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Balance, inside, and physics - Page 3

post #61 of 82
Quote:
Originally Posted by mrzinwin View Post
It depends on your frame of reference. From a third party observer, a person making a turn is being accelerated towards the inside of the turn. However, from the perspective of the skier, he is being accelerated towards the outside of the turn. (When a car makes a turn, you are thrown towards the outside of the turn, not the inside).

Since we want to take the perspective of the skier, we must thus assume that we are being pulled to the outside of the turn. During a turn, our edges must be at an angle with relation to the ground. Since our legs are attached to our skis, and our bodies are attached to our legs, this necessitates that our bodies are also at an angle to the ground. However, at a certain point, this causes us to tip over. The goal of skiing is to prevent us from tipping over. There are really four things that keep a skier from falling over when he/she is turning. They are: 1) the slope of the mountain (which affects the degree of edging needed) 2) angulation 3) centripetal forces (which are a function of how fast you're moving and how sharp you are turning) and 4) the degree of edging.

These are the only factors that are at work here.
What you feel in a car is the car being accelerated inward in a turn by the centripetal acceleration. Your body is neutral, but senses the car accelerating inward, thus you perceive being thrown to the outside. It's not really what's happening. The car is accelerating inward around you.

A skier is not being accelerated to the outside of the turn either. The centripetal acceleration acts inward only. You are detecting the forward motion and the "reaction" forces on the skis which may trick you into thinking you're being pushed outward, but it's not the case.

Don't forget, acceleration is the rate of change of velocity. If you were being accelerated outward, your sideways velocity would eventually be changing to an outward direction. In reality, our velocity is increasing towards the inside direction (hence the result of the turn). Just think about it -- if you are going straight (no sideways velocity) and then make a turn to the left, obviously you generate a component of velocity to the left. That means an acceleration to the left (inside the turn) has taken place.

When you get into physics, it's important not to get tricked by what your body feels. The case of the car is a prime example. Your body is *not* being thrown outward, the car is being accelerated inward!

Your point about the frame of reference is important. When we talk about acceleration, it's important to do it from an *unaccelerated* frame of reference to keep things sane. That's why we look at the car, skier, etc, from a *fixed* frame (the earth). If you want to go into the accelerated frame of reference (say fixed to the car or fixed to the skier) it complicates the problem quite a bit, beyond what we have touched on here. And I don't see a benefit to it for the present type of discussion.
post #62 of 82
The skis/snow push the skier around the turn, and the body pushes back on the skis/snow in rebellion, trying to continue on the path it was on.

Mix in gravity, and you have a skier whose rebellious push back happens at some angle between horizontal and vertical, depending on the shape and speed of the turn. Balance is simply a matter connecting your Center of Mass to your skis along that push back line.

On the snow is where the science of forces meet the art of balance. Learn the many ways to shape a turn, and the body along the way learns how to maintain balance while doing it. Science tells us where the forces are coming from, and how to theoretically manage and manipulate them. On snow the body feels them, and learns on the fly, in real time, how to survive and use them.

These discussions serve good use when is simple terms they help people understand how to maintain balance via angulation,,, and how by intentionally disrupting balance you can flow effortlessly from turn to turn.
post #63 of 82
Quote:
Originally Posted by skier219 View Post
What you feel in a car is the car being accelerated inward in a turn by the centripetal acceleration. Your body is neutral, but senses the car accelerating inward, thus you perceive being thrown to the outside. It's not really what's happening. The car is accelerating inward around you.

A skier is not being accelerated to the outside of the turn either. The centripetal acceleration acts inward only. You are detecting the forward motion and the "reaction" forces on the skis which may trick you into thinking you're being pushed outward, but it's not the case.

Don't forget, acceleration is the rate of change of velocity. If you were being accelerated outward, your sideways velocity would eventually be changing to an outward direction. In reality, our velocity is increasing towards the inside direction (hence the result of the turn). Just think about it -- if you are going straight (no sideways velocity) and then make a turn to the left, obviously you generate a component of velocity to the left. That means an acceleration to the left (inside the turn) has taken place.

