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# A little quiz

Here's a little quiz to stimulate some thought:

How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?

FASTMAN
: : : :

Is this a real question or are you just throwing big words at me to make me feel stupid?:

^Im kidding
The more you shift weight and thus add pressure to your inside ski, the more your CM will move laterally backwards.
Quote:
 Originally Posted by Rick Here's a little quiz to stimulate some thought: How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?FASTMAN
This question is somewhat open ended and I am the stupidest epicski poster for taking the first stab at it without the benefit of hindsight that comes from the future posters. Them thar posters (and you KNOW who you are) will surely take the high road on explanations by feeding off my explanation, at my expense.

First let's cut down the open endedness so as to foil the highroaders a bit. Let's separate the fore and aft component of CM movments from the lateral CM movements and say that lateral only means straight sideways to the skis. That being the case, the anatomy of the human body would suggest that changing the pressure distribution foot to foot directly controls the lateral movement of the CM no matter how efficiently its done.

Somehow I doubt this is what you wanted but one must be cautions as the first poster to a question such as yours. A moron alert should be issued as the next poster could capture the intent wrong when viewed in hindsight of the future posts. Sorry, I am a bit punchy this morning. My first reaction was "Oh bouy, where is this one going? I am suspicious when I know the extent of the knowledge of the thread originator
Rick: Re presure and lateral movement.

The reason why the skier moves the CM laterally (to the inside of the turn) is to balance against lateral forces which are trying to pull the skier to the outside of the turn. The same move happens when riding a bicycle-you lean to the inside. This position of the CM to the inside of the turn is called inclination, and it`s main purpose is balance, and it is essential when making a parallel turn-that is it is essential to incline. BUT it is also essential to do something else and that is keep most of our weight on the outside ski. It`s a tricky thing when you think about it-you must move to the INSIDE but keep most of your weight on the OUSIDE. Hmmmmmm.

Fortunately our bodies can bend. We can incline to the inside with our lower bodies (from the hips down) and bend our upper bodies slightly to the outside. This will keep weight on that outside ski. This bending is called agulation. When we see good skiers with hips to the inside of the turn at the same time as their shoulders are fairly level, we are seeing inclination and angulation working together. It is this angulation move that helps us distribute most of the pressure to the downhill ski while maintaining our CM to the inside of the turn. As pressure increases during the turn, we see good skiers continually moving their shoulders and upper bodies forward to the outside to help them stay over that ski.

So in response to your question, I think that one implication of pressure and the lateral movement of the CM is the need to be able to angulate effectively.

cdnguy
What is CM?
Center of Mass
cdnguy, i think you hit the nail on the head. Good post!

Roto, that was seriously priceless. Keep up the good work (Not picking a fight this time as this is a great topic, it just temporarily amused me).

Good topic Rick. I'm going to offer a little insight to this one that i dont think has been touched on as of yet. Most have related the turn to where the CM should be, but not where the CM should be in relation to the pressure on the ski (by this im assuming youre talking in terms of outside ski pressure). As the angle increases i think it is safe to say that outside ski pressure (in theory) should also increase. Thus, the center of mass should move further inside of the turn to counter act the motion that cdnguy talked about above (downhill tendancy). This should effectively brace your weight and resulting force from the turn against the outside ski. Of course you cannot do this in an upright position, so you have to move youre CM inside the turn or fall over.

So i would say that as your turn gets tighter (smaller than the skis natural radius) that your angulation should increase as a result of you moving your center of mass further into the turn, and thus lowering it (CM) to counter act the natural motion for your body to want to move in a stright line directly out of the turn (not always down the hill, as the direction of the turn changes, so does you tendancy to move in a particular direction). Keep in mind that the upper body must be square or even slightly leaned toward to outside (weighted) ski (pinch) in order to maintain the weight on the outside ski and keep it on the snow during the turn.

In this kind of situation i think that you have to assume that the skier is centered on the skis, and has a good idea of the seperation of upper and lower body throughout the turn. Also, you should assume that the transition of the turn is not going to take a long time, as that you can prevent dependancy on the inside ski as you initiate the turn. If a skier were to favor the inside ski as they initiated the turn they would end up with their CM inside the turn, but not having their weight counteracted with the outside leg bracing the turn.

