This project shows the effect of gravity, when one large mass acts on a collection of smaller masses. (The smaller masses do not act on each other, nor do they act on the larger mass.) The large mass is represented by the mouse pointer, each small mass by a turtle. Notice the conic sections marked out by each of the smaller bodies around the large mass.
For detailed instructions on using the program, take a look at the info window from the Gravitation model.
Click on one of the pictures to see a quicktime movie of the model: