dx = partB.x - partA.x;
  dy = partB.y - partA.y;
  distSQ = dx*dx + dy*dy;
  distance = Math.sqrt(distSQ);
  force = partA.mass * partB.mass / distSQ;
  ax = force * dx / distance;
  ay = force * dy / distance;
  partA.vx += ax / partA.mass;
  partA.vy += ay / partA.mass;
  partB.vx += ax / partB.mass;
  partB.vy += ay / partB.mass;