The solution is really easy ... I'll give you an example ... imagine that you get 30 fps on the HTC Desire and 40 on Ipad 2
System | Compare two values : fps = 30
Apply Physics impulse 30 toward (Touch.X , Touch.Y )
System | Compare two values : fps = 40
Apply Physics impulse 8 toward (Touch.X , Touch.Y )
No need for an Mathematical Physics Conundrum !
Edit : If you need a .capx , just reply "@Whiteclaws"