Day 4: 14 Trials and PASCO

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The team used 7 balls with 2 different backgrounds resulting a total of 14 video recorded trials in slow motion. In the trials, a member of the team dropped these balls individually as another member recorded the action with an iPhone 7 at 240 fps. During this process, the third member searched for additional clamps for the apparatus used to hold the iPhone 7. Several materials including steel, lead, wood, cork, rubber, and metal balls as well as a golf ball were dropped. The balls can be seen in Figure 1.


Figure 1. The 7 Balls

Figure 2 shows the iPhone 7 used along with the clamp to hold it securely to a horizontal metal stand.

Figure 2. iPhone 7 Holder and clamps


The first background was a wooden closet and the second background was a light blue brick wall. We used different backgrounds to see in which videos the visibility is best. Video 1 displays one of the seven recorded videos with a wooden closet background. Notice an error in the trial as the member of the team dropped the ball directly in front of her instead of dropping it over the wooden closet background.

Video 1. Golf ball with wooden closet background

The error from Video 1 was corrected in Video 2 by having a wooden ball dropped within the wooden closet background.

Video 2. Wooden ball with wooden background

The next video recorded in slow motion was of a cork ball. You can see the ball correctly drop in front of the wooden closet background; however, the ball passes through a white sticker/label on the wooden closet. See Video 3.

Video 3. Cork ball with wooden closet background

Video 4 and Video 5 presents balls (steel and lead) dropped with a light blue brick wall background. There is a paper taped to the wall that labels the material of the balls since it is difficult to tell the steel and lead balls apart through viewing the video. However, the quality of the video does not allow the viewer to read what the paper says for both videos 4 and 5. To correct this error, we will use a thick sharpie marker instead of a thin pen to label the paper of the ball's material being tested. Also, the white paper interrupts the uniform light blue brick wall background as the balls passes through it.

Video 4. First unknown ball with light blue brick wall background

Video 5. Second unknown ball with light blue brick wall background


After recording the 14 trials, PASCO program allowed us to edit the videos with information and create graphs to find the acceleration due to gravity.
Directions to use PASCO program:
1. Open program.
2. Select movie (double click).
3. Open movie file.
4. Select video.
5. Click on the second button on the top tab entitled, "enter video analysis mode."
6. The words, "audio playback is not supported" may appear on center of screen. It should fade away as you move your mouse.
7. Use the calibration tool to drag the measurement. In our case, we dragged it to the meter stick in the video for scaling.
8. Click the "display" tab.
9. Next "properties."
10. Then "overlay."
11.a. Change "frame increment" to "5."
11.b. Next "movie playback"
11.c. Change Playback frame rate to "240."
Note: We tried to change the frame increment to 3, but the time ball won't advance.
12. Click "Ok."
13. Select "magnify video" tool which is the 6th tab.
14. Click on the position of moving object. Make sure you click on the center of the ball.
15. Keep clicking on the center of moving object (ball) as it moves (falls). Those are the data points.
16. Select the graph icon on the right side tool bar. Drag it to the working area of the screen.
17. Click on "select measurement."
Note: x-axis is time and y-axis (for first graph) is "y. Object #1 (m)" which is distance.
18. Name the graph. Example: Position vs. Time of a Falling Golf Ball
19. Drag and hold the x-axis and y-axis to adjust the size of the graph to make it fit properly.
20. Drag in another (second) graph.
Note: For second graph, y-axis is Object #1: "vy. Object #1 (m/s)" and x-axis is time.
21: Name the second graph.
22. Adjust the second graph size by dragging the x-axis and y-axis.  
23. Select "highlight range of point in active data" for the second graph. Drag and select the desired data points.
24. Go to "apply selected curve fits" for the second graph.
25. Open drop menu and select "linear: mt+b."
Note: m is the slope (acceleration due to gravity).


For Video 1, of the golf ball, the slope is -9.82 ± 0.13 m/s2 .   What happened?  Yesterday, our measured g for the golf ball was significantly lower.

We believe that our inconsistency is due to the fact that we were not careful dropping the balls at the same distance from the camera that the meter stick used for calibration was placed.  This would cause our length scale in the video to be incorrect.  We will test this hypothesis tomorrow.

Blogged by Jennifer Mikulko

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