When you get into physics, it's important not to get tricked by what your body feels. The case of the car is a prime example. Your body is *not* being thrown outward, the car is being accelerated inward!

Your point about the frame of reference is important. When we talk about acceleration, it's important to do it from an *unaccelerated* frame of reference to keep things sane. That's why we look at the car, skier, etc, from a *fixed* frame (the earth). If you want to go into the accelerated frame of reference (say fixed to the car or fixed to the skier) it complicates the problem quite a bit, beyond what we have touched on here. And I don't see a benefit to it for the present type of discussion.
again--I really think it depends on the frame of reference. During a turn in a car, if you consider yourself to be stationary, then the car is accelerating towards you towards the inside of the turn. But if you consider the car to be stationary, then you are accelerating outwards toward the outside of the car.

In the end, this is all semantics. THe bottom line, is that centripetal forces keep you from tipping over. And that's all that matters. For every skier, there's a maximum about of tipping you can do without falling over. If you want to develop greater edge angles beyond this point, there are only three ways to do it--ski on a steeper hill, angulate more, or increase centripetal force (ski faster, make a tighter turn). That's it.
post #64 of 82
Just be careful -- if your frame of reference is accelerating (ie, the car, the ball) it changes things a bit. It's best to reference everything to a fixed, non accelerating frame. When you don't, all sorts of pseudo forces need to be added in to make everything work (and that's where centrifugal "force" and a boatload of associated confusion enter the building). This is a good reference that explains it clearer than I can:

http://physics.nmt.edu/~raymond/clas...ok/node61.html

As an example, if your frame of reference was the center of the ball whirling on the string, the ball is never moving/accelerating relative to it's own frame of reference, so you have to do some hand waving and add pseudo forces to make F=ma work.
post #65 of 82
Quote:
Originally Posted by skier219 View Post
Just be careful -- if your frame of reference is accelerating (ie, the car, the ball) it changes things a bit. It's best to reference everything to a fixed, non accelerating frame. When you don't, all sorts of pseudo forces need to be added in to make everything work (and that's where centrifugal "force" and a boatload of associated confusion enter the building). This is a good reference that explains it clearer than I can:

http://physics.nmt.edu/~raymond/classes/ph13xbook/node61.html

As an example, if your frame of reference was the center of the ball whirling on the string, the ball is never moving/accelerating relative to it's own frame of reference, so you have to do some hand waving and add pseudo forces to make F=ma work.
well, this is off topic, but I think it depends on what you're trying to accomplish. If you're trying to solve physics equations, it's probably not a good idea to see an accelerating object as stationary. But if I'm trying to describe my experience as a passenger of a car, it certainly makes sense. After all, when you're sitting in a car, doesn't it feel like there's this "pseudoforce" pushing you against the car door? Yes! So the same thing is true with skiing. When you're making a tight turn, it feels like there's a strange force that keeps you from falling over, even though your body is nearly horizontal. You can call that a "pseudoforce" or whatever, but it's definitely part of your experience as a human being.
post #66 of 82
Again, I think most here have the idea and any differences in text are probably a result of perspective, choice of Frames and degree of depth in the analysis.


Here is a different (though interesting) perspective to consider. Visualize a well angulated ski racer in a fast turn having mostly straight legs (tipped to 45-degrees) with the upper-body mostly vertical (WRT Gravity) ... what does this skier experience?

If it were me I think I'd experience a 'pull' on my upper body to the outside of the turn while feeling a 'push' coming up from my feet. From this I might conclude that centripetal force pushing on my feet while centrifugal force is tugging at my upper body. Perceptually, this is very different from an external analysis that describes only the Centripetal (or Centrifugal) Force acting on the skier.


This perceptual discrepancy highlights the problem JASP indirectly alludes to with ...
Quote:
To conclude that we would move with the skis along a tangential path (when the ski edges are released ) is inaccurate. You are assuming that the skier would move as a unit.
In the past I've brought up the idea of modeling the skier as a set of individual Mass-Segments connected by Joints. Unfortunately, this can get complicated quite quickly and we seem to have enough problems communicating about the motion of Rigid Bodies.

