High School Tuned mass dampers in Taipei 101

[anonymous24]

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.

 

[NascentOxygen]

A heavy mass up near the top so that when the base is shaken the building doesn't sway as much as it otherwise would. I don't know specifically about tuning, just that you would need to be mindful of its natural resonances.

 

[A.T.]

I mean the movement of the damper should not shift the centre of mass if the whole thing.

Of course it does, but that isn't the point. Rather the delay and thus movement in counter phase to dampen the oscillation.

 

[anonymous24]

Of course it does, but that isn't the point. Rather the delay and thus movement in counter phase to dampen the oscillation.

why would the centre of mass of the damper+the building change if we just move the damper, aren't they the internal force of the same system

 

[A.T.]

why would the centre of mass of the damper+the building change if we just move the damper

If only the damper moves, then the centre of mass of damper+building also moves. But that isn't even what is happening or the point of the system.