public static void applyForceBetweenNodes(double f, Node node1, Node node2) {
		// on calcule la projection de la force sur les axes x et y
		// et on l'applique

		double dist = Link.getDistance(node1, node2);
		if (dist <= 0)
			return;

		double delta_x = node1.pos_x - node2.pos_x;
		double delta_y = node1.pos_y - node2.pos_y;

		double cos_theta = delta_x / dist;
		double sin_theta = delta_y / dist;

		node1.for_x += f*cos_theta;
		node2.for_x -= f*cos_theta;

		node2.for_y -= f*sin_theta;
		node1.for_y += f*sin_theta;

		return;
	}