This Demonstration lets you simulate several systems acted upon by Newtonian gravitation. You can start or stop a simulation with the "start" and "stop" buttons and use the "presets" drop-down menu to load predefined situations. Planets can be created by clicking their positions and dragging to giving them initial velocities. The mass of created planets can be controlled with the "" slider. Planets are assumed to undergo an inelastic collision when they touch. The mass of a planet is represented by color and radius. The gravitational constant is set equal to 0.00002 for convenience.