APS Excellence in Physics Education Award
November 2019

Education Prize Logo
Science SPORE Prize
November 2011

NSF Logo
The Open Source Physics Project is supported by NSF DUE-0442581.

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

Return to the Measuring g in simple free fall (incorrect values from fits) thread
Login to post to this thread

strange fluctuations in basketball throw vx graph
lookang Avatar
lookang
222 Posts

hi tracker community, anyone knows why there are strange fluctuations from -4.8 to -6.2 m/s in basketball throw vx graph https://www.dropbox.com/s/gr3ehmk0sgxelha/wangyuxing_312_28_tracker.trz

 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 http://youtu.be/M0N5P0zgFaA without addressing the question, just general graph interpreting using tracker

 

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.


OSP@SG blog
OSP@SG Digital Library

Replies to strange fluctuations in basketball throw vx graph

Re: strange fluctuations in basketball throw vx graph -
Douglas Brown Avatar
Douglas Brown
339 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.

Hope this helps!  Doug

Attached File: basketball.gif



Re: Re: strange fluctuations in basketball throw vx graph -
lookang Avatar
lookang
222 Posts

Agreed with your analysis.
Thank you!


OSP@SG blog
OSP@SG Digital Library


Re: Re: strange fluctuations in basketball throw vx graph -
lookang Avatar
lookang
222 Posts

dear prof doug,

based on our student's understanding, it is not the camera but the compression process by iMovie that caused the weird unequal vx.

thought you should know :)


OSP@SG blog
OSP@SG Digital Library


Re: Re: Re: strange fluctuations in basketball throw vx graph -
Paul Nord
26 Posts

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.

Paul



OSP Projects:
Open Source Physics - EJS Modeling
Tracker
Physlet Physics
Physlet Quantum Physics
STP Book