Jason Cozy from Smith Micro explains physics in the latest official Anime Studio Webinar (hold on March 23, 2012, see here) with lots of examples. Great introduction, a must-see for beginners with the Anime Studio physics engine.
(3:10) Basic setup with layers and shapes, enabling the physics engine.
(9:30) How bones and physics interact (skip to 10:50 to see the example)
(12:00) Angle Constraints and Bone Dynamics (example at 14:10)
(18:00) Non-moving objects and initial moving directions.
(21:45) Object properties: Friction, Springiness, Density
(25:00) Layer origin points (complex example at 28:20)
(31:40) Pinball example, Force fields
(38:40) Washing machine example
(40:20) Example with real cartoon character and background scene: guy falling down a hill. Jason creates it from scratch with elements from the library and answers some additional questions.