Hello!!
I don´t know really how to do that fully functional.
But you can change so the value always start at 0 (zero), but the slider will be in the middle as always.
**Go to this folder that contain the physics files: Standard: "C:\Program Files (x86)\Google\Google SketchUp 8\Plugins\SketchyPhysics3"
Then open the "ControllerCommands.rb" file with a text editor or something like that! (ex. notepad in Windows)
Scroll down to line "11", probably you don´t have to scroll.
The line: def slider(sname,defaultValue=0.5,min=0.0,max=1.0)#command#
Change the line to: def slider(sname,defaultValue=0.0,min=0.0,max=1.0)#command#
Then save and restart Sketchup!
When you create a slider or motor or anything at all right now, with a slider, and then start it will show 0.0 and nothing will move.
!BUT the slider itself will be at the middle of the line! Don´t touch it yet!
Press pause and pull down the sliders to zero, then it will work!**
/hope this will solve the problem, otherwise, I recommend that you learn to script with "RUBY".
Perhaps we can help you with the basics, if I get time! Otherwise, I recommend this website!