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is there a bug in the acceleration vector in the world view in polar coordinate system? post and replies

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is there a bug in the acceleration vector in the world view in polar coordinate system?
lookang Avatar
lookang
222 Posts

Dear Doug, thank you very much for creating this wonderful tool, which it i have created with many teachers and students open educational resources in Singapore. http://iwant2study.org/ospsg/index.php/component/tags/tag/60-tracker

 

i notice in the circular model i built to fit the real life data of a fan, the acceleration vector is fluctuating in length (magnitude) as well as direction ( towards the center of the circular path). i speculate is not correct as the theory suggests a vector of equal magnitude and pointed towards center of circular path.

the right size graph of a vs t is constant ( which is correct).

maybe the Tracker 4.92 currently calculates the vector of a incorrectly in polar coordinates thus resulting in the view as in attached picture?

 

Post edited February 16, 2016 at 11:56 PM EST.

Attached File: 2016-02-17_1248.png


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Replies to is there a bug in the acceleration vector in the world view in polar coordinate system?

Re: is there a bug in the acceleration vector in the world view in polar coordinate system? -
Douglas Brown Avatar
Douglas Brown
339 Posts

Loo Kang, can you attach a TRK file with your circular motion model or else attach a screenshot of the model builder? I'd like to reproduce it in order to trouble-shoot.  Thanks!



Re: Re: is there a bug in the acceleration vector in the world view in polar coordinate system? -
lookang Avatar
lookang
222 Posts

The source trz is available herehttp://iwant2study.org/ospsg/index.php/interactive-resources/physics/02-newtonian-mechanics/05-circle/209-tracker-modeling-in-uniform-circular-motion-of-fan

Thank you for looking into this matter.
:)


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Re: Re: Re: is there a bug in the acceleration vector in the world view in polar coordinate system? -
Douglas Brown Avatar
Douglas Brown
339 Posts

This is not a bug in the way Tracker calculates the acceleration, but instead a result of the way it stretches vectors. I'm guessing you have the stretch set to 24 or so, and the problem occurs because the stretch is applied by (a) first determining the normal (unstretched) PIXEL position of the tip of the vector, and then (b) moving that pixel position by the desired stretch. Since the pixel position is rounded from the true sub-pixel coordinates of the tip, this rounding error is magnified as well (enormously, in this case!).  You can see that if you zoom in on the video then the vectors point more and more nearly toward the center as they should because the pixel rounding is a smaller percent of the original pixel length of the vectors.

It is indeed a bug, though ;-)  I'll have to spend some time seeing how I can fix it.  Thank you for bringing it to my attention!



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