wrote:
Thanks for the feedback! You make a good point about pushing off on
skis -- the normal force during the pushoff probably exceeds the
skiers weight by a fair margin. That was not something I had thought
of. On the other hand, I was concerned that a friction coefficient of
0.025 was possibly underestimating modern waxing improvements. Having
experienced the joy of Cera F on a warm day it seems completely
possible that it glides more than twice as well as whatever was
available in 1976.

Twice as good on wet snow, sure, but 1976-era skis didn't get even close
to maximum glide on those conditions, i.e. the starting point probably
wasn't 0.05.

Secondly, the base power calculation assumes that all energy output
actually is used in the direction of travel, to gain altitude.

Well, energy must go *somewhere.* If you treat the skier as a closed
system, then the effects of that system's work are to move it forward
and up the hill at a certain rate depending on power output, which is
where gravity/air resistance/ski friction come in.

But it also moves significant parts of his body sideways, back & forth
across the hill, and this is mass that has to be started and stopped on
each stride.

As for how efficient that system is at producing energy (a human body
making the nordic skiing motion vs a human pedaling a bike), that is a
whole new issue! I certainly agree with you that a load-bearing sport
like skiing (or running) is going to be much less efficient than a
supported sport like cycling -- that is, Soedergren has to keep a lot
of muscles flexed just to keep himself upright (unlike a cyclist), and
that work is "wasted" outside the system, as it doesn't move him
forward or up the hill.

Exactly right.

Terje

--
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"almost all programming can be viewed as an exercise in caching"