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

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strange fluctuations in basketball throw vx graph - May 12, 2014 at 4:09AM
lookang Avatar
190 Posts

hi tracker community, anyone knows why there are strange fluctuations from -4.8 to -6.2 m/s in basketball throw vx graph

 my current answers are:

Question by wangyuxing 312 28 tracker: for the first 1 second of my video, the ball's velocity in the x direction fluctuates greatly, may I know why this is so? possible reasons are:

possible reasons are:


  1. compression has a kind of rounding off error to create a sharp image of the basketball.
  2. camera used has some unknown object smoothing causing this effect.
  3. unlikely tracker software has a bug in the calculation of vx.

by the way, I made a video tutorial to explain the physics without addressing the question, just general graph interpreting using tracker


<iframe src="//" frameborder="0" width="560" height="315"></iframe>

Post edited May 12, 2014 at 4:10 AM EST.

Post edited May 12, 2014 at 4:11 AM EST.

Post edited May 12, 2014 at 4:21 AM EST.

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Current Replies - View all
Re: strange fluctuations ...   (Douglas Brown - May 12, 2014 at 11:48AM)
Re: Re: strange fluctuati...   (lookang - May 12, 2014 at 12:17PM)
Re: Re: strange fluctuati...   (lookang - May 19, 2014 at 1:17AM)
Re: Re: Re: strange fluct...   (Paul Nord - May 19, 2014 at 11:47AM)
Measuring g in simple free fall (incorrect values from fits) - Feb 9, 2014 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?

Current Replies - View all
Re: Measuring g in simple...   (Douglas Brown - Feb 9, 2014 at 10:01PM)
Re: Measuring g in simple...   (Armando Pisani - Feb 13, 2014 at 2:49AM)
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