In order to build a dynamic particle model of a ball that rolls (no slip) along a small slope, I need to calculate the following force: fx = m*g*sin(teta)-0.4*m*ax, where a should be the recent acceleration.
The problem is that "ax" is not a valid variable. How can this be done?

Explanation for the above expression:
* The total forces at the x-axis are: m*g*sin(teta)-fs, where fs is the static force between the slope and the ball, which generates the roll of the ball.
* The torque is fs*R=I*alpha ==> fs*R=0.4*m*R*R*alpha ==> fs*R=0.4*m*R*R*ax/R
* This gives fs=0.4*m*ax, which is what I need.

A temporary workaround I did is to replace m*ax with fx, which results in: fx=m*g*sin(teta)/1.4. The result is pretty close to the real measurements.

I welcome any type of recommendation how to model such experiment by Tracker.

Thanks a lot,
Eitan