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:
compression has a kind of rounding off error to create a sharp image of the basketball.
camera used has some unknown object smoothing causing this effect.
unlikely tracker software has a bug in the calculation of vx.
by the way, I made a video tutorial to explain the physics http://youtu.be/M0N5P0zgFaA without addressing the question, just general graph interpreting using tracker
Re: strange fluctuations in basketball throw vx graph -
Douglas Brown
452 Posts
I've attached a screenshot that shows the first 12 positions marked with vertical lines so the horizontal positions are easy to see. It looks like the camera is not recording images at regular intervals. I think that many phones and maybe even tablets do this when they can't keep up a steady frame rate. I would try a different camera.
I've seen iMovie do some very strange things to videos. It can make a frame-by-frame analysis impossible. This even happens in come commercially produced video sources. Google "Todd Frazier Loses Bat, Hits Home Run". Those who have analyzed the motion carefully have found that there is a frame missing. You have to work around that to correctly determine the forces involved.
Suggestion: Don't use iMovie on videos you want to analyze.
