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# Bullet Proof Short Turns - PMTS - Video - Page 26

### Wow!

Does all of this have sometime to do with using ILE to reach a (baseline) neutral that is not over flexed? I hope so.
IMHO...
In the apex of a high edge angle arc turn... the inside leg is squashed and the hips are very LOW. (Following OLR) ILE must be used to have the hips reach a higher position in neutral. Sometimes the neutral is lower than other times but a repeatable/habituated neutral depends on MOVEMENTS (or skills call it what you like). Bolter

### Gear mentioned in this thread:

Quote:
 Originally Posted by Bode Klammer Remind me never to let you put up the B fences for the Super G. 1. The green arrow is a reaction to the the centripetal force created at the base of support. If you remove the base of support, the green vector becomes zero. " the green vector is reduced to zero" is just another way of saying "remove the base of supprt" or "relax the outside leg." That vector is irrelevant to the motion of the skier once it is removed. 2. All those vectors are force vectors, not velocity vectors. Velocity after release will be determined be the initial veloctiy and gravity, which is the only remaining force. 3. Regardless of the frame of reference of the forces, the discussion of motion here is from the point of view of the camera, that is, stationary. Does the skier fall down? or does the earth rush up to him? Since you persist in your error, you are dismissed from class. You will not graduate. You are destined for menial jobs in the food service industry, or even worse, you may be forced to work as a ski instructor to indulge your related compulsions to ski and to talk about vectors without making sense. Professor Klammer
Professor:
You may be a professor of English literature or perhaps religious philosophy, but certainly not physics (not even Natural Philosphy ). In fact I have already graduated, several times, and I doubt you have any degrees remotely connected to physics or engineering (well maybe electrical engineering ).

It's time to look at Newton's third law of motion, even though you still don't grasp the second law. It is true that for every force there is an equal in magnitude, but opposite in direction "reaction force", but this reaction force acts on a separate object.

The green vector is not the reaction force. The green vector is a centrifugal force which exists in the frame of reference that is moving with the skier, and tries to accelerate the skier in the centrifugal direction. Perhaps it is too much to ask you to use the frame of reference in the picture.

As viewed from the camera, there is a force acting on the skier in the opposite direction of the green vector. The "reaction force" to this vector is acting on the snow, not the skier. This force, not the reaction force, and gravity together act as a net force on the skier. This net force accelerates the skier towards the centre. The component of this force that is acting towards the centre of the curve is called the centripetal force.

Dr. Ghost P.Eng.
.....and no matter where you go.....there you are!

Do I hear 800 posts? Will we be setting a new EpicSki record? Will the Highway Star MA thread withstand the onslaught of this new contender?

Inquiring minds want to know!!!!
Quote:
 Originally Posted by Si RicB, Let me start out by saying I'd classify myself as agnostic on these issues (whatever that means : ). From there then, I'd answer your first question by saying that I do see different physiological mechanisms (muscle activation, recruitment patterns) for tipping vs. twisting of the ski. However, I would add that I don't think these are totally independent, that is, the recruitment patterns may overlap (to a lesser or greater extent depending on the situation). Furthermore, I have never seen anything published on the nature of the relationship between tipping and twisting (applying a rotary force to the ski in the plane of the ski) based on objective quantitative assessment and study, although I expect there are motion analysis models out there that could do so. Maybe DavidM's stuff has some relationship but I gave up on his writing long ago. Most of what we hear in this regard are people's own impressions, MA "experts" who make very different claims about video and photo montages that are analyzed, people who follow ATS, PMTS, ... gospel. None of these are convincing to me as there are clearly multiple interpretations for the same situations and without more data (perhaps the most important would be to have EMG or other muscle activation data, the second most important probably being sensors for measuring the human/ski interaction) I remain agnostic. My present thinking is that a fairly exclusive focus on tipping movements as prescribed by PMTS probably is the most effective for most people. That's because (as you've stated) it provides for adequate hip and subsequently foot rotation to at least follow the ski (and perhaps in some situations actively guide the ski . I don't understand the need to individually identify and focus on twisting the foot and ski and blend this with tipping as some here advocate. Sure there are times where I use more twisting, but if I have the balance and skills to employ the tipping movements and skills I strive toward, I think that twisting, when necessary, can be easily (and pretty automatically) produced without much effort at all. I certainly face this necessity regularly in my skiing based upon my level of skill, the terrain I ski, and the speeds I employ. Now, if you or anyone else can guide me to measurement based published studies on the relationship of recruitment patterns for tipping and twisting a ski I am very interested. For myself, however, I feel like it may be a moot point for the reasons presented above.
Well simply put, this supports PMTS claim for secondary or resulting rotary, but in my mind it also supports the idea of kinetic chain recruitment from a foot steering intent. The same muscles are introducing rotary force in both situations from a far distance away from the source of the intent. the difference lies in the angle of the said muscles and not there being different muscles invovled. This also delves into intramuscluature (within) versus intermuscluature (between) recruitment. Most of the muscles in the hip region serve several roles depending on intent and the position of the bone structure. I agree it is a moot point, as long as people don't speak in absolutes. Personally I find it all very interesting. FWIW I personally find that as an organization PSIA is a little behind the eight ball wiht regards bio-mechanics.