Still, when taking about Forces, Vectors, Accelerations, <etc> I think it best to keep in mind the inconvenient realities we're judiciously neglecting out of what I'll call 'descriptive convenience'.

.ma

PS: I'm not sure it's ever helpful to debate whether or not a certain force is fictitious. Any given analysis may exclude a Force not needed in that investigation. When figuring how a cannon ball travels we could claim that Electromagnetism is 'fictitious' since it isn't necessary in our standard figuring. Of course, if we want to predict the flight of an iron cannon ball past a large magnet...
post #67 of 82
michaelA,

Predicting the motion of rigid bodies is a much different problem than the movements of segmented bodies. It's not in the same league.
post #68 of 82
I was pointing out the model that assumes we move as a unit is flawed. Nothing more. I feel the platform move in response to movements I make. I also feel the snow pushing back up through the skis. For me balancing on that moving platform means I need to move with the skis yet changing what the skis are doing requires me to move seperately from the skis. I never would describe that as being pushed to the outside of the turn.
post #69 of 82
The ability to control the separate paths of cm and skis is a key marker to advanced skiing. A two part model is probably as complicated as most folk would want to go. We have release of cm and and edge release occurring separately at different degrees. While skiing, typically I like to think of my feet and skis as a unit, hips and above as another unit. The area between is kind of fuzzy; though it all contributes to the cm, we have two cm's and varying amounts of mass attributed to each. This to me is where the raw physics stops being as useful as other analogies; the complexity is such that it would make a fun computer model, but most people aren't going to integrate mass and momentum terms in their heads.
post #70 of 82
I don't see a point going to more complexity when we can't even agree on the basic physics of a simple model. In reality, the simple model would give us a very good first order understanding of the problem, and provide a mathematical relationship between centripetal acceleration and gravity that gets to the core of the problem. It's what I would call a 90% solution.

There are a couple good papers that make the rigid body assumption and come up with some great analysis. They're on my computer at work, and I will try to dig them up next week and post links.
post #71 of 82

It's all relativ(ity)

Wow. Centrifugal force exists. No, it doesn't. Yes it does--you don't understand physics. No, it doesn't--you don't understand reality. It's all a matter of perspective, of your frame of reference. It's a pseudoforce--not real. It's something someone just "made up" to make the math work. No it isn't. Yes it is. It's a fake force. It only exists from the accelerated, rotating frame of reference of the skier, but we shouldn't use that reference frame. You're just stupid, and know nothing about Newton's laws. No, you're stupid, and know nothing about Newton's laws. Oh, forget it--just shut up and ski. Don't just tell me to forget it--this is important and I like to talk about it. OK, but you're stupid. I know you are, but so am I. ....

And so it goes, around and around and around.

This had to come up again, didn't it?

Everyone, please read Tom/PhysicsMan's post in the thread that MichaelA linked to here. Read Tom's post, #1, and ignore the rest, which sounds a lot like this thread. And read Ghost's posts here--I think he speaks truth!

Of course, I disagree with Tom and Ghost--and a few others here--on one point: while it may be "simpler" to describe and analyze skiing from a "stationary observer's reference frame" (Ghost's "fixed map," in which we define all movements and accelerations as relative to the surface of the earth, and in which centrifugal force does not apply), the undeniable fact is that we each experience skiing from our own "accelerated frame of reference," in which centrifugal force is very, very real.

So unlike Tom, Ghost, SharpEdges, et al, I think it is often preferable, and in many ways simpler, to describe ski technique from the skier's perspective. (Tom would agree that neither perspective is more "correct" than the other. This is not a matter of "truth" or even opinion--merely one of personal preference.) When I feel a force pulling me toward the outside of my turn, it is real to me, measurable, and something I must deal with. Why should I care that, from your perspective as a "stationary" observer, you can explain these forces another (not incorrect) way?

