs obtained in parts a and b. f. Describe the difference between the graphs obtained in parts c and d. g. Describe the difference between the graphs obtained in parts a and c.
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4. Consider the following motion: a person starts at 0.5 m, walks away from the detector slowly and steadily for 5 seconds, stops for 3 seconds, and then walks toward the detector quickly and steadily for the last 2 seconds. First, make a prediction by sketching the expected distance vs . time plot on the blank graph below to the left. Then have your lab partner perform the actual motion and sketch what appears on the screen on the blank graph to the right. Be sure to discuss your prediction with your lab partner(s). prediction actual
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0 2 4 6 8 time (seconds) time (seconds) a. For this activity, is your prediction the same as what appears on the computer screen? b. If not describe how you would move to make a graph that looks like your prediction.
5. Make up your own distance vs . time graph below. Use straight lines (no curves) and sketch with a dashed line. Now see how well your lab partner can duplicate this graph using the motion detector. Sketch the best attempt by your lab partners on this same graph using a solid line.
Label both graphs.
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6. Perform the motion necessary to produce the following three distance vs . time graphs. It may take several attempts to get a good match.
Time Time Time
Graph 1 Graph 2 Graph 3 a. Describe how you moved to produce a distance
Graph 1
Graph 2 vs . time graph, for each of the graphs.
Graph 3 b. What is the general difference between motions which result in a straight distance vs . time graph and those that result in a curved distance vs . time graph?
For activities 7-9, you will use the motion detector to observe a velocity vs . time graph. Click on the word “Position” on the graph and a Y-axis selection box appears. Select “Velocity”. Change the vertical scale by clicking on the highest number and enter 1.0. Then click on the lowest number and enter -1.0. Change the Time axis to run from 0 to 5 seconds. The LoggerPro software can approximate a true velocity vs . time graph by calculating the average velocity over the time interval between two successive position measurements, and by then plotting this value as a function of the mid point of the time interval for each interval. Using short time intervals, the graph displayed by the LoggerPro software should be reasonably close to the actual velocity vs . time of the measured object.
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7. Constant velocity . Perform the "motion" that a. produces a constant zero velocity. b. produces a constant, but small positive velocity. c. produces a constant, but larger positive velocity. d. produces a constant, but small negative velocity. e. produces a constant, but larger negative velocity.
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a, b & c
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d & e
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Time (seconds) Time (seconds) f. Describe the motion that was performed to obtain the graph for each part: part a part b part c g. What is the most important difference between the graphs made in parts b and c? d and e? b and d? h. How are the velocity vs.
time graphs different for positive motion and negative motion?
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8. Changing velocity . Perform the "motion" that a. produces an increasing velocity. b. produces a decreasing velocity. c. produces an increasing, then decreasing velocity. a & b
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Time (seconds) Time (seconds) d. Describe the motion that was performed to obtain the graph for each part: part a part b part c e. Are these motions related? In what way?
9. Consider the following motion: a person starts at 0.5 m, walks away from the detector slowly and steadily for 5 seconds, stops for 3 seconds, and then walks toward the detector quickly and steadily for the last 2 seconds. First, make a prediction by sketching the expected velocity vs . time plot on the blank graph below to the left. (Keep in mind that you are given information regarding initial position and time, and you must predict what will happen in the Velocity graph!)
Then have your lab partner perform the actual motion and sketch what appears on the screen on the blank graph to the right. Be sure to discuss your prediction with your lab partner(s).
Note: The time range is now changed to 10 s.
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prediction
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Time (seconds) Time (seconds) a. For this activity, is your prediction the same as what appears on the computer screen? b. If not describe how you would move to make a graph that looks like your prediction.
10. Now we will attempt to produce the following velocity vs . time graph.
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Fig. 2
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Time (seconds) a. What motion must be performed to duplicate this graph? Describe for each portion of the graph what motion was performed. b. Now confirm your prediction by matching your predicted motion to the graph displayed above. You may need to change your prediction several times before you get a close match. Sketch your best result on the graph above. c. What was the difference in the way you moved to produce the two differently sloped segments of the graph? d. What motion did you have to perform to obtain the horizontal portions of the graph?
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