It might be good to label the red belly vector "velocity" . I thought it was a force first and was comparing it to the "air drag".

I was also wondering it the air drag was static length, just showing direction. I could see by stopping (which also zooms in) that it wasn't. Since the beginning of the process is quite interesting and hard to see now, it would be interesting to have a zoomed in version that just showed the velocity with the red vector (maybe plus some

      O
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    [u]  /\   [/u]) [move]markings on the slope[/move]
that would sweep by.)