Center of mass with gymnastics as an example

[blaisem]

Hi there, I was recently watching a gymnastics video, and the three gymnasts balance on each other in a crazy tower. Here is the part:

http://youtu.be/Q5L4MT8LdOQ?t=2m10s

Does the person on the bottom feel a shift in the center of mass due to the top person leaning so far back, making it easier to support the other two? In other words, is the center of mass of the three people independent of where they are connected?

The person in the middle I would expect to experience a backwards-rotating torque as a result of the top girl, but it seems from the angle of her arms that she is pushing herself backwards, which would be the opposite direction to counteract the torque I expected. Am I missing some force that causes her to push in that direction?

Does the top person having her legs out wide play any role in helping her to balance?

I tried to explain this in my head, and these are the questions I came up with. Would appreciate anyone helping me improve my understanding of mechanical statics. Thanks!

 

[MostlyHarmless]

The center of mass of the gymnasts depends on the "shape" the are making.

By spreading her legs out like that, she lower's her center of mass closer to the other girls leg, which in turn, reduces the torque the she has to compensate for.

Answering out of order here..

Yes, when the top person leans back, she is, again, lowering her center of gravity. Making it easier in general, to stay balanced.

Whenever you're talking about balance, what makes something balance, is having its center of gravity "over" its base. Whenever that center of gravity moves "outside" of the base, that is was things fall. So, by widening their "base" or the area on which they cover, the gymnasts are making their tower more stable.