Algodoo is a unique 2D-simulation software from Algoryx Simulation AB.
It mix a easy to use interface with a complex system of available simulations «objects» (or scenes, or tools) you can use for educational or testing purposes.
Formerly born as Phun, and free for many platforms, was turned to a pay version called “Algodoo” for a while, and now available for free (but only for Win and Mac)
For the Linux version you have to search for the “phun for linux” version still available online.

The Interface is clean and easy to understand. You can place objects and give them proprierties; add gears, hinges, and turn them in motors; control speed, sense of rotation, force, etc.
Turn solid into water, or place round objects to an inclined plane, to see rolling, bouncing, etc.

For fun, but also for simulations, educational purposes, etc.