3/16 Steve: modeling friction

I plan to modify the traffic model to design a model of how friction and forces work.  I will have a box and three sliders.  The sliders will control mass, coefficient of friction, and force.  When the force on the box is greater than the friction force, the box will start moving.  If the force is less than the friction force, the box will not move.

This is related to NGSS standard K PS2-1 and K PS2-2:

Plan and conduct an investigation to compare the effects of different strengths or different directions of pushes and pulls on the motion of an object. Analyze data to determine if a design solution works as intended to change the speed or direction of an object with a push or a pull. So far I have made the sliders, the background colors, changed it to just one turtle and made the turtle a box shape. After I get the acceleration functionality working I will try to include an arrow that is scaled with the force slider.