A) Setting up and Shutting down

Setting up

1. start Windows (if necessary; if the DOS Shell is open, press the Shift and F9 keys simultaneously, then type "win" at the DOS prompt and press Enter)
2. double click on the icon that says MPLI or MPLI Program for Windows
3. double click on the MPLI for Windows icon within that window
4. Plug the sonic ranger probe into slot A of the blue box labeled MultiPurpose Lab Interface.

1) remove the sonic ranger from the blue box... DO NOT PULL ON THE CORD. Grasp the plug and pull it out.

2) Ask your instructor if you should leave the program running or exit the program.

Once you have reached the MPLI window:

1) under File, select Open, and then double click on the digital.exp file

2) press <Enter> (or click on the green Start button). The software displays the distance to the nearest solid object, in meters.

3) Your job in this part is to experimentally determine the approximate minimum and maximum distances that the sonic ranger measures correctly (allow for a 1 or 2 cm uncertainty). You'll need a meter stick to know the "accurate" distances. Record the results for the workable range of your Motion Detector in your lab journal. Check with the instructor to see if your answers are reasonable.

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1) Decide which graphs to display. In general, you should always display the Distance (which really should be called "Position") graph, and you may add the Velocity and Acceleration graphs ONLY IF you have predicted them first in your lab book.

2) To add graphs: under Window, select Add Window (select the lower of the 2 options, so that the windows are arranged vertically one above the other) and then further select Plotter Graph

3) Change the name of the new window you just added to plot Velocity (or Acceleration): double click on the new window, and then select More Y-axis Options. Select the appropriate quantity to be plotted in your new window, and de-select all other quantities. Click on the OK buttons twice.

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C) Position, time, & velocity

1. under File select Open, and then double click on the motion.exp file. Once the file opens, close the Velocity vs. Time and Acceleration vs. Time graphs and the Text Window. Use the Timing button (on the upper right of the screen) to set the Experiment Length to 5 seconds.
2. While standing still, aim the sonic ranger at a wall. Before asking the software to produce a graph of the wall’s position vs time, predict what you think that graph will look like (don't worry about putting numbers on your axes; you only need to predict the shape of the graph). Make a sketch of your predicted graph in your lab journal at the top of a new page; use the whole top half of the page for this prediction and reserve the bottom half for the velocity-vs-time graph that will come later. Label your axes: position and time.
3. Once you've predicted the shape of the graph, aim the sonic ranger at the wall and click on the Start button to get the software to plot the graph. [Remember that only the distance-vs-time plot should be showing on the screen.] Does your prediction agree with the plot? To set the proper scaling, double click on the graph, select More Y-Axis Options, then select Manual Scaling and type in 0 for the Bottom Limit and 3 meters for the Top Limit.
4. You probably have some familiarity with the term velocity. What is the velocity of the wall (with respect to the sonic ranger) for the situation described in #2 above? What would the velocity-vs-time graph for the wall look like? Make a prediction and sketch it in your lab book right below the position-vs-time graph. Remember to label your axes with words.
5. We hope that you answered zero (or some equivalent) to the previous question. Now go back to the computer and check your prediction. The instructions for adding a graph are on the 1st page.
6. Look back at the position-vs-time graph for the wall that you predicted and the computer plotted. Is there anything zero about the position-vs-time graph? Talk it over with your partner.
7. We trust that your answer to the previous question, without looking ahead, was something like "the slope of the line or graph is zero." Now, is it a coincidence that both the velocity and the slope of the line are zero? In your lab journal, write a general definition of the slope of a line. Do not use the symbols x and/or y in your answer, but you may use words like horizontal, vertical, rise, and run. This is the last we'll remind you to write only in your lab journal. In the future, we expect you to do this automatically.
8. If we now apply your definition of slope specifically to a position-vs-time graph, we see that the slope of the graph is the change in position divided by the change in time. And this is exactly the definition of velocity.
9. Now let's try something that moves. One person will walk away from the sonic ranger at a fairly slow constant rate. But before you try this, sketch a prediction of the position-vs-time graph for the slow-walking person. Again, label the axes (but don’t worry about numbers other than the origin on your graph). Compare your prediction to your partner's.
10. Now collect data -- but remember to turn the velocity-vs-time graph off first (you haven’t predicted it yet, right?) The person who walks should begin at a distance from the sonic ranger that is roughly equal to the minimum distance for which the ranger works; you might also want to set the maximum distance to 3 meters. Record the computer-generated graph. Remember that double-clicking on a graph allows you to select various options for that graph. NOTE: If you occasionally get spikes (exceptionally high or low points) on the graph, this is due to the failure of the ranger to detect your reflected signal. It sometimes helps to hold a reflector in front of the ranger as you are walking. This could be a book or other hard, flat surface.
11. Did your prediction match reality? Check with the instructor if you're not sure. Estimate the slope of the position-vs-time graph by reading two ordered pairs (time, position) of coordinates at the opposite ends of your line and calculating the slope. Avoid using points that seem to be noticeably off from the general trend. To obtain coordinates, under Analysis, select Examine; then drag the mouse around your graph. Show your work, and include units (m, s, m/s) whenever you write a position, time, or velocity number.
12. Once you have calculated the slope of your position-vs-time graph: under Analysis, select Tangent Line. Then drag the mouse along your position-vs-time line. How do the Tangent Line values compare to the slope you calculated ?
13. Now for a new prediction. What would the velocity-vs-time graph look like for the walk done above? Sketch your prediction (again, don't worry about numbers on your graph). [Hint: Was the velocity of the walker constant? How could you tell from the position-vs-time graph?]
14. After you have sketched your velocity-vs-time graph prediction in your lab book, go back to the computer and add the velocity-vs-time graph. Leave the position-vs-time graph on. You might also set the minimum velocity equal to zero and the maximum velocity equal to 2 m/s. Was your velocity-vs-time graph prediction correct?
15. The other partner will now try a faster walk away from the sonic ranger. Before this happens, however, make a prediction of the position-vs-time graph on the same axes as your previous prediction for the slow walk. Label the first prediction SLOW and the second one FAST. Also predict the velocity-vs-time graph (don’t worry about numbers here) for the FAST walk and record it on the same velocity graph for the slow walk. Label both lines.
16. After predicting both position-vs-time and velocity-vs-time graphs for the fast walk, collect data for that walk, remembering to start at about the minimum working distance of the sonic ranger. Did your predictions match expectations?
17. Summarize what you have learned so far about how velocity information shows up on an object's position-vs-time graph.

