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.

Help, Vy Pretty Smooth Line but Ay is erratic post and replies

Return to the Help, Vy Pretty Smooth Line but Ay is erratic thread
Login to post to this thread

Help, Vy Pretty Smooth Line but Ay
Robert Francis
1 Posts

I am conducting free-fall experiments with a control and with magnets. I originally was using two accelerometers, an Arduino Nano 33 BLE Rev2 BMI270 IMU and an Adafruit BNO055 IMU. They give seemingly good results but I was told by some physicists who read about my experiment that I should be using a video camera and frame analyzation software.

I am using a Samsung A25 5G cellphone with Slow Motion recording at 240fps 720p which I speed up to realtime with Shotcut.

When I analyze the video in Tracker the Vy plot results are a pretty smooth line but the Ay plot results are jagged up and down with acceleration rates as high as 40m/s2 to -10m/s2.

I must be doing something wrong, the accelerometers delivered far smoother acceleration rates.

Any help is appreciated, thank you,
Robert


Replies to Help, Vy Pretty Smooth Line but Ay

Re: Help, Vy Pretty Smooth Line but Ay -
Paul Nord
30 Posts

Robert,

This is a feature of data analysis.  The program is calculating derivatives from the difference between individual points in the y vs. t graph.  When the difference in the pixel value is small, the derivative may vary a lot.  And the second derivative will be even noisier.  If you're just looking at a constant acceleration, it's probably good enough to fit a line to the entire Vy vs. t data set.

Another way to eliminate this noise is to skip frames.  At 240 frames per second, the motion may be very small.  This will give you a lot more noise in the v and a calculations.

Also... being more careful about marking the data points can reduce some of this noise.  Use the mouse wheel to zoom in and select precisely the same point on the object you're measuring.  Depending on the size of the object and the video quality, you may even be able to estimate the center at the level of a fraction of a pixel.

Paul



Re: Help, Vy Pretty Smooth Line but Ay -
Douglas Brown Avatar
Douglas Brown
454 Posts

This may be a case where more data is not better. (When the displacements between positions are very small then the relative uncertainties in those positions are large.)  What happens when you set the step size to 6, say? Then it only analyzes every 6th frame.

If you want to use all the data you could double click the plot to open the velocity-time data in Data Tool, then fit a line or other curve to it (depending on how constant you expect the acceleration to be). Once you have a fitted curve you can easily find its derivative.


> When I analyze the video
> in Tracker the Vy plot results are pretty smooth
> but the Ay plot results are jagged up and down
> with acceleration rates as high as 40m/s2 to -10m/s2
>
>
> I must be doing something wrong, the accelerometers
> delivered far smoother acceleration rates.



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