Physics Experiment

Motion with Constant Velocity



In this lab, you will use a Pasco motion detector to gather data on the motion of a constant-speed cart. You will then use DataStudio software to transform the data into graphs of position versus time, velocity versus time, and acceleration versus time for the motion, and analyze the graphs using both the DataStudio software and "pencil and paper" calculations.


Pasco ScienceWorkshopTM 500 Interface (CI-6760) Pasco USB/Serial Converter (CI-6759)
Pasco Motion Sensor II (003-06758) Pasco Variable Speed Motorized Cart (ME-9781)


  1. Set up the interface.
    1. The power supply connects to the wall outlet, and the plug goes in the socket at the center of the back of the interface box.
    2. When the switch on the rear of the interface is set to "ON", the green LED on the front of the interface should light.
    3. Connect the round plug of the USB/Serial converter to the "TO COMPUTER" socket on the back of the interface box. The flat end of the converter cable connects to a USB port of your computer.
  2. Set up the software.
    1. Find the DataStudio folder inside the Applications folder of your computer.
    2. Inside the DataStudio folder, double-click on "PasPortal".
    3. In the PasPortal application window, click on "Launch DataStudio".
    4. If no dialog appears automatically, select "New Activity" from the file menu. Otherwise, in the DataStudio dialog that appears, select "Create Experiment".
    5. If the Experiment Setup window of DataStudio does not display a ScienceWorkshopTM 500 interface, or if there is an error message that the interface was not found or is not connected, click on the "Change" button in the menu bar and select "SW 500 ". This should bring up a picture of the interface you are using in the Experiment Setup window. (If the Experiment Setup" window isn't visible, click the "Set up" button in the DataStudio toolbar>
  3. Set up the motion detector.
    1. Double-click on "Motion Sensor" from the list of sensors on the left side of the Setup Window.
    2. The picture of the interface in the Setup window shows where to put the yellow and black plugs. ( The yellow plug goes on the left (Ch 1) as you face the front of the interface box, and the black plug goes in the second socket (Ch 2).)
    3. The white plug on the other end of the cord goes in the side of the motion sensor.
    4. Set the switch on top of the motion sensor to "NEAR".
    5. Adjust the motion sensor so that the front (metal grill) is vertical (more or less).
    6. If you get a message "DataStudio could not connect ..." or the Start button in the DataStudio toolbar is "grayed out" so you can't select it, click the "Connect" button in the Setup window toolbar.
  4. Test the setup.
    1. In the (top) Data Window, drag the first line (Position Ch 1 & 2 (m)) down to the Displays Window, and drop it on "Graph". A graph window will open showing position in meters on the vertical axis and time in seconds on the horizontal axis. You can enlarge this graph window by pressing the "+" button at the top, or just drag the sides/bottom.
    2. Press "Start" in the button bar at the top of the screen.
    3. You should hear a rapid clicking coming from the motion detector. When you wave your hand back and forth (horizontally) in front of the motion detector, your hand motion should register on the graph. When you hold your hand stationary in front of the motion detector (and at least 15-20 cm from it) your hand position should register as a steady line on the graph.
    4. If not, click "Stop", check your connections and setup, and try again.
    5. Once everything is working, delete any data runs you made while testing the motion sensor by selecting "Delete ALL Data Runs" from the Experiment menu. (Yes, you can throw them away - they aren't actually experimental data.)
  5. Set up the experiment.
    1. Drag the "Velocity Ch 1&2 (m/s)" and "Acceleration Ch 1&2 (m/s/s)" lines (one at a time) in the Data Window to "Graph" in the Displays Window, just as you did for position.
    2. Now, drag all three (Position, Velocity, and Acceleration) entries in the Data Window to the "Table" entry in the Displays Window. With this setup, the DataStudio software will automatically create data tables and time graphs for these three quantities.
    3. Be sure that you have a clear area one to two meters long in front of the motion detector.
    4. Try a practice run to be sure that you can get reasonably-good data. If the data points "jump all over the place" on your graph, try:
      1. Adjust the aim of the motion detector.
      2. Be sure there are no objects near the path of the motion detector that could reflect spurious signals back to it.
      3. Tape an index card on the back of your cart to provide a bigger target for the motion detector.
    5. Once you are satisfied with your experimental setup, delete all of the test data runs.
  6. Run the experiment.
    1. Place the cart approximately 15-20 cm in front of the motion detector, so that it will move away from the detector. Turn the speed-control knob on the cart to produce a fairly slow speed for the first trial. Release the cart and press the "Start" button on the interface screen.
    2. Collect data for ten to fifteen seconds, then press "Stop".
    3. Repeat steps "6a" and "6b" two more times, using a medium, and then a "high" speed for the cart.
  7. Save the experiment/data file.


