www.algodoo.com/
Algodoo, by Algoryx Simulation, is a 2D-simulation environment for creating interactive scenes in a playful, cartoony manner, making use of the physics that we use to explain our real world. Algodoo is designed to encourage young peoples own creativity, ability and motivation to construct knowledge. The synergy of science and art makes Algodoo as educational as it is entertaining. Algodoo applies a constructionistic learning paradigm - learning by designing, constructing and exploring physical systems.
This is a video tutorial showing how to get started with Algodoo, and how to deal with gears and collisions.