private Vec2D mousePos;
private AttractionBehavior2D mouseAttractor;
private void addParticle() {
VerletParticle2D p = new VerletParticle2D(Vec2D.randomVector().scale(5)
.addSelf(width * 0.5f, 0));
physics.addParticle(p);
// add a negative attraction force field around the new particle
physics.addBehavior(new AttractionBehavior2D(p, 30, -1.2f, 0.01f));
}