The brain works like a conductor over a symphony, to serve our ideas and our intent. Like in BigE's outcomes thread, whatever works for a given individual is OK by me.
Quote:
 Originally Posted by FastMan relax the old outside leg and tip into the new turn over the old inside leg
Quote:
 Originally Posted by nolo Rick, either you have an inside where you meant to have an outside or you have an old where you meant to have a new in that sentence. It makes no sense as it reads now, to me anyway:
Nolo, the old outside ski/leg is the stance ski/leg of the current turn you're in. If you relax that leg at the end of the turn, pressure transfers to the old inside leg (the inside/swing leg of the current turn), the skier enters that temporary state of imbalance you were expounding the virtues of mastering in a recent post, and the CM begins to pendulum (tip) over the old inside foot and into the new turn.

No terms were reversed in my statement, and the concept is very much in this universe. It was you who were space traveling I suspect.
Quote:
 Originally Posted by Bolter Does all of this have sometime to do with using ILE to reach a (baseline) neutral that is not over flexed? I hope so. IMHO... In the apex of a high edge angle arc turn... the inside leg is squashed and the hips are very LOW. (Following OLR) ILE must be used to have the hips reach a higher position in neutral. Sometimes the neutral is lower than other times but a repeatable/habituated neutral depends on MOVEMENTS (or skills call it what you like). Bolter
As in these freeskiing clips, where extension of the old inside leg prior to neutral is clearly visible.

http://youcanski.com/video/video_index_en.htm
Once Ghost explained what you were trying to say, Rick, it made sense, and yes, it is exactly what I said in other words in another thread. Thank you for clarifying as well.
Quote:
 Originally Posted by nolo Once Ghost explained what you were trying to say, Rick, it made sense, and yes, it is exactly what I said in other words in another thread. Thank you for clarifying as well.
You're most welcome, Nolo. The confusion was really my fault. I should have said PIE and CRUST ski.
Pie and crust?
Quote:
 Originally Posted by borntoski683 Pie and crust?
Pie/inside, crust/outside ski.
Just came home a little bit ago to find things had traveled far (but along which Arrow seems to be in question). Gee - and I thought I might get into trouble saying what I said...

Ghost,
Kinda surprised that you didn't jump on my bandwagon here but considering your posted statement that the reference arrows are moving with the skier rather than a snapshot in time maybe I'll have to think about it a bit more...

...OK, all thunk - nope sorry, still doesn't fly. Thought maybe we could somehow pin down the CM's trajectory along that Green path as the skier moves forward while turning since a line moving with the skier would essentially rotate with the skier... But in the case of a full double-retraction the skier will not be turning any longer! (And besides - if the arrow set is always moving with the skier - how could the skier's path ever follow any of those lines? Whichever way the skier moves, the lines would keep their relationship to the skier by moving with him!)

BK
There's no need to be hostile when you're right! Slamming only causes us grasshoppers to hunker down and defend ourselves at all cost. You came to the same conclusions I did, and probably in the same way. If Ghost has another point of view then he likely has a reason for it. If we can figure out where he and others are coming from, we can decide if they're on to something - or just on something.

Ghost
your car example includes a continuation of the Turn. Lift all four tires off the road, then take your finger off the ball bearing. Now which way does the bearing travel?

---
*ALL* - be sure to look again at the *diagrams colors* before stating your point! Methinks people are posting correct thoughts but mentioning the wrong arrow (possibly) and causing some of the frustration.

---
Sounds like RicB is on this particular bandwagon also. Anyone else?

