/// <summary>
/// Initializes a particle with a random position and velocity.
/// The position is uniform in the unit box; the velocity in
/// either direciton is chosen uniformly at random.
/// </summary>
public Particle(System.Random random)
{
_rx = random.NextDoubleRange(0.0, 1.0);
_ry = random.NextDoubleRange(0.0f, 1.0f);
_vx = random.NextDoubleRange(-0.01f, 0.01f);
_vy = random.NextDoubleRange(-0.01f, 0.01f);
_radius = random.NextDoubleRange(0.01f, 0.07f);
_mass = _radius * 10.0;
_color = Color.black;
}