Re: Why can't human statue stand?

On Fri, 28 Apr 2006 mmeron@xxxxxxxxxxxxxxxxxx wrote:

Timo Nieminen <uqtniemi@xxxxxxxxxxxxxxxxx> writes:
On Sat, 28 Apr 2006, PD wrote:
neo wrote:
Suppose there is human statue of cement in standing position. We human
being can remain in standing position. But why can't this statue remain
standing even if it is exact like our body?

There are two types of equilibrium: stable and unstable. Both are
slope=0 points in a potential energy curve, but one curves up and the
other curves down. A standing human is better modeled with unstable
equilibrium.

Why? We stand with our centre of mass over our feet, which constitute a
platform of finite extent, and thus the equilibrium is stable unless a
perturbation is sufficiently large.

I can recall reading some stuff about how miraculous the act of standing
up is, with continual corrections required, but the main things are that
the CoM is over the feet (and hence forward of the ankles), and the torque
about the ankles is balanced by the elasticity of the Achilles
tendon/calves. Nature has "chosen" a simple rubber-band based model over
some complex computer-controlled continual correction model - hardly a
surprise.

Indeed, Mother Nature designs well. Moreover (just anticipating what
you say next), the system is designed for moving, not standing. Other
than in some highly contrived social situations (ceremonies and the
like) there is hardly ever a call for standing motionless for longer
than few minutes.

If you need to "stand" for a long time, it's usually easiest to lie down.

One past housemate of mine was very, very lazy. I came home from work one day and saw he was lying down on the sofa to watch TV. I asked him why, and he said sitting up made him too tired. The only exercise he ever got was sitting on the sofa to watch TV, and walking to the kitchen for food or to the toilet. So I told him that he'd better sit up, or else he'd reach the stage where even lying down to watch TV would make him tired. The plasticity of the human body.

With periodic small corrections, an unstable equilibrium
can be maintained, but a statue cannot make those corrections.

Of course, humans move, and take advantage of moving the CoM out of the
stable region when we walk. I've heard it said that walking (at what
speed?) on flat ground uses less energy than standing (is this true?).

Try standing at attention for an hour. Then walk for an hour.
Compare how it feels. I'm not sure that more energy is used in the
first case, overall, but you certainly end up more tired.

I don't doubt that efficient walking (exactly what that might be?) is easier than standing; the easiest walk is a controlled fall. I wonder how fast one needs to walk to use more energy than standing? I'd just also add that standing at attention is not a very natural way to sand.

Some measurements were made of energy used by quadrapeds - mammals vs reptiles - in walking and running. The researchers were surprised to find that lizards were more efficient than mammals, since everybody knows that mammals are vastly more advanced than reptiles. But reptiles move side to side, while mammals move up and down and need to do work against gravity. Kangaroos are about as efficient as reptiles because they store this energy elastically and get it back when they spring back up, but other mammals are less efficient.

Efficiency isn't the only thing that's important - why cars don't have Stirling engines!

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
.

Relevant Pages

  • Re: Why cant human statue stand?
    ... I don't doubt that efficient walking is easier than standing; the easiest walk is a controlled fall. ... I wonder how fast one needs to walk to use more energy than standing? ... The researchers were surprised to find that lizards were more efficient than mammals, since everybody knows that mammals are vastly more advanced than reptiles. ...
    (sci.physics)
  • Re: Why cant human statue stand?
    ... being can remain in standing position. ... There are two types of equilibrium: ... on flat ground uses less energy than standing. ... mammals are vastly more advanced than reptiles. ...
    (sci.physics)
  • Re: Why cant human statue stand?
    ... being can remain in standing position. ... but a statue cannot make those corrections. ... on flat ground uses less energy than standing. ...
    (sci.physics)
  • Re: Why cant human statue stand?
    ... being can remain in standing position. ... but a statue cannot make those corrections. ... Of course, humans move, and take advantage of moving the CoM out of the ... on flat ground uses less energy than standing. ...
    (sci.physics)