Hello, When I was in lesson today, our physics teacher mentioned a device called tuned mass dampers in Taipei 101 to reduced the effect of earthquakes. What he said is that when the building shifts to the right, and there will be a mechanism for the big ball (the damper) to shift to the left to counteract, and vice versa. The question I have is that wouldn't the shift of the damper be an internal force, so how does that changed anything resulted by the earthquakes waves, an external force. I mean the movement of the damper should not shift the centre of mass if the whole thing. Thanks in advance.