A simple gravity simulator.
It can simulate multiple objects of different masses and trace their paths.
To add an object to the simulation, click on the the ADD_OBJECT button and drag the object onto the sandbox. You can click on any object to select it. This is represented by a circle around the object. When selected, you can view the properties of and object, as well as edit them. The TRACING button toggles tracing.
This is an eclipse project that can be imported. Alternatively, you could run
$ java gravsim.GravitySimulator
The project is public domain and free to use for any purpose.