Now if I move my center of mass, so
if I go like this, now my center of mass has shifted,
I have to use something to keep me upright.
And so in fact what we are really keeping over the support surface is not so
much the center of mass, but remember that force equals mass times acceleration,
so what we are really keeping over the support surface is the center of force.
And what's the force that we use?
The force that we use is muscle force, muscle contraction,
the force of muscle contraction.
And so let's just think about this.
If I reduce my support surface by standing on one leg,
it is now more challenging to keep me upright,
to keep my center of force over my support surface.
Another way to think about this is that if I have a small support surface and
I elevate my center of mass, it's also harder.
So, if I had weights and I were holding them up, that would be harder to stand.
If I stand like this,