We worked on that last season in slalom as i had a tendancy to get my CM too far inside the turn when skiing a slalom course - thus i wasnt keeping my upper body and most notably by inside hand up during the turn. This caused all kinds of problems from energy in the turn, to slow transitions, etc. Luckily now i think i have the problem fixed in the course, and have effectively brought more of my free skiing technique into the course.

Later

GREG
Heluva ... glad you "got" my post. I wrote it to amuse you and your pal fastman. Honest. No sarcasm here. (Though I honestly didn't know what CM meant. I would have thought CG, center of gravity.)
Quote:
 Originally Posted by cdnguy Rick: Re presure and lateral movement. The reason why the skier moves the CM laterally (to the inside of the turn) is to balance against lateral forces which are trying to pull the skier to the outside of the turn. The same move happens when riding a bicycle-you lean to the inside. This position of the CM to the inside of the turn is called inclination, and it`s main purpose is balance, and it is essential when making a parallel turn-that is it is essential to incline. BUT it is also essential to do something else and that is keep most of our weight on the outside ski. It`s a tricky thing when you think about it-you must move to the INSIDE but keep most of your weight on the OUSIDE. Hmmmmmm. Fortunately our bodies can bend. We can incline to the inside with our lower bodies (from the hips down) and bend our upper bodies slightly to the outside. This will keep weight on that outside ski. This bending is called agulation. When we see good skiers with hips to the inside of the turn at the same time as their shoulders are fairly level, we are seeing inclination and angulation working together. It is this angulation move that helps us distribute most of the pressure to the downhill ski while maintaining our CM to the inside of the turn. As pressure increases during the turn, we see good skiers continually moving their shoulders and upper bodies forward to the outside to help them stay over that ski. So in response to your question, I think that one implication of pressure and the lateral movement of the CM is the need to be able to angulate effectively. cdnguy
I'd tweak that explaination just a bit. Start with the order of events. The first goal is to turn. To turn efficently, you want to get your skis up on edge. This requires that your lower body move to the inside of the turn. This is the first reason you move to the inside. If you just moved your whole body to the inside, that's inclination as you point out. That may not be that efficient as you may not have enough centrifical force in the turn to balance the entire weight of your body being inside. Therefore, if you just inclinate, you can't get as high an edge angle. That's where angulation comes it. Angulation is moving the lower body into the turn more than the upper body. What angulation allows you to do is get your skis at a higher edge angle while your center of mass balanced against the turn.
I thaught lateral moovement ment moovement in the direction of the skiis....
I wasnt sure at first either - but even if you think of a globe - latitude is across (ie, back and forth). And if you think if the fore-aft movement in the turn related to the pressure distribution... it doesnt make sense. Moving only foreaft, you cant really pressure your skis - without the turning force of your body weight on the skis. So, i would read it as an across-hill movement.
Later
GREG
Quote:
 Originally Posted by Rick Here's a little quiz to stimulate some thought: How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?FASTMAN
Interesting question.

I have this conjecture:

Suppose pressure is to be increased on the shovels at the start of the turn.

Lateral movement "inside" is then coupled with forward movement of the CM to apply pressure to the shovels of the skiis. The CM should track at a slight diagonal with respect to forwards (ski direction) and laterally (90 degrees sideways.) Say 10:30/11:00 on a left had turn, with skiis pointing at 12:00.

As the turn builds, the CM will become oriented more laterally. Pressure is equalized between shovel and tail when skiis are parallel to the fall line, CM is at max lateral position.

As the turn completes, it is necessary to ensure that the CM begins to move forwards again, so that pressure may once again be established on the shovels.

Increasing pressure on the tail of the outside ski is coupled with the forward movement of the CM (otherwise a backseat posture will result). The key implication is that without transferring the pressure to the tail of the ski, the CM will continue backing off from the shovel, and you'll get into the back seat. The next implication is that it is easier to move the CM forwards again if it remains sligtly ahead of fully lateral at fall-line.

The forward movement of the CM continues through completion, pressure on the tail, but CM forwards of lateral, while the inside leg is extended. The point of pressure is transferred to uphill ski, the initial pressure distribution on the uphill ski is centered on the uphill edge.

The CM begins to topple diagonally downhill. The pressure incerases towards the shovels.

The new outside edge engages and the forward position of the CM begins to back off so that an even pressure distribution can occur again in the fall line.

The key implication is that the CM is ahead of the feet at all times.

How's that?

### The cart is before the horse!

If you are dealing with skiing as an intermediate looks at it, you are all right on the money...

