These programs simulate and draw the 2-dimensional motion of particles
interacting via gravity. The size of the particles is proportional to their
mass. Each particle is drawn with a red arrow indicating its velocity, and 
a blue arrow indicating the direction of the total force acting on the particle.

n-bodies.py simulates n free particles with random starting positions/velocities.

fixed-body.py simulates n particles symmetrically distributed around a fixed central mass.

The trajectories are propogated with a Runge-Kutta 4 integrator.

To run either file, you must have pygame installed, and you must invoke a 32-bit version 
of python, for example:

prompt$ python2.7-32 n-bodies.py