VideoPointTM Analysis

Free Fall



Discussion:

VideoPointTM software gives you the ability to analyze motions captured in digital movies. In this lab, you will analyze the motion of a ball in free fall.


Procedure:

1. Load the VideoPointTM software. (This software is not available on all computers.) On the initial screen, click on "Open Movie", then navigate to the file "vp_freefall.mov" located in your Group Shared folder.

VideoPoint splash screen

2. You will be tracking a single ball in this video, so press "OK" to select one object.

VideoPoint number of points screen

3. This is the main screen of the VideoPointTM software, but there is not much here of interest to us now. Go to the Movie menu, and select "Double Size" or "Fill Screen" to get the largest movie possible (You can also press <Apple><3> to do this.).

VideoPoint initial screen

4. Use the movie controls to play the movie. (<Apple><R> rewinds, or you can select "Rewind" from the Movie menu.) Repeat until you can follow the path of the ball as it is thrown into the air. (It's not so easy - I wonder who thought that a blue ball against a blue background was a good idea...)

5. Notice that the cursor looks like . Center the cursor on the ball in the first frame and click the mouse. The movie will automatically advance to the next frame. Locate the ball in this frame, and continue for the rest of the movie. (There are 29 frames in this movie.)

6. If you make a mistake in a frame, you can go back and drag the locator with the mouse to the correct position, but wait till you track the whole movie to make changes.

7. Now, you need to tell the software how to scale your measurements - in other words, how much real distance a "movie distance" represents. To do this, click the Scale icon (VideoPoint scale icon) from the toolbar on the left side of the screen, or select "Scale Movie..." from the Movie menu. The dialog shown at right will appear.

8. Since the movie contains a meter stick, you can leave the "Known Length:" equal to 1.00 m - in fact, you do not need to make any changes in this dialog box at all - just click on "Continue".

9. You will be instructed to click the mouse at one end of the known length (either end will do), and then click the mouse at the other end. That's all there is to it.

10. You can now select "Normal Size" from the Movie menu (or press <Apple><1>). Notice that the coordinates of the ball's position have been entered into the data table for you.

VideoPoint Scale Dialog

11. To make graphs of position vs. time, velocity vs. time, and acceleration vs. time for the ball, click on the Graph icon (VideoPoint graph icon) on the toolbar, or select "New Graph..." from the View menu (or press <Apple><G>).

12. Change the Vertical Axis to "y" (as shown at right) and select Position, Velocity, and Acceleration (hold down the <Shift> key for the second and third selections.). Then click "OK".

VideoPoint plot dialog

13. Enlarge your graph window to a reasonable size. Notice the shape of the velocity time graph - it is (hopefully) a straight line, more or less. The VideoPointTM software can calculate the equation of this line for you. Click on the Fit icon (VideoPoint fit icon) to the right of the velocity graph, or select "Add/Edit Fit" from the Graph menu (the Graph must be the selected window).

14. Since the graph appears to be a straight line, press "Apply", and the equation of the line that best fits your velocity vs. time data will appear above the graph. Press "OK" to get the dialog box to disappear.

15 Repeat steps 12 and 13 for the acceleration vs. time graph, which (hopefully) also appears to be a straight line, more or less.

VideoPoint curve fit dialog

16. Print a copy of your graphs.

17. Select the data table window, and enlarge it to show all of the position and velocity data. Then, print this window.

18. Save your analysis file in case you need to refer back to it later.

19. Quit the VideoPointTM program.


Analysis & Questions:

  1. Write a brief description of the position vs. time graph for the ball's motion.
  2. On your position vs. time graph, identify the portion of the graph that represents the time period when the ball was moving upward, and the portion of the graph that represents the time period when the ball was moving downward. Also, identify the instant (as nearly as you can) when the ball was at its highest point.
  3. Write a brief description of the velocity vs. time graph for the ball's motion.
  4. On your velocity vs. time graph, identify the portion of the graph that represents the time period when the ball was moving upward, and the portion of the graph that represents the time period when the ball was moving downward. Also, identify the instant (as nearly as you can) when the ball was at its highest point. (These time periods probably should match the times in question 1, shouldn't they?)
  5. Write a short description of what the velocity was doing as the ball was going upward, and when the ball was going downward. What was the ball's velocity when it was at its highest point?
  6. According to the best-fit equation, what is the slope of the velocity vs. time graph for the ball's motion? Why does it have that slope?
  7. Did the ball have a different acceleration as it was going upward than when it was going downward? How do you know?
  8. What was the ball's acceleration when it was at its highest point? How do you know this?
  9. Write a brief description of the acceleration vs. time graph for the ball's motion. What does the graph tell you about the acceleration of the ball while it was in the air?
  10. Looking at the best-fit equation for your acceleration vs. time data, what is the physical significance of the y-intercept of the line? What is the physical significance of the slope of this line?


last update October 10, 2001 by JL Stanbrough