The physics demostrated in this application are simple Newtonian analogues. By default, the cursor acts as a massive object to which the particles are attracted via the inverse square law (e.g., gravity) and repulsed by a stronger force acting via the inverse cube law. Thus, the two forces cancel each other out at a set radius, with the repulsive force dominating within said radius and the "gravitational" force dominating beyond.
To alter particle behavior, access the Control Panel and edit the simulation state variables. Pressing the Apply button, resets the simulation and applies the new state variables.
This application demonstrates some of the functionality of the HTML5 canvas element.
3D version: HTML5 3D Particle Simulator