Motion is relative. Something must be defined as "stationary"--whether it is the sun, the surface of the earth, the deck of a cruise ship, or your seat in an airplane. Yes, you can be "motionless" while sleeping on an airplane (from your frame of reference), and at the same time be going very fast from the perspective of someone on the ground. Do you walk forward as you head to the tail of the plane--or is it an illusion, and you are "actually" moving backwards at 600 mph? Neither is more or less "real" than the other. Nor is it incorrect to say that you're "actually" going "east," as the earth rotates, from the perspective of someone on the moon. Or that you're really going some millions of miles a minute around the sun....

They're all correct. They're all "real." And yet, motions and acclerations and forces appear very different from each of these perspectives.

Again, the only frame of reference that really matters to me when I'm making a skiing turn is my own. Interestingly, from my own perspective, I'm not moving at all. Not turning, not accelerating, not going from "here to there." Barring a true out-of-body experience, from my frame of reference, I am always "here." (Just ask me--any time! How about right now...yep, I'm here.) It's only from your perspective that I can possibly be anywhere called "there."

Why is this important? This frame of reference actually allows me to make sense of the physicist's definition of balance as "equilibrium"--a state in which all forces acting on me add to zero--which they must, of course, since I'm not accelerating, since I am (by definition of my frame of reference) stationary, and always "here." This is the frame of reference from which we--every one of us--experience life. (Yes, the world really does revolve around me!) This is where balance happens!

Conversely, from a "stationary observer's" frame of reference, I am never in balance--unless I'm standing still, or moving at a constant speed in a straight line. From that perspective, analyzing skiing is all about the unbalanced motive forces pushing me around the mountain, making me speed up, slow down, and turn. Not unreal, not wrong, but not the self-centered perspective we all know and love!

Some may feel that this "fixed map" perspective is the only "real" perspective, the only one that really makes sense. But from this "stationary observer's" frame of reference, a balanced skier can't turn, and a turning skier is not balanced! That's not the way most skiers talk, is it?

From the frame of reference of the skier--and only from that frame of reference--ski technique is the constant and simple struggle to maintain balance, to keep my equilibrium. I balance the real, measurable force I feel pulling me toward the outside of the turn (which you may call by whatever name you choose, but it actually has a long and well-established name: it's called centrifugal force) by leaning in against it. (So do you.)

---

Ahh....what's the point? You either understand this stuff (relativity) and nod, or you don't and it sounds ridiculous. It either makes total sense, or you think I'm nuts to suggest that you could be constantly standing still, while everything else moves about you. (Of course, I maintain that we actually think and speak this way more than most realize. Have you ever felt you were going so fast that the trees "flew by in a blur?" Not so odd, is it?)

It only matters, really, when a misunderstanding goes so far as to make us do things that aren't right--like decide not to lean into a turn because "centrifugal force doesn't exist." If that happens, it really is time to stop thinking about it and to recognize that, understand it or not, from the neck down our bodies know all about it!

Best regards,
Bob
post #72 of 82
Those are good points Bob, and I also know what it's like to feel the snow "resisting" our ski edges. That's an inward push which makes us feel like we're being driven outward against the snow. Same case where a cornering vehicle accelerating to the inside of the turn makes us feel like we're being pushed outward against the door. These perceptions are part of how our body registers what is happening.

The body is remarkable in that the muscles and the brain manage the balance we achieve on our skis, and they work out the interaction and equilibrium between centripetal acceleration and the acceleration of gravity, on the fly, as we ski. There is little to no analytical physics happening in that body/thought process, in fact it's generally happening at a level removed from where we would think about the physics. By trying to break down the physics and look at the associated math, we're actually complicating the whole thing unnecessarily from the standpoint of the person/skier.

It's different when we want to look at the physics and develop a math model. As any pilot will tell you, what the body perceives is not necessarily what you want to use to make decisions or guide the dynamics of flight. There is a reason they use gyros and instruments and don't rely on the body. Even a simple accelerometer can provide misleading information in flight. In that situation, as in the case when you want to analyze the physics, the choice of the reference and the notion of fixed, inertial, etc, frames becomes very important.

