Re: Definition of "Swing" (was: Interesting theory on dance)

cs_posting@xxxxxxxxxxx wrote:
Ron N. wrote:
The reason Waltz has "swing" is that the foot is horizontally
powered while directly under the c.g.
....
This is true only for the acceleration due to muscular input. Where
the path is horizontal, push from the legs will most directly create
horizontal acceleration. But it's completely untrue for the horizontal
acceleration resulting from recovered downswing energy.

Any acceleration of the c.g. of a body must be due to forces
from outside that body. This is true even for a mechanical
pendulum recovering potential energy. The force of gravity
accelerates the pendulum bob downwards, and the pivot provides
a force vector with a sideways component (except at the very
bottom of the swing) which accelerates the pendulum bob back
and forth.

So even if you think that some of a Waltz dancers horizontal
momentum was recovered from potential energy, the actual
force leading to c.g. acceleration must come from the floor.
Thus the leg muscles must engage and move to match that
force for a portion of the step. Thus a Waltz basic must
be horizontally powered by the legs, especially because it's
a lossy system where only a fraction of the potential energy
can be recovered.

Why doesn't a pendulum's suspension do any net work?
Because the the support string doesn't change in length.
What portion of a Waltz basic could be danced with legs,
ankle and foot absolutely locked in length? Perhaps a small
metronomic portion near the top, but that's not where the
highest horizontal acceleration occurs on a "swinging"
Waltz basic.


IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M

.

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