Basics of Forces
- Every action force has an equal and opposite reactive force.
Newton's Third Law.
- The skier's center of mass can be moved from side-to-side only
by applying force with something against the snow.
(Normally that something must be either the tip of the pole or the
side of the ski. Pushing down on the base of one ski can help
only a little, and even that little is possible only if the skier's
center of mass is not currently directly over that ski).
Since Force requires Work (see below),
this implies that moving the skier's body weight from side to side
always requires Work.
- Acceleration equals Force divided by Mass
This implies that
- - the quicker you want to get all or part of your body go
get moving, the more force it takes.
- - the more of your body you want to get started
moving in some direction, the more force it takes.
- - when the skier's body is moving forward at constant speed
- - the more parts of a skier's body which remain more close
to constant forward motion, the less force needed for
acceleration.
- De-acceleration is a kind of acceleration
So also if the skier wants to slow something down --
something which is not otherwise slowing down due to some natural
force like friction or air resistance -- then the skier must apply a
force.
To move the body from side to side requires the skier to apply a
force to start the body moving toward the other ski
(acceleration). And then another force to slow and stop the body
(de-acceleration) from moving on past the other ski.
Since it requires two forces, it doubles the Work
(see below). But since these
sideways forces are not pushing the skier forward, this double
work is a wasted, another source of inefficiency.
To move the body weight up and down requires the skier to apply a
force to start the body moving upward (acceleration). And then
another force to slow and stop (de-acceleration) the body's downward
motion from gravity -- into stable position over the ski -- rather
than crashing down into the snow (in which case the snow and ground
supply the force to stop the downward motion). Since it requires
two forces, it doubles the Work (see below,
and also Double
Cost of Extra Down-Force). But since these vertical forces
are not pushing the skier forward, this double work is a wasted,
another source of inefficiency.
- The ski cannot feel where the skier's upper body is.
The ski can only feel the forces as they come through the
binding.
Therefore it is possible to temporarily "fool" the ski
into feeling the skier's body-weight as over the center of the grip
zone.
This implies:
- - It is possible to start the kick early, even while
the hip is behind the ankle, even while the toe is far in front of the
skier's center of mass. This is important for both advanced
classic stride (see below under Work,
and the Striding
Smooth and Long "secret") and advanced hill bound for climbing
up a steep hill.
Friction
- Friction is proportional to downward Force (other things being
equal)
Very important for getting more grip friction for Classic
stride.
- With a ski on snow, friction is greater when the distribution of
the down-force pressure is uneven
If the entire weight of the skier is all on one ski, the best glide
is with the ski riding flat with no edging forces applied.
But the best glide is with the skier's weight evenly distributed
between the two skis (and each ski riding flat on the snow with
no edging forces).
The ski will have more friction for grip if edging force is applied
and one edge is digging into the snow. This is true even if the
ski is not angled away from the direction of motion. (So
it is possible to get some of the "herringbone" effect, even
without the herringbone angle).
- Friction to prevent movement from starting is different from
friction during motion
The first kind is called static friction, and the second
kind is dynamic friction. The magic of grip wax for
Classic skis is that it has higher static friction and lower dynamic
friction -- so that can glide as well as grip.
Therefore the skier can get more effective work out of each stroke
either by pushing more intensely or by pushing through a longer
ground-contact distance.
The only Force that counts for effectively moving the skier forward
is the component of force in the forward (or backward) direction --
not any up or down component.
Implies:
- - Early start to leg-push helps.
- - Full finish to leg-push helps
- - Lower body position can help (lengthen snow-contact
distance)
But
- - it only helps to lengthen leg-stroke to in ways that
permit applying additional force. (e.g. rotating the hips about
the vertical axis can lengthen the leg-stroke, but those
vertical-rotator muscles are not very effective at applying
force).
- Power equals Cadence times
Work-per-stroke
[ to be added ]
(Nothing about glide.)
Implies:
- - not wasting Time matters
- - Cadence matters
Biomechanics
- Higher peak force puts disproportionately greater stress on a
muscle
Ski Design
- Camber -- the more the weight and down-force of the skier are
focused on the center of the wax pocket, the greater proportion of
snow-pressure will go through the grip zone of the ski base.
- Side-cut (or "carve")
[ to be added ] Easier Turning versus Classic-stride Grip
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More Complicated stuff
Sustainable Speed
This gets more complicated.
[ to be added ]
(Nothing in there about glide.)
back to Top
Double cost of extra down-force
[ to be added ]
(combines reactive force with gravity)
see
Double Cost
of Extra Down-Force
Dead spots in the stroke cycle
[ to be added ]
see
Is more Glide good?
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