In all cases, it's possible to reconcile the various frames and models through a proper transformation. And that's where pseudo forces enter the picture. What you might describe as the feeling of being thrown outward in a turn, and the feeling of the snow pressing on your edges, is the centrifugal acceleration/force "convenience" concept used to reconcile the problem from the frame moving with the body. There is nothing wrong with that, but you sure as heck need to transform back to the fixed earth frame if you want to talk about the actual turn, because that's where the turn takes place. In the skier frame, there is no turn, no turn shape, etc. The skier frame is extremely useless and limiting in the overall picture. I don't think it's what we as skiers really use, because we spend a *lot* of time planning and directing our motions relative to the earth's fixed frame. If we were doing it entirely from the skier's internal frame, we'd be incapable of anything more than instantaneous reaction. I'd say our bodies feel real-time forces/accelerations in the skier frame, but we're really thinking/skiing/performing in the earth's frame overall. That's where the turns happen.

I will leave off with this thought to drive the point home: so you feel like you're being driven outward in a turn when you just detect the forces happening to your body. But what direction are you moving in? Where are you going? You're going into the turn! Relative to the no-turn case (going straight down the hill with only gravity entering into the picture) what direction are you accelerating? Into the turn! When you step back to the big picture, which could be looking uphill at your tracks or looking down hill to visualize the line, how does that correlate to the physics? You're always accelerating into the turn! Relative to that hypothetical "gravity only" straight line down the hill, lateral acceleration is in phase with the turn direction from the beginning to the midpoint of the turn, and then you reverse it in preparation to go into the next turn. I don't think there's any question that centripetal acceleration is what's driving us *into* turns.
post #73 of 82
Quote:
I don't think there's any question that centripetal acceleration is what's driving us into turns.
That is true, by definition. Or at least, turns are "centripetal acceleration," and the force that causes turns--regardless of its source (gravity, ski-snow interaction, wind, etc.)--is centripetal force.

Quote:
That's an inward push which makes us feel like we're being driven outward against the snow. Same case where a cornering vehicle accelerating to the inside of the turn makes us feel like we're being pushed outward against the door.
These are not just "perceptions." We actually are "being driven outward against the snow" in ski turns, and being "pushed outward against the door" when a car turns--from our frame of reference as skier or driver/passenger. It (centrifugal force) is as real as the centripetal force that causes the skier or car to turn, from the frame of reference of a "stationary" observer. When your cell phone goes flying across the dash board in a hard left turn, it is really moving (accelerating) to your right--and a real force is the only thing that can cause that. (It matters not that, from another perspective, you and your car are moving left, while the cell phone simply continues in a straight line, and entirely different motive forces must be described. It's true, yes, but it is simply a different perspective--not a better or more real one!)

It is a mistake to assume that one of these frames of reference is more "real," more valid, more natural, or more important than the other. They are equal, in that respect. But it is a bigger--and much more common--mistake to assume that a person with whom we're trying to communicate is seeing the issue from the same frame of reference as ouselves. Virtually all of the arguments about centripetal/centrifugal force--at least by those with some understanding--stem from disparate reference frames, usually without the parties being aware of it.

Quote:
There is nothing wrong with that, but you sure as heck need to transform back to the fixed earth frame if you want to talk about the actual turn, because that's where the turn takes place. In the skier frame, there is no turn, no turn shape, etc. The skier frame is extremely useless and limiting in the overall picture.
Well, you don't really have to do that--just recognize that other frames exist, and that, sure enough, if you want to figure out what external forces moved you along your path from one place to another, it is more convenient to think of the "map" as stationary, with you moving on it. But if you want to analyze the techniques and movements you used to manage and deal with those forces, I maintain that it's an easier job when viewed from your own "accelerated" reference frame. Why do you incline into a turn? It has nothing to do with the motive forces pushing on your center of mass, causing you to turn. It has everything to do with the "inertial forces" (centrifugal forces) that result from the turn--and your need to balance against them.