1. Use a dynamics cart, loaded with weights, for these activities. Set the ranger on the floor, aimed at the cardboard reflector taped to the back of the cart. Change the software settings to give you only a position-vs-time graph; set the maximum position to 3 meters.
2. In this part you will give the cart a push away from the ranger. Try it first without the sonic ranger recording data (just turn the ranger face down on the floor temporarily). Then predict & obtain the position-vs-time graph. If the cart slows down as it moves, how is that seen on the graph? Did it slow down?
3. Based on your position-vs-time graph, predict what the velocity-vs-time graph of the same motion would look like. [Hint: As the slope of the position-vs-time graph changes, how does the velocity-vs-time graph change?]
4. Turn on the velocity-vs-time graph and check your prediction in 3). Sketch the graph you obtain. Were you correct?

6) Next you'll look at a collision. If you push the cart so that the plunger end strikes a wall, the cart will push back off and return. Set up the ranger facing the wall and about 2 meters away. Put the cart about a half-meter from the ranger, ready to be pushed toward the wall. But wait! That's right, make your predictions of the position-vs-time and velocity-vs-time graphs. (If you don’t think you can do both together, do the position one first, verify, then predict the second.) You can, however, try the experiment without having the sonic ranger recording, if that will help you visualize the position and velocity graphs. Once you've predicted, push the cart and record results.

7) When the cart turned around at the wall, how did the position-vs-time graph change? Be sure to be specific in discussing the slope of the line.

8) How did the velocity-vs-time graph change? Again, be specific.

9) Something to note at this point is that changes of velocity are caused by the application of forces. When the moving cart slows down, that is due to frictional forces. When the cart changes direction, that is due to the push of the wall on the cart. (It may seem strange to think of a solid, immovable wall as exerting a push. Later we'll see that walls, floors, etc., can exert such forces.)

10) Something else to note is that there are two ways that velocity can change. One way is when the object speeds up or slows down. The other is when the object changes direction. You probably noticed that the velocity of the cart changed faster when it bounced off the wall than when it was slowing due to friction. This rate of change of velocity, termed acceleration, is investigated in the next section.

E) Velocity, time, & acceleration

1) In this part, you'll look more closely at acceleration. Use the same cart as before, but allow it to roll down an inclined ramp. You'll find a variety of boards in the lab to serve as ramps. Start with an incline angle of just a few degrees. Continue to make predictions before performing each of the experiments.

2) Here, you'll start the cart at the top of the ramp. Position the sonic ranger aimed down the ramp so that the cart will move away from the ranger while rolling down the ramp. Obtain position-vs-time and velocity-vs-time graphs and record them. You may need to change scales if the maximum values are too small/large.

3) Describe in words each of the graphs you just obtained. Use words like constant, increasing, decreasing, slope, positive, negative, zero.

4) How do you think you could determine the acceleration of the cart? [Hint: Think about how you found velocity from a position-vs-time graph. How can you find acceleration from a velocity-vs-time graph?]

5) We hope that you realized that the slope of the line on the velocity-vs-time graph is the acceleration. You can calculate a value for it by reading two (time, velocity) coordinate pairs from the graph and using the slope formula. Do that now, and be sure to carry units throughout the calculation. Note that the units of acceleration are m/s per s or m/s² for short. Once again, avoid using points that seem to not follow the overall trend.

6) Add a graph of acceleration-vs-time below the velocity-vs-time graph. You may see a lot of bumps in this graph, because the process of calculating accelerations from the velocity-time data introduces error in addition to those introduced in calculating velocities from the position-time data. Do any time intervals on the graph have constant acceleration?

Conclusions