But how does an elite skier view what you just asked? That skier would be confused! Let's reorient our way of thinking about this...

Let me start by using an analogy-

When you walk, do you move your legs/feet first, and then try to make the CM catch up with it? Or do you get the CM moving in the direction you want, then keep the legs/feet moving to support it?

So- when you ask what the relationship between the CM and pressure is, it's easy-!

To direct/redirect the CM where you want it to go requires some sort of force (def- "a push or a pull"). Since the skis do an absolutely pitiful job of "pulling" on us, I'll suggest it's a "push" (ie-pressure -[def-"force divided by the area over which it acts"]). (By the way- gravity does a great job of pulling!)

Since our (supposed) goal (mission) is to move the CM in a particular direction or arc, the pressure must be somewhat offset to that direction/arc. The tighter the arc (the greater/more abrupt the redirection), the greater the pressure, and vice versa, given a constant speed.

At this point, the discussion would normally move into vector analysis, using Conservation of Momentum as its core. But thats more then we need to get into here.

What it boils down to- elite skiers (and Bode is a perfect example of this) are more concerned about where they are moving their CM to (the "mission"), than worrying about how they position any particular part of their body, provided the body/legs/feet are supporting that mission.
Geez Vail Snopro I thought it was only me.

### Counteracting move

I have found that I tend sometimes to hook the tip of my left ski when I transition from a left to a right turn, especially if I come across the fall line too much. Building on the counteracting concept that Heluva developed and the reorientation of the CM that vsp discussed, it seems that the hooking is because I haven't shifted my balance over to the old inside, new outside ski. When I do that, it braces the new outside leg. This allows me to reorientate the pelvis (CM) in the new direction. Are we then coming back to Rick's description of the inside leg extension?
So in other words, the goal is to use the CM to control the pressure. While it makes perfect sense, it does (kind of) kick the kinetic chain theory in the nuts. For those of you who are not familiar, the kinetic chain refers to the technique of starting movement (such as a turn) with the feet.

I would say that pressure distribution can control CM position and viceversa. Let the circumstance dictate what is the best approach.

### It's all depends upon your perspective....

TomB-
Yes, it does kind of blow up the "kinetic chain" theory, doesn't it? The KC idea is great for describing static biomechanics, but it does not accurately, nor adequately, describe what upper level skiing is about!

For those who do not have the training or the natural sense of body that elite athletes have, they must rely upon the mechanical aspects, ie- do this, then do that... The average "good" or "advanced" skier uses this to a very slight degree, where as elite skiers use it almost exclusively!

But this concept can be learned/developed. I will not deny that a certain amount of understanding of how to effect the edges and the movements necessary to benefit this "mission" is required. But with good coaching, this is not a difficult concept to acheive!

And the idea is not for the CM to control the pressure, but rather for the CM to dictate what, when, and where that pressure should exist! Then, as I previously stated, the legs/feet come into a supporting role, to make what the CM needs, happen.

But back to an old theme around here..."Intent dictates technique".
If I want to go "there", then what do I do first? In the case of an initial movement, I must harness some form of effort to begin the movement. That could be gravity, or my own muscles. But once I am moving, the most efficient technique is to harness the energy/forces which already exist.
So can I move my CM by doing something with my legs/feet? Sure. But it's likely to be some awkward, arbitrary direction and amount. But if I were to predetermine where I want my CM (core) to go, then it automatically programs my legs/feet to do only what is necessary to achieve that outcome. No more, no less. And no longer arbitrary!

Why do people have a hard time skiing bumps? Because they can not control their CM to an effective degree. They get pitched up and down, side to side.
Why do others have a hard time on ice? Because, once again, they do not control their CM, and make only the necessary movements required to effectively hold.
Look at a truly great powder skier, you'll see the degree of CM control I refer to.

In each and everyone of these cases, you will see and sense, a certain smoothness/flow existing in this skiing. It is not static, it is not mechanical. It is where skiing becomes something more than a physical sport.

It is where the "ZONE" begins!
Good explanation Vail Snopro. I think the theme of "Intent dictates technique" says it all.

Thanks!