Centripetal force is defined as the force that causes turns. But centrifugal force, which results from those turns, is the phenomenon we deal with in ski technique. Regardless of your preferred frame of reference, we still need to "lean right" in a right rurn, and vice-versa. Why? For balance, of course. From the inertial (stationary oberserver's) perspective, that "lean right" is actually the turn itself--it is your body (center of mass) moving in the direction of the turn.

Yes, you could say that we need to somehow generate the forces we need to cause our line to go where we choose, and technique is how we do it. But it is equally accurate and valid to look at technique as how we deal with and balance against those forces, given that they occur.

Again, to me, it is a more simple, direct, and relevant route to explain technique from the skier's perspective. And when you think about how most people describe ski technique, that really is the reference frame they usually assume. We "lean in" to keep our balance in a turn. We feel pressure as we "push" against the snow (from the inertial frame of reference, of course, it is the snow pushing against us, causing the turn). We speak of moving forward or back over our skis--when from the "inertial" frame of reference, our skis and our bodies are generally both moving forward. As I noted in my Encyclopedia (page 12), when an instructor suggests that a student "keep your head still," or "hold your hands still," do you suppose he really wants you to leave them behind?

Clearly, "still" is defined here according to the moving perspective of the skier. We very commonly describe--and experience--skiing from that moving ("accelerated") frame of reference. And from that perspective, centrifugal force is absolutely, positively, measurably real.

As I've suggested, though, it is perfectly valid and acceptable to prefer a different reference frame when analyzing the motion of skiing--as long as you're consistent and clear about it. It is the root of chaos to choose a different frame, or to switch frames, without knowing it or making it clear to others we're communicating with...as discussions like this thread demonstrate!

Best regards,
Bob
post #74 of 82
Nothing wrong with your perspective Bob.

It's just if your going to use it you must understand it, as you obviously do. The recommendation is for those who cannot understand the accelerated frame of reference. It is better for them to stick to the simple frame of reference, if they cannot accept from the very real observations of their own senses forces that exist in that frame of reference. As Einstein said in explaining his theory of relativity with the train and the flashes of lightning, they are instantaneous because the observer observes them at what is to him the same time. The forces are real because the observer feels them.

That being said, it is also a good idea to stick to the fixed plan, as we often think ahead and plan our next movements down the hill based on the fixed-in place map.

In the heat of battle, we make no calculations, but we may make decisions based on knowledge gained though past analysis. In practice and preparation and in deciding on tactics and analyzing past performance so we can improve it, all views are used.
post #75 of 82
In my view the frame of reference should not be forced into one box or the other for any skier at any level. There are moments when we clearly feel inertial forces pushing us outward, which we created by changing our direction, and there are moments when we feel the skis push back at us significantly and move us into the new direction. Its counter productive to try to categorize everything into one box or the other and even try to think in these terms.

Think about skiing. Think about the movements. Think about different feelings of pressure at different phases of the turn and under different circumstances and what you will do to harness those forces, balance against them, etc..

Saying that centrifugal forces are imaginary forces is only relevant in a physics classroom where a particular method of analyzing forces in nature is being utilized and perhaps some greater good comes from calling centrifugal forces imaginary. But on a pair of skis, they are not imaginary. We change direction, our inertia creates outward force that we feel. If we edge effectively, then the friction created by that will create centripetal forces back. We feel them both at various times while skiing.
post #76 of 82
True enough, Ghost. There's a reason why relativity and alternate frames of reference may not come up in an introductory physics class!

But regardless of whether or not someone truly grasps the concepts, I think it is still critically important to understand at least that these concepts exist--and that failure to recognize that leads to chaos and confusion.

And for those who do grasp the concept of the relativity of motion, it is important to realize that people actually do experience, and often describe, skiing (and other activities) from their own, personal, accelerated frame of reference. They describe the forces they feel, and the movements of their body parts ("ski technique") relative to each other, far more commonly than they describe them relative to "stationary" objects on the snow.

