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.

velocity not smooth in tracker 4.85 dynamic model for free fall with air drag post and replies

Return to the velocity not smooth in tracker 4.85 dynamic model for free fall with air drag thread
Login to post to this thread

Jumps in velocity in tracker 4.85 dynamic modle for free fall.
Ami B
9 Posts

Hi, to the Tracker experts ,
as high school physics teacher I use Tracker a lot
I am trying to study   "free fall with air drag" using "Tracker  (Window 4.85) "
capturing video with mobile camera (30fps).
I have some difficulty with the model the velocity is not smooth.

I am using dynamic model with several masses of the same shape  attached are the model and graph for  5 gram and 20.3 gram:
M (Kg) V (m/s) kl        kt          Reynolds
0.005 2.10925 0.023231 0          0
0.0203 3.794713 0.034049 0.004843 0.539689

remarks
I did excel  model using Euler method and got smooth curve. (this is the simplest)

also I suggest to estimate the error by cheking the differnce of the model and the real X,Y values and Velocity .

also may be using more points for the model will help ( half or third of the time)?

Post edited July 30, 2014 at 5:22 AM EST.

Attached File: TRACKER_PROBLEM.ZIP


Replies to Jumps in velocity in tracker 4.85 dynamic modle for free fall.

Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
lookang Avatar
lookang
222 Posts

can upload your TRZ file? easier to debug.

anyway, for info: my drag force model is good with smooth vy :)

my TRZ is available via this link

https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/lookangejss/02_newtonianmechanics_2kinematics/trz/airresistanceshuttlecock.trz

Post edited July 30, 2014 at 9:33 AM EST.

Post edited July 30, 2014 at 9:36 AM EST.

Attached File: 2014-07-30_2129.png


OSP@SG blog
OSP@SG Digital Library


Re: Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
Ami B
9 Posts

Loo
Thanks very much for your quick response  

I have put the TRZ file  here:
https://drive.google.com/folderview?id=0B7GLVisbKUvKanNKeGNhczQ4V0E&usp=sharing

you realy do not need the movie to check the model you can create the model on any move
here are the parameters:
and check vh Vx graph

fx: m*g-kt*vx^2-kl*vx

m 0.005
g 9.8
kl 0.023231009
kt 0

also other parameters will create the same problem

this equation is more complicated than your example it has both velocity and square of velocity parte (laminar and turbolent).



Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
Douglas Brown Avatar
Douglas Brown
339 Posts

Hi ??

My drag_viscous.trz file with your model and no video at all is at http://www.cabrillo.edu/~dbrown/tracker/test/drag_viscous.trz.

Your freeFall_5g.trz file wouldn't open for me and was not even recognized as a valid zip file, so it must have gotten corrupted.

Open my drag_viscous.trz and check video clip and model settings. They're almost identical to your 5g numbers but you can see the plots are all nice and smooth.

The difference is in the missing times that you see clearly on your position plot. I suspect your video camera has dropped occasional frames (many do this automatically when they can't keep up 30fps). This means the deltaT is not constant which is not what the dynamic model assumes.

Using a video with no dropped frames would fix the problem...

Hope this is helpful.  Doug

PS I see your name as ??? ??????. Is this just my computer? Can you change it to something more widely readable?



Re: Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
Ami B
9 Posts

Thanks Doug the time is sometimes double the 0.033 seconds ,

that could lead to the problem, I will try with differnet camera.

ami



Re: Re: Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
lookang Avatar
lookang
222 Posts

dear ami b,

the problem seems to be the video as doug mentioned.

the model u created is perfectly smooth :)

rename the attachment back to trz from zip and it will work in tracker again.

Attached File: freeFall_5g.zip


OSP@SG blog
OSP@SG Digital Library


Re: Re: Re: Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
lookang Avatar
lookang
222 Posts

see picture

Green is model smooth

Red is real Data points

:)

Post edited July 31, 2014 at 10:25 AM EST.

Attached File: model_is_smooth_real_object_is_unequally_spaced_due_to_compression_20140731_2216.png


OSP@SG blog
OSP@SG Digital Library


Re: Re: Re: Re: Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
Ami B
9 Posts

loo,doug thanks very much .
both are rigth the problem is becuase of the cammera.
using an older mobile cam with constant times  the problem disappeared.

I have some questsions/suggestions for future releases:
1. loo how did you change the times for the model  - this could be a good solution?
2. suggest that you add pair wise least square differece to estimate the model fit.
3. as you see I have 2 constants ( linearly connected  Kt=mg/v^2 -kl/v
v is the terminal velocity ) i did find the best pair by minimizing the difference /

thanks ami



Re: Jumps in velocity in tracker 4.85 dynamic modle for free fall. -
Douglas Brown Avatar
Douglas Brown
339 Posts

Hi Ami,

Glad to hear you were able to fix the problem by using an older camera with constant times.

I like your suggestion to add an easy way to find the rms deviation between a model and marked points. I'll see if I can add that to a future release :-)

best regards, Doug



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