Well, no snow in sight around here. So I’m digging through old threads to see what trouble I can get myself into.
I stumbled into this old thread and figured I’d have a go at
hacker’s question.
hacker wrote:…
Can someone explain how "gravity and balance pull the skis into the falline".
A simple force diagram would help. What are the rotational forces turning the skier/skis?
I don't believe gravity by itself will 'turn' the skis as gravity will be pulling on all of the skis and the skier.
I am thinking some opposing forces are in the mix?
We’ll look at a single ski, but the physics is basically the same for two. So you can look at these diagrams as either a one-footed release or half of a two-footed release.
Let’s start from the start, before we release, standing on the snow, skis sideways to the slope, edges dug in.
In the above diagram the skier’s weight is directly over the center of the ski (as indicated by the round checkerboard center of gravity symbol).
The force of gravity pulls the skier down, effectively at their CG.
Since the edges are set, the snow is pushing back with a force equal and opposite to the force of gravity. Of course the snow is pushing all along the length of ski, but it behaves the same as a single force applied at the average point (in this case the center). This force is labeled as Ffs (force of static friction).
The forces cancel each other out so there is no motion.
Next, we release the edge, flattening the ski on the snow.
Now that the edge is released the ski begins to move because the friction force exerted by the snow on the ski’s base (Ffd, the force of dynamic friction) is smaller than the force exerted by the skier’s weight.
Note that even though the ski is slipping sideways down the hill it is
not turning. There is no rotational force in this scenario because the opposing forces are aligned.
This is the situation in the exercise “Release to Sideslip” (ACBES1, section 4-1 or PMTS Instructor Manual, section GB-1).
With some practice you can sideslip all the way down the hill without turning at all.
Next, instead of releasing with our weight directly over the center of the ski we move our weight forward, so that our CG is in front of the center of the ski.
Here we have the same forces at work (sliding friction on the base and our weight), with the same magnitude as in the last case but the gravitational force is in a different location.
The two forces, now separated by a distance, create a torque.
This,
hacker, is the turning force that you were looking for. It causes the ski to rotate, in this case clockwise, as it slides down the hill.
The force of gravity pulls the ski down the hill. The force of friction is always opposite the direction of motion. As the angle of the ski approaches parallel alignment with the forces, the distance between the centerlines of the forces (moment arm) gets smaller and smaller. Once the distance becomes zero the torque goes to zero as well. The ski now heads straight down the fall line.
If there were some overshoot or other angular disturbance, the angle would generate a restoring torque and the ski would tend to realign with the fall line.
The ski will continue straight down until we use edging to change direction, run out of hill or find a tree.
The important thing about the about the above example is that even though it’s a highly simplified case, is still shows a fundamental characteristic of good skiing, that you’re making your speed and direction changes purely by balance and tipping.
You do not apply any twisting or turning forces to the ski!All turning forces occur at the ski/snow interface. Your job is to tip it and balance on it.
But I have to disagree just
a little bit with Harald on one statement:
h.harb wrote:To those who are confused about the TFR, this is not rocket science.....
The physics here is really similar to that of at fin-stabilized ballistic projectile, i.e. a
rocket (or an arrow or a dart or…).
In the case of the rocket, we balance the CG against the forces of aerodynamic pressure, rather than snow friction. And the rocket acts in three dimensions rather than two. But the directional stability principles are the same.
pc.