### center of mass

Quote:
 Originally Posted by RotoFury Heluva ... glad you "got" my post. I wrote it to amuse you and your pal fastman. Honest. No sarcasm here. (Though I honestly didn't know what CM meant. I would have thought CG, center of gravity.)
Roto: You were right, basically. Center of gravity is pretty much the same as the centre of mass. Perhaps not to a physics professor, but for purposes of discussing skiing the terms really can be used interchangably. Center of mass (CM) and base of support (BS), also called the feet, are two terms that all ski instructors learn right away, and it becomes part of the jargon.

Instructors understand CM as the point. or location in space where all the forces acting on the body converge. Take gravity, for example. It acts downward on our feet, legs, hips, each arm, and on and on. But it is convenient to speak of one point where it acts on the entire body. This is the CM.

And by the way-I didn`t really begin to understand upper and lower body separation until I had been teaching 5 years. My point being that one knows what one knows, doesn`t know what one doesn`t know, and all that is OK. So keep asking questions.

cdnguy
cdnguy: Instructors understand CM as the point or location in space where all the forces acting on the body converge.

Actually CM is the point at which all the mass of a body may be considered to be concentrated in order to analyze its motion. For a sphere is is easy - CM is in the center. But for a human being hucking off a cliff with skis on, it is less obvious. And it will change positions as the hucker twists or flips through the air.

But if the object is static, a good way of identifying the CM is the point where that body will balance on an axis perpendicular to the ground.
Actually, it's the acceleration of the CM that relates more directly to the forces on the edges. Yes, I can put more pressure on a forward edge by having my weight forward and to the side, but if I really want to dig in an edge I throw my weight forward and catch myself with the edge, stopping my forward motion and accelerating the cm backwards due to a force applied by the snow at the edge (assuming the snow is able to resist an equal and opposite force).

Acceleration changes velocity which changes position, so I guess the position of the CM limits the forces you can apply while skiing without getting into weird yoga positions.

### CM ahead of the feet

Quote:
 Originally Posted by BigE Interesting question. The key implication is that the CM is ahead of the feet at all times. How's that?
Big E:

I suppose it depends on what you mean by ahead of the feet. I have always thought that the whole idea is to move in such a way that all the forces acting on the CM pass through the feet at all times. For example, lean forward in your ski boots without your skis on. You will soon begen to topple forward when your CM has moved ahead of your feet. Now do this with your skis on. You will be able to lean much further forward without falling. But your CM will have still moved beyond your feet, and you will be out of balance, although not falling because the front of your skis are supporting you.

Perhaps you mean that the skier is balancing on the front part of their feet. Certainly good skiers do adjust their fore-aft balance. They might, for example move so that their weight is on the balls of her feet at initiation.
This puts pressure sightly forward, which in turn pressures the forebody of the ski, which helps get the turning process started. Later in the turn, in anticipating the release, the skier may allow weight to shift towards the heels, faciltating the turn exit.

In summary, I believe skiers balance on different places on their feet, moving their CM and feet relative to each other depending on intent. The CM, however, would not be deliberately moved forward of the feet. Good skiers use their skis for many things, but not as an extended base of support. They still balance on their feet.

Anyway, my \$00.2 worth!
cdnguy

### Chicken and the Egg

Quote:
 Originally Posted by TomB So in other words, the goal is to use the CM to control the pressure. While it makes perfect sense, it does (kind of) kick the kinetic chain theory in the nuts. For those of you who are not familiar, the kinetic chain refers to the technique of starting movement (such as a turn) with the feet. I would say that pressure distribution can control CM position and viceversa. Let the circumstance dictate what is the best approach.
Not much viceversa here. This isn't chicken and the egg. Your body is connected and resisting forces at your feet. By changing pressure mix between the feet you shift forces that will move your CM.

If not there, you might be able to change your CM by what, waving your arms? Bending at the waist? The kenetic chain is still intact and not a theory. You use it to stand and walk. (sing along - foot bone connected to the ankle bone - etc) That's all it means. It's not magic or a black box.

If you wan't to just move your CM and don't do it with your feet, what are you moving?

Rick's question was and is the point. To control your lateral CM movements in relation to the skis, your greatest tool is the pressure balance between the feet. You can also distrub your balance sideways (which is another way to say the same thing) by tipping a foot to it's LTE strongly. This will disturb your lateral balance while leaving the pressure the same between the two feet.

