physics: working sliders demo with deceleration vs friction.

I need more description that relates the gravity example prior (which is constant acceleration) to this example which uses constant deceleration. Also I need to fine-tune the friction number.
physics: note on made up number for gravitational constant
physics: fix a bug in sliders where acceleration could get eliminated; clean up wording.
physics: fall-with-soft-landing simulation, as an example of combining simulations.
physics: three examples based on falling dialogs