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Free fall - value of g too big
Christof Wiedemair
3 Posts

I filmed a free fall of a metal ball and analyzed the movement with tracker. However, no matter what I do I always get ca. 11 m/s^2 for g (parabolic fit to y-t-data). What am I doing wrong? Do I have to keep a minimum distance to the scene to prevent distortion? I filmed it from ca. 4 m distance. free fall distance was about 80 cm.
I use an Iphone 12 and the latest version of Tracker (6.0.10).
Thank you in advance

Attached File: free_fall_tracker_g.jpg

Replies to Free fall - value of g too big

Re: Free fall - value of g too big -
168 Posts

It is possible you do not have the correct value for # frames per second.  Film a stopwatch for a known amount of time and check to see if the time on the stopwatch and Tracker agree.  

Some video clips may have a variable frame rate. The Handbreak video converter allows you to set the frame rate to be constant.  Try converting your video to mp4 with the H264 codec and a constant frame rate.

Re: Free fall - value of g too big -
Christof Wiedemair
3 Posts

Hi Wolfgang,
thank you for your suggestion.
The video I used for my analysis was half a year old. I filmed a free fall today and got better results (ca. 10.2 m/s^2). Maybe they released an update for iOS during the last months which fixed a bug. I also found an iOS setting saying Auto-FPS, which I turned off. Apple says that this setting reduces frame rate in the case of low light condition. However, turning it off does not seem to affect the results.
I filmed a clock with my phone with the Auto-FPS setting on and off. Both times the clocks seemd in sync.
I'll keep you posted.
Best regards

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