Laboratory Activity 1: Position Graphs
/ Group member names:-Rebecca Wade
-Jed Peterson
-Stephanie Peterson
Objectives
· Learn how to use a motion detector.
· Explore how motions are related to position-time graphs
Equipment
· Computer
· Ultrasonic motion sensor
· USB Link
Activity One: Simple graphs
Prediction 1: Using the line tool on the drawing toolbar of Word, draw a line on the graph below predicting the shape of a curve for a person walking away at a slow, steady pace. If you don’t see the drawing toolbar (usually located at the bottom of the window), check to see that it is enabled with the drawing toolbar button .
An ultrasonic motion detector is a device that can be used to record an object’s motion as a function of time. It sends out a series of sound pulses (too high frequency to hear). These pulses reflect from objects in the vicinity and the reflected pulses return to the sensor. The computer can record the time it takes the reflected pulses to return to the computer, and using the speed of sound, calculate the position of the object.
When using a motion sensor:
· Do not get closer than 0.3 meters (one foot) from the sensor because it cannot record reflected pulses that come back too soon after they are sent.
· The ultrasonic waves spread out in a cone of about 15° as they travel. They will “see” the closest object. Be sure there is a clear path between the object whose motion you want to track and the motion sensor.
· The motion sensor is very sensitive and will detect slight motions. You can try to glide smoothly along the floor, but don’t be surprised to see small bumps in position graphs and even larger bumps later in velocity and acceleration graphs.
· Some objects like bulky sweaters are good sound absorbers and may not be “seen” very well by a motion sensor. You may want to hold a book in front of you if you have loose clothing on.
Set up the equipment: Obtain a USB Link and a motion detector as pictured. Start up the computer if it is not already running. Plug the USB link into the back of the computer as shown. Note that there is only one direction in which the cable will plug in. Then plug the cable of the motion detector into the USB Link. Note that there is again only one direction in which it will plug in.
Within a few seconds the computer should announce, “Sensor detected” and ask you what to do. Select “Open DataStudio” and wait until it has finished loading and detected the sensor. Click on the “Start” button along the bar at the top and move something away and towards the motion detector to verify that it is working. You should hear it making a clicking sound and see the line being graphed move up and down. Click on “Stop” (the same button as “Start”) and clear the data by going to “Experiment” on the top menu, selecting “Delete Last Data Run” and answering “OK” to the prompt. Open the experiment file called Distance.ds to display distance (position) vs. time axes. You can find it by going to the networked disk “PLAB” (usually disk G:), in that opening the folder “Plab”, then finding the folder for Physics 201, and then looking in the folder for Lab1.
Experiment: Set the detector on the edge of the table and have one member in your group stand about 0.5 meter (½ yard) away from the detector and walk away backwards at a slow, steady pace, while watching the graph on the screen. It may help to get a smoother graph to hold a book in front of you away from your body at the level of the detector. Let everyone have a chance. When you have a nice graph, copy the graph and paste it in the section below. If you have a graph that messes up, you can delete it by selecting “Delete last data run” from the “Experiment” menu. If you get so many runs it is hard to see, you can remove them all with the “Delete all data runs” from the Experiment menu. To copy the graph to MS Word, first click on the graph in DataStudio to make sure it is selected, then select “Copy” from the “Edit” menu in DataStudio. Then click in the space below and then do a “Paste Special” in the “Edit” menu of Word. Select the bitmap image option and click OK.
-paste graph here
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Question 1: How did the result compare to your prediction? If it was different, do you now understand why it is the way it is? Note: you should not go back and change predictions. There is absolutely nothing wrong with making a prediction and then finding out it wasn’t quite correct. In fact, that can show that you are learning.
The graph from our experiment matched our prediction fairly well. The only true difference is the slope of the line, indicating that we walked at a different pace than that we initially graphed.Prediction 2: How will the graph be different if you walk away from the detector at a faster pace? How will the graph be different if you start further from the detector and walk toward it?