  1. Print a copy of each data table for your lab record.
  2. Press the "Scale to Fit" button (the left-most button) in the Graph Window toolbar for each graph. This will generally produce a good view of your graph.
  3. It would probably be a good idea to pull down the "Settings" menu at the top of each graph and un-check "Connected Lines".
  4. Use the Text button at the top of each Graph Window to place a suitable title at the top of each graph.
    1. Click the "A" button.
    2. Click at the approximate location of the graph title.
    3. Enter the title text in the Annotation Properties dialog that appears. It doesn't hurt to put your name in the graph title so that you can find it more easily in the printer.
    4. Before you press "OK", select "None" from the Data Pointer drop-down menu. You can make up your own mind about the "border" and "opaque" text boxes.
    5. Click "OK".
  5. Draw best-fit lines for all of your data sets.
    1. Click on the "run" number in the graph legend.
    2. Select "Linear" from the "Fit" pull-down menu in the Graph Window toolbar.
    3. You can drag the statistics box with the mouse to put it somewhere out of the way.
  6. Print a copy of each graph for your lab record.
  7. Calculate the area under your velocity versus time graph.
    1. Uncheck runs #2 and #3 in the "Data" pull-down menu in the Graph window toolbar for the velocity versus time graph. Only the graph for run #1 should be showing. (Don't worry, nothing happened to the other data.)
    2. Using the cursor, drag a selection rectangle around the data points in some time interval (say from 1 second to 10 seconds). The selected data points will be highlighted in yellow. If you didn't get them all, just try again until you do.
    3. Select "Area" from the Statistics pull-down menu in the Graph Window toolbar. (The statistics icon is the Greek letter "sigma".) The program will shade and calculate the area between the graph and the horizontal axis. You can drag the report window to a convenient spot on the graph.
    4. Print this graph for your lab record.
    5. Repeat steps "6a" through "6d" for runs #2 and #3 for the velocity versus time graph only.
  8. Now, some "pencil and paper" work. Go to the printout of your position versus time graph. For each run, select (and label) two points on the best-fit line and calculate the slope of each line. Be respectful of units. Also, be sure to show a sample calculation in your lab record.
  9. Repeat step 7 for your velocity versus time graphs. You don't need to show another sample calculation (because it's the same calculation) but be sure to indicate and label the points on the graph that you selected for your calculation.
  10. Now, for each of the three velocity versus time graphs that you created in step 6, calculate the shaded area by considering it to be a rectangle bordered on the top by the best-fit line, the bottom by the time axis, and by the starting and ending time values on the left and right. Calculate the area of this rectangle for each run. Show a sample calculation. Respect the units.


Here are some questions to help you draw some useful conclusions from this experiment:

  1. Why is the position versus time graph for this motion a straight line?
  2. Why is the velocity versus time graph for this motion a horizontal line?
  3. Why is the acceleration versus time graph for this motion a horizontal line along the time axis?
  4. Did your hand-calculated slopes for the graphs agree with the automatically calculated values? How well? Do you think that you can trust the software-calculated slopes?
  5. What is the significance of the slope of the position versus time graph? (The units of the slope are a big hint.)
  6. What is the significance of the slope of the velocity versus time graph? (The units of the slope are a big hint.)
  7. What is the significance of the area between the velocity versus time graph and the time axis (the area "under the graph")? (Again, the units of the area are a big hint.)
  8. The area "under" the acceleration versus time graph over most any time interval will be approximately zero, right? What does this say about this motion?
  9. Suppose we took the cart and placed it about 2m from the motion detector, and let it move toward the detector. What would the position versus time, velocity versus time, and acceleration versus time graphs for this motion look like? (Yes, of course it's ok to try it and see if your prediction is correct!)

last update August 27, 2009 by JL Stanbrough