On May 7, 5:12 am, Terje Mathisen
wrote:

I liked the power budget calculations, but I believe the friction
numbers might severely understate the case:

As the article noted, it was a warm day, and they were skiing a steep
uphill, right?

In such conditions the actual friction almost certainly increase when
you need to angle the skis more and push off harder, i.e. the 0.025
value is too optimistic.

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

This might be a reasonable assumption for a fit cyclist, sitting on his
seat, but not for a skier, no matter how good his technique.

I.e. I'm pretty certain the article numbers are most useful as an
absolute minimum value, but the real energy usage might have been
significantly higher.

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

(Author here)

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.

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.

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.