The graph would be different if we walked at a faster pace. More distance would be covered in the same amount of time, meaning that the slope of the line would be more steep. If we walked towards the detector, the slope would be negative instead of positive as it was in our initial experiment.Experiment: Without erasing your last data run, start about ½ meter from the detector and walk away at twice the rate. Then take another run, starting away from the detector and walking toward it. Label each run as to which it is by double clicking on the labels “Data Run 1” in the menu list on the upper left-hand window and giving them more descriptive names. Then copy and paste your graph in the space below the same way as above.
Question 2: What does the magnitude of the slope (how fast the over-all line is increasing or decreasing) tell you about the motion? What does the direction of the slope (increasing or decreasing) tell you about the motion? What would a graph look like for someone standing still?
The faster one walks, the greater the slope of the line will be. If someone walks away from the motion detector, the graph will show a positive slope. If someone walks towards the detector, the graph will show a negative slope. If someone stands still, the graph will show a slope of 0, or a horizontal line.Activity Two: More complicated motion
Prediction 3: Sketch on the graph below a prediction for the following motion: walking away from the detector slowly, stopping for a while, and then walking faster away from the detector.
Question 3: Try walking the way described. How does it compare to your predictions?
Save a walk that lasted a little longer when done, the graphs look very similar.Activity Three: matching graphs
(Note: if your group had some technical difficulties and are running out of class time, skip to the summary questions and come back and finish as much of this activity as you can.)
Open the experiment file called PositionMatch.ds. A position graph like that shown below will appear on the screen.
(from RealTime Physics (Electronic Version) by Prisilla Laws, et al., John Wiley and Sons, NY, 1999)
Prediction 4: Describe in words how you would move in front of the motion detector in order to create the graph.
A person should stand still for 6 seconds. Then, he should walk away from the motion detector at the pace of .75m/s for 2 seconds. After that, he should stand still again for 4 seconds, only to walk away again at .25m/s for 4 seconds. Then, the person should stand still for the remaining 4 seconds.Experiment: Walk in front of the motion detector and try to create the graph. You may try a number of times. It helps to work in a team. Get the times right. Get the positions right. Each person should take a turn.
Experiment: Try walking to produce a graph like each of these bellow. First discuss with your team how you would need to move to produce each graph, and then work as a team to do each.
(from RealTime Physics (Electronic Version) by Prisilla Laws, et al., John Wiley and Sons, NY, 1999)
Question 5: How do you need to walk to match graph A?
One would need to walk away from the detector slowly, and then accelerate at a constant pace.How do you need to walk to match graph B?
One must decelerate at a constant pace, walking away from the motion detector.How do you need to walk to match graph C?
One should walk towards the motion detector, accelerating the entire way.Summary
The following questions will help you get the main ideas out of this lab. You should find these straightforward questions, but take the time to talk it over with your team and write complete answers to these questions. You may find your answers here to be the most useful part of this lab down the road.
Summary 1: In a position graph, what does the horizontal distance from the axis of the graph mean? What doe vertical distance from the axis mean?
The horizontal axis of the graph shows time elapsed. The vertical axis of the graph indicates distance from the motion detector.Summary 2: In a position graph, what does the slope of the line mean? What does a line sloping down mean? What about a line sloping up? A horizontal line? What does the steepness (how fast it is increasing/decreasing mean)?
The slope of the line indicates velocity. The larger the slope, the greater the velocity. The smaller the slope, the lesser the velocity. If a line slopes downwards, the subject is moving towards the detector. If the line slopes upward, the subject is moving away from the detector. If the line is horizontal, the subject is standing still.Summary 3: How do you know your above statements are true? What experimental observations and/or logical reasoning can you give to justify what you said in summary questions 1&2?
Through our experimental runs, we were able to support what logic already told us. Throughout the experiment in its entirety, when we walked away, there was a upward slope. When we walked towards, there was a negative slope. When we remained still, there was a slope of 0, and the faster we walked, the larger the slope.This activity was adopted from materials in the following curricula: Workshop Physics for Life Sciences, Beth Ann Thacker, RealTime Physics, David Sokolof, Ronald Thorton and Priscilla Laws, and University of Northern Iowa General Physics Activities, Larry Escalada and Malcomber.