Hey all, wetting my feet by programming gravitation/circle collisions. This works alright but circles will often get stuck inside of each other, especially when there are a lot of them acting upon each other or when they're coming to a standstill.

Code:

void checkCollisions (ArrayList bodies) {
  for (int i = 0; i < bodies.size(); i++) {
    Body b = (Body) bodies.get(i);
    
    if (b == this) continue;
    
    PVector radius = PVector.sub(pos, b.pos);
    if ( radius.mag() < (dia + b.dia)/2 ) {
	PVector sNorm = radius.get();
	sNorm.normalize();
	  
	sNorm.mult(2 * vel.dot(sNorm));
	vel.sub(sNorm);
	vel.mult(COR);
    }
  }
}
 

Where dia is the radius (float), vel is the velocity (PVector), and COR is the coefficient of restitution (float). This is called every frame, after acceleration and velocity have been calculated. Pretty standard, but obviously not robust. How can I improve it?