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Measuring g in simple free fall (incorrect values from fits) post and replies

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Measuring g in simple free fall (incorrect values from fits) - Feb 9 at 7:12PM
Zach Kost-Smith
1 Posts

We are attempting to make a simple measurement of g via a free fall for a high school class.  The values given by fits are consistently high (around 11-12 m/s^2 rather than 9.8 m/s^2).

Using the video of projectile motion provided on the website gives the correct value for g. Because of this, we doubt this is a problem with the software itself.

We have tried many things to get this to work including different cameras, orientations (of the camera and meter stick), different balls, locations, backgrounds, lighting levels. We suspect the video source, in particular, we suspect that there is something with the frame rate that causes this issue, but we are unsure. I even went so far as to extract the frames myself (via ffmpeg), mark them via an image editor, and fit the data resulting a value of 12.4 m/s^2 for g.

It looks like the problem is with the video or with the video import, except that the video appears to be fine and looks good after importing into Tracker.

Is there anything that I am missing regarding how to get usable video data into Tracker for quantitative results?


Replies to Measuring g in simple free fall (incorrect values from fits)

Re: Measuring g in simple free fall (incorrect values from fits) - Feb 09 10:01PM
Douglas Brown Avatar
Douglas Brown
110 Posts

Hi Zach,

One possibility is that the camera uses a CMOS sensor with a "rolling shutter" (see the Wikipedia article http://en.wikipedia.org/wiki/Rolling_shutter). This causes the camera to record the lower section of each frame later than the upper section, resulting in a measured vertical acceleration that is too high.

However, if a rolling shutter is the problem, then turning the camera sideways should fix the problem, and upside down it should give a lower acceleration. So if you tried this and it had no effect, it must be something else :-(

I can think of only 2 other possibilities:

  1. the calibration is wrong because the calibration object (meter stick) is further from the camera than the ball.
  2. the frame rate is not what you (or Tracker) think it is. You might want to confirm it by capturing a video of a digital stopwatch for a few seconds. Set the video player to display time and see if the time tracks the watch. You change the frame duration in the clip inspector if needed.

I hope this is helpful!  Doug



Re: Measuring g in simple free fall (incorrect values from fits) - Feb 13 2:49AM
Armando Pisani
1 Posts

Dear Zach,

  try to analyze the balldrop video you can find at this url:

http://jabryan.iweb.bsu.edu/videoanalysis/

I got a value for the vertical acceleration that is 9.9 +/- 0.5 m/s^2

even if the fps is 30 and the image of the ball degrades quite quickly. The fit is anyway quite good.

  However I must admit that in my own video I generally get a larger value for the gravitational acceleration as you can see in the video I recorded when analysing a falling basket ball by tracker in my lab/lecture. You can find it at this url:

http://www.opendante.com/OpenDante/Physics.html

I think this is generally due to misalignment between the plane containing the calibration stick  and the vertical motion of the ball and the plane of the image. This is likely to cause an overestimate of the lengths and of the acceleration as well.

Best regards, yours Armando P.



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