It would be unusual, indeed, to hear someone trying to describe a hand or knee movement relative to an object on the snow. Can you imagine trying to describe skiing movements from your "fixed map" perspective? "Move your knees 6 inches west as they move 20 feet down the hill on a curved path of 16-meter radius, and move your hips in an arcing path inside the line of your feet, also 20 feet down the hill, while moving your head and hands exactly 21 feet down the hill...." seems to me a lot more complex than "move your knees to your left and balance as you enter the turn, and hold your head and hands still."

Whether it's "simpler" or not is immaterial--the accelerated frame of reference is where people live--especially people with little understanding of physics or the concept of alternate frames of reference. And it's likely where they're coming from when they describe their skiing.

Best regards,
Bob
post #77 of 82
Being the self-centered guy I am, I think of one force only -- the snow pushing on me. So, when I don't want the snow to deflect me any more, I "let" my CM move to the other side, so that the snow can deflect me the other way....
post #78 of 82
Quote:
Whether it's "simpler" or not is immaterial--the accelerated frame of reference is where people live--especially people with little understanding of physics or the concept of alternate frames of reference. And it's likely where they're coming from when they describe their skiing.
Actually, in rethinking that statement, it's likely that they'll describe skiing from both frames of reference simultaneously ("Hold your hands still while making turns from here to that sign down there....") Usually, we understand what they mean, but if you take that statement literally, how confusing!

From the "fixed map" perspective, their "still" hands must never leave the start. From their accelerated frame of reference, they'll never get "there"!

Best regards,
Bob
post #79 of 82
It's all the same thing.

We "feel" the skis pushing us in a new direction because we "feel" ourselves pushing back in retaliation. If we locate our center of mass in the correct position we push back into our feet. If we locate our CM wrongly, we push back over the top of our feet and get launched over our skis, or push back inside of our feet and end up tipping over inside and falling to the snow. If we did not push back (centrifugal) the skis would not bend, there would be no turn, and there would be no centripetal force. Centripetal/centrifugal only exist if they co-exist, and the force we feel is one. Compression, if we have our CM in the proper place,,, and big trouble if we don't.
post #80 of 82
Its very interesting to read through the thread. a lot of NEW understanding actually. My new understanding is that:

When carving the skier is describing an arc rather than going straight therefore the snow is actually deflecting the path of the skier. If we don't look at CF it is possible to see that the skis is experiencing a different force than the upper body. That is:

1. the skis is being deflecting from its direction of travelling by an amount depending on the radius of the gripping.
2. the upper body is being deflected by the lower body, which in turn deflected by the logic in 1 above.

So when in high upper C if the skier engaged very early, the snow push back to the skis that is the exact point where the skier experience a force pushing him inward. Now if the skier is not careful enough he would simply resist the force by instinct. My guess is that the skier should anticipate or act very fast to react to this force because its not acting on the CM.
post #81 of 82
Quote:
Originally Posted by Rick View Post
If we locate our center of mass in the correct position we push back into our feet. If we locate our CM wrongly, we push back over the top of our feet and get launched over our skis
Yes Rick. Exactly. When we're at the part of the turn where we are stacked and long, then we will experience what you described, Centripetal forces pushing in as much as centrifugal is pushing out. Like when you're in a car that goes around a corner, you feel the centrifugal forces slam you up against the door but then once you are stabilized against the door, you feel the door pressing into your shoulder and its hard to tell who is doing the pushing except that your neck is fighting to hold your head up straight which probably makes you sense it more as centrifugal, but the point is, when you reach that stable state against the door, it does become sort of all the same.

On the other hand, as we finish a turn, release our rigid/long body, and go into transition we will distinctly feel centrifugal forces carry us across, particularly in a cross-over transition. On the third hand, in a cross-under transition we may feel a centripital sensation as our skis sling shot across underneath our stable body.
post #82 of 82
Yep, BTS, those transition sensation contrasts you describe are quite how I experience them too. And I've got to get one of those "third hands". Imagine the possibilities!
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