One of the ways Arcmeister was teaching us to play with this lateral CM movement was the idea of getting your body in place for balance ahead of the time you need that balance. At the end of a turn instead of feeling the g's build, release the pressure from your downhill leg which will cause the CM to move over the skis and get you where you need to be (which is down the hill) by the time the top of the new turn starts. Turning this way removes that big g force increase at the bottom of the turn and makes the maxiumum g force actually be at the fall line of the turn. The whole pressure gradiant moves up. (hey arc - if I got any of this wrong my apologies)

Dealing with fore/aft balance as the turn accelerates and decelerates harkens back to that earlier thread of pulling the inside ski back as the turn develops to keep your CM in good fore/aft position. But I don't think Rick in his specificity of "lateral" CM movement was that concerned with that in this mental exercise.

Just a couple thoughts. First, for what it's worth, Center of Mass (CM) and Center of Gravity (CG) really are the same thing, for all practical purposes.

Second, I'm intrigued by this quote from TomB:

Quote:
 So in other words, the goal is to use the CM to control the pressure.
Tom--you've brought up what may be at the heart of one of the deeper-rooted philosophical differences that cause much disagreement here at EpicSki, regarding technique. Is technique an end in itself, such that our movements are measured in terms of their technical correctness against a standard of some preferred technique? Or is technique merely the tool we use to accomplish a task, and movements are measured in reference to their effectiveness toward that end?

I strongly lean toward the second answer, personally. Which means that, to me, we most definitely do not use the CM to control pressure--we use pressure to control our Center of Mass! My goal is not to pressure my skis in some particular "correct" way, at least not usually. My goals vary, but they tend to be much more task or outcome-based, rather than technique-based--to ski the path I choose, to control my speed, to get a good time in a race course, or to generate some desired sensation (ie. "fun").

Ric (VailSnoPro) has alluded to this notion as well. "Intent dictates technique" is a running theme of our EpicSki Academy events (including the Eastern Tune-Up and Front Range Workshop at Eldora). The point of pressure on one ski or the other, ultimately, is to accomplish some particular outcome--to "go that way," or to control speed, generally.

And this thought relates to my third point. Remember that "pressure" really refers to a force--as Ric says, a "push or a pull." It is, among other things, the force that pushes us around on the hill, determining our line, speeding us up, and slowing us down. If you want to change direction, you NEED a force to cause it, which generally comes in the form of pressure on the ski(s). This law has been enforced since the reign of Isaac Newton, and it's pretty hard to break it even today.

But, in response to John Mason's post above, it's important to note that "movement" (motion) does NOT require a force. This can include the "crossover" of the CM from one turn to the next. Newton had something to say about this as well--"an object in motion will remain in constant motion UNLESS acted on by an external net force." It is only a CHANGE in motion ("acceleration") that requires force. So, if the feet and the CM are going in two different directions, the crossover can occur without the need to generate any additional force to disrupt the motion of either.

There are times (many) when we do need to change motion, but there are also times when we don't. Shifts of weight (pressure) from one ski to the other do affect the motion of the center of mass, and they are one of the most important tools we have to accomplish that. Likewise, a change in the motion of the center of mass can cause a weight shift--think of a car turning left and right.

The question is--are you looking for the shift in weight as a technical end in itself, or are you more concerned with the result, letting the desired outcome ("intent") dictate the technique? The choice is yours!

Best regards,
Bob Barnes
Another thought, regarding the "kinetic chain":

The concept of movements that originate in the feet (i.e. low in the kinetic chain) is very important in skiing, as in many things. But it is crucial to recognize that many movements can originate in the feet ONLY if the Center of Mass is already in the right place and moving in the right direction. In other words, if you're seriously out of balance, it's usually futile to make subtle little movements way down in the feet. A common example is the crossover--should we focus on "moving the Center of Mass across the skis" (high in the kinetic chain), or on tipping the feet--the inside foot, in particular? The answer is--the feet . . . IF (big if!) the center of mass is in the right place and moving the right direction. If you're out of balance, you can't "pull yourself across the skis" by simply tipping your feet. You've got to move your center of mass, which may involve a weight transfer, perhaps even a widening of the stance.

Or, as Peter Ralston says in his excellent book, The Principles of Effortless Power, "Every action should have its source in the feet and its direction coming from the center." He's writing about martial arts, but his reference to "every" action is no accident!

