float a2bX = a->position.x - b->position.x;
            float a2bY = a->position.y - b->position.y;
            float a2bZ = a->position.z - b->position.z;


            float a2bDistanceSquared = a2bX*a2bX + a2bY*a2bY + a2bZ*a2bZ;

            if ( a2bDistanceSquared < distanceMinSquared )
                a2bDistanceSquared = distanceMinSquared;

            float force = k * a->mass * b->mass / a2bDistanceSquared;

            float length = (float)sqrt( a2bDistanceSquared );

            // make unit vector

            a2bX /= length;
            a2bY /= length;
            a2bZ /= length;

            // multiply by force

            a2bX *= force;
            a2bY *= force;
            a2bZ *= force;

            // apply

            if ( !a->fixed )a->force -= ofVec3f( -a2bX, -a2bY, -a2bZ );
            if ( !b->fixed )b->force += ofVec3f( a2bX, a2bY, a2bZ );

        }
    }