Video analsyis with the bicycle

Some data:

• Video used: frames_of_reference.avi (1.3Mb)
• Video produced at: Norfolk State University
• Producer: C. G. Henderson
• Cyclist: Jan Klijn
• Video analysis software used: vidshell 98 Beta (Tb60_32r.exe) by Doyle Davis (Download the latest version of Vidshell ! Free!)

Exercise

1) Start the program Tb60_32r.exe (or Tb60_16r.exe) on the computer.

2) The computer asks for a *.tbk file. Choose vidshell.tbk and click on OK.

3) Click anywhere on the purple screen to start.

4) You see a grey screen with some buttons and two black windows. Click on the green button with the text show button info. Now the computer shows a grey window with information about each button.

5) To open a video: click on the grey button with the video recorder icon to display a list of available videos.
Button with video camera.

6) Choose other source, since the video is not a library video.

7) Browse trough the directories to find the file frame_of_reference.avi. Select the file frame_of_reference.avi and press the OK button.

8) The computer fills the left black window with the video clip of a cyclist. Press the Rewind button just below the video window to reset the video clip. If you wish, you can play the video clip again py pressing the play button.

9) After rewinding the video, the next step is defining distances in the clip. The computer doesn't know the distances in the clip, so you must set them yourself. Press the button with the icon of the computer mouse to scale the video.
Button with mouse icon.

10) An explanation window opens. Read it and click on OK.

11) On the video window you see two small red Coca Cola cans. The distance between them (centre to centre) is 2.93 meter. Go with the mouse to the first Cola can and click the left mouse button without releasing it. Hold it and drag it to the other Cola can. Release the mouse button now.

12) The computer asks for the length of this line. Type 2.93 and click OK.

13) The units of the measurements are in meters, so type "m" and click on OK.

14) Now it is time to set the frame of reference. The computer can measure the displacement only relative to a point. On the left side of the screen you will find a block of 5 rows of buttons plus a row with color buttons. Choose the first button on the third row with the black plus and the letters x and y. Press this button to draw a set of standard (non-rotated) coordinate axes.
Button to set the origin.

15) Read the important message and click OK.

16) Put the mouse on the axis of the front wheel of the bicycle and click on the left mouse button.

17) Now you have yellow cross on the screen with the crossing as origin. After this you have to define the positive directions for both the x- and y-axis. Choose right for the x-axis and Upward for the y-axis. This gives a standard X-Y coordinate system.

18) Click now on the button Show data on the right of the screen.

19) Choose a x, y, t data table. You see an empty table with five columns.

20) The video window might not show the video now. This is an error in the program and might happen more times. To restore the video, press the button Hide video. This button changes now in Show video. Press the button again. The video is visible now.

21) Press now the button to collect data in the table. It is marked with the three columns and nine rows, in the left lower part of the screen.
Button to collect data in the table.

22) The computer asks for the time difference between two frames in seconds. A frame is a picture and the video plays 30 of these frames a second. So enter 0.03333 (i.e. 1/30) and click OK.

23) The computer asks for the kind or data to collect. Select x, y, t.

24) Click on one of the eight color buttons on bottom of the screen. Best is to select a bright color with a large contrast with the video. Let's choose red.
Color buttons.

25) Click on the axis of the front wheel of the bicycle.A red dot appears in the video window.

26) In the table right of the video windows, you see the values of your first marked point. The table shows

• The number of the point (n)
• The number of the frame (pos#)
• The horizontal position relative to the origin (marked with the yellow cross) (x)
• The vertical position (y)
• The time (t)

27) Click on the Step forward button below the video window and click again on the axis of the front wheel. It has changed a little.
Step forward button.

28) repeat step 27 until the front wheel axis is no longer on the screen.

29) Make in your notebook a table with five columns and copy the data from the table on the monitor to your notebook. With the scrollbars, you can make all rows of the table visible.

30) Press the Rewind button below the video to reset it. The points and yellow lines disappear.
Rewind button.

31) Press the button with the three columns and nine rows at the bottom of the screen again. The points and lines reappear, but the video is gone.

32) Select another color by pressing a color button on the bottom of the screen. Let's choose blue.
Color Buttons.

33) Click on the Hide video button and then on the same button to Show video. Video and points are visible now.

34) Click on the white reflector on the front wheel of the bicycle. A blue point appears.

35) Click on the Step forward button below the video window and click again on the reflector.
Step forward button.

36) repeat step 35 until the reflector is no longer on the screen.

37) You see now, that the table contains much more datapoints than before. Find by scrolling the last row with data of the axis of the front wheel (see the number of the last line in in your notebook). It should be halfway your table. Make a new table in your notebook of SEVEN columns and copy the new rows from the table in the computer to the table in your notebook. Keep the last two columns empty.

38) Make a graph with the x-position of the wheel axis on the vertical axis and the time on the horizontal axis.

39) Plot the in the same figure the x-position of the reflector.

40) Make also a graph of the y position of the axis of the wheel on the vertical axis and the time on the horizontal axis. Add the y-position of the reflector to this figure.

41) QUESTION
Why is the graph of the x position of the axis of the front wheel versus time a straight line?

42) QUESTION
Why is the graph of the y position of the front wheel axis almost horizontal?

43) QUESTION
Why has the graph of the y position of the reflector a wave shape around 0?

44) It is now possible to fill the last to columns of the second table. Fill the sixth column with numbers obtained in the following way:

• Go to the first row in your table with data of the reflector. Look for the time in the fifth column.
• Now find the same time in the table with the data of the front wheel axis.
• Take the x position of the reflector (2nd table) and substract the x position of the wheel axis (1st table). Write the answer in the sixth colunm.

45) Do the same for all rows in the second table.

46) Fill the seventh column in the same way, but for the y's.
So to fill them:

• Go to the first row in your table with data of the reflector. Look for the time in the fifth column.
• Now find the same time in the table with the data of the front wheel axis.
• Take the y position of the reflector (2nd table) and substract the y position of the wheel axis (1st table). Write the answer in the seventh colunm.

45) Do the same for all rows in the second table.

46) Make a new plot. On the horizontal axis the numbers in the sixth column of your table and on the vertical axis the seventh column.

47) QUESTION
What kind of figure do you find?

48) QUESTION
What is the explanation for this figure?

49) QUESTION
Can you estimate the distance between the reflector and the axis of the front wheel from your plots? How?

50) QUESTION
In this exercise, we used two different frames of reference. Can you say which two frames we used?

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