Best regards,
Bob Barnes
Quote:
 Originally Posted by cdnguy Big E: I suppose it depends on what you mean by ahead of the feet. I have always thought that the whole idea is to move in such a way that all the forces acting on the CM pass through the feet at all times ..snip... Perhaps you mean that the skier is balancing on the front part of their feet. Certainly good skiers do adjust their fore-aft balance. They might, for example move so that their weight is on the balls of her feet at initiation. This puts pressure sightly forward, which in turn pressures the forebody of the ski, which helps get the turning process started. Later in the turn, in anticipating the release, the skier may allow weight to shift towards the heels, faciltating the turn exit. In summary, I believe skiers balance on different places on their feet, moving their CM and feet relative to each other depending on intent. The CM, however, would not be deliberately moved forward of the feet. Good skiers use their skis for many things, but not as an extended base of support. They still balance on their feet.
Your two cents is quite accurate.

I was thinking of the balance point when I posted, but I ignored it, since the question was ONLY about the CM and pressure distribution, not balance point.

If the pressure from fall-line to completion of the turn is moved towards the tail, the balance point moves to the heel of the foot. To keep the ski from shooting out forwards, one must apply a force against the rebound energy.

How? The CM should not be directly above the heel, as the application of the force against rebound would be very difficult; the rebound would manifest as a torque, with pelvis as fulcrum and the skier would have trouble pulling the ski back against this torque.

Can the CM be directly above mid foot? Yes, the skier has a bit more of a mechanical advantage in trying to keep the ski back in this position. But not that much... also, the skier may be just about to transfer weight from outside to inside ski.

So, moving the CM over the forefoot if not entirely in front while keeping the balance point at the heel, will yeild maximum mechanical advantage over the rebound energy of the ski. I'm trying to describe an alignment of the body such that the rebound pushes more directly against the CM. That's why I think that the CM can be ahead of the foot.

Anyway, that's my original line of reasoning.

Does that make sense?

### CM vs pressure

Here's my take from a physical perspective. Consider the inertial reference frame of the slope. The turning skier is moving in a circle. Assume for now constant speed and radius of curvature. For this circular motion to occur the net force on the skier must be parallel to the slope and directed towards the center of the circle (centripetal force). The magnitude of the centripetal force is proportional to the skier's speed and inversely proportional to the square of the radius of curvature. The centripetal force is provided by the force of the snow on the skis, primarily on the outside ski. This force of the snow must also counteract gravity, which points straight down towards the center of the earth. Thus the snow force must be directed towards the inside of the turn and upwards from the snow. The skier's center of mass must be aligned with the snow force vector. Otherwise the snow force exerts a torque on the center of mass causing the skier to roll towards the inside or outside of the turn. This rolling torque, which the skier senses as being out of balance, will cause the skier to fall if it is sufficiently large. To align with the snow force, the skier's center of mass must move laterally towards the inside of the turn. The extent of this lateral move depends on the direction of the snow force vector. The pressure that the skier senses in the ski is proportional to the magnitude of the snow force vector. Both the magnitude and direction depend on the skier's speed and turn radius, and these ultimately determine the relationship between lateral motion of the center of mass and pressure on the skis. We could also consider other complicating factors such as changing speed, changing radius of curvature, and weighting/unweighting motion. Each of these would change the magnitude and direction of the snow force vector. Thus they would affect the position of the CM required for balance (both inside/outside and fore/aft) and pressure on the skis.
I agree with Bob; to me the end is controlling my position during my ski down the hill. But I have to admit that there is a bit of "feedback loop" or circuitousness for lack of a better term.

Timing is everything. We accelerate our CM using forces (which are pressure integrated over the ski edges) to go where we want to go, but we also have some choice on at which point in the turn to exert the most force and when to exert the least. We can turn using a greater force from the tip, or we can steer with the tail for example. A judicious adjustment of the acceleration of our CM, for example increasing the acceleration, is linked by Isaac's laws to greater forces at the ski, which we can use to bend or shape the ski (and also sometimes to force it to dig into the snow and then be able to generate more force), in effect altering the resulting force vector that from integrating the pressure along the ski's new shape.

Thus we play at accelerating our cm in balance with forces at the ski's edges. These forces ultimately dictate the motion of our cm, but it bears noting that these forces also shape the skis and affect the ski-snow interaction and thus change the forces themselves.

I hope this doesn't make anyone dizzy.
A practical example:
One reason for skiiers to loose their balance and be caught in the back seat during a turn is that they at the end of the turn shift too much weight too quickly to the uphill/inside ski without compensating with leaning forward. With today short skiis its a bigger problem than before. Look at any WC slalom race for some examples.
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