The best case statement so far is "Take away the centripetal force (Blue) and you're left with Gravity (Yellow) so the CM moves straight ahead."

I would revise this to say "...so the CM initially moves perpendicular to BOTH the Yellow and (former) Blue arrows, then accelerates downward in the Plane of the Yellow arrow."

The Green arrow is history and no longer relevant the moment our feet leave the snow because our lost foot-contact removes the centripetal force, and thereby the centrifugal force.

---
Yes - this does eventually get back to ILE and OLR relevance. But first the idea being debated must be pinned down. Then, we gotta pin down the 'Flexing causes Crossover' idea. Only then can we meaningfully examine some new variations on ILE and OLR ideas.

---
I'm beat after going up today. If I'm not lured back up there tomorrow I'll try to put some JPG's together that might provide better insight than words seem to be doing here.

.ma
Quote:
 Yes - this does eventually get back to ILE and OLR relevance. But first the idea being debated must be pinned down. Then, we gotta pin down the 'Flexing causes Crossover' idea. Only then can we meaningfully examine some new variations on ILE and OLR ideas.
I always wonder why we focus so much on centrifugal force and so little on centripetal force. I like to approach it from the perspective that I am in charge of creating and utilizing the forces, as opposed to reacting to or resisting them. These line diagrams over the skier always seem to portray the need to react and resist rather than the action of create and manage.

Here's a quote I made last year that sums up my own perspective and approach FWIW:
Quote:
 "We make movements that create forces, we make movements to manage the forces, and we make movements to release the forces so we can start all over again in the other direction. I don't agree with the idea that all movements are to counteract forces. All forces have an opposite an equal reaction for sure, but there are two sides to this coin. To me the ability to create, mothers the ability to counteract and manage "effectively and efficiently". I want to be on the proactive side of the coin rather than the reactive side. When I'm accurate in creating the forces then the balance and counteracting are built in. Imbalances only feel threatening when I'm not responsible in creating them. Gravity is the only given. Centrifugal and centripetal forces I want to be the conductor of. I want to lead the symphony. Just my way of looking at it."
It is a yin and yang thing for me, living within the rule of opposites, holding them in harmoning. Long leg short leg. When one starts to get long the other needs to get short. When in the turn this happens is critical, but so too is this symbiotic relationship between the opposites, the long leg short leg.

Shortening the outside leg (OLR) is the release of the forces we created, and the lengthening of the inside leg (ILE) is the birth of the new turn. The creation of new turning forces. To me neither is instantaneous, and both work together in harmony. How we manage their relationship determines how quickly we move into the new, and where we direct the CoM.

Anytime we remove all pressure from both skis at the same time we lose all ability to manage our direction, until we reconnect. Hence the usefullness of ILE. Maintaining the connection of the new outside ski to the snow as we move into the new turn. Doesn't need to be a strong pressure movement. Even a very subtle maintaining of the connection gives us reference and the constant ability to influence our path into the next turn.

My \$.02 for the day.
Quote:
 Originally Posted by RicB Well simply put, this supports PMTS claim for secondary or resulting rotary, but in my mind it also supports the idea of kinetic chain recruitment from a foot steering intent. The same muscles are introducing rotary force in both situations from a far distance away from the source of the intent. the difference lies in the angle of the said muscles and not there being different muscles involved. This also delves into intramuscluature (within) versus intermuscluature (between) recruitment. Most of the muscles in the hip region serve several roles depending on intent and the position of the bone structure. I agree it is a moot point, as long as people don't speak in absolutes. Personally I find it all very interesting. FWIW I personally find that as an organization PSIA is a little behind the eight ball wiht regards bio-mechanics. .
This post's content (and Si's) is the reason I (we) stuck with it for 700+.

If our intent is to pivot/twist the skis our muscles do it. If our intent is to tilt our muscles do that. My juniors/kids/son use edging and balancing movements to turn. They do not focus on twisting the skis. They ski all the mountain in all conditions with no problem. IMO if the movement intent/focus was on twisting they could ski the whole mountain (not as well) but they would not arc the way they do now.
Intent is a function of desired outcome. They know that tilting is faster (more glide) and easier to do. BTW, I know of several other programs that have the same experience/results.

Quote:
 The brain works like a conductor over a symphony, to serve our ideas and our intent. Like in BigE's outcomes thread, whatever works for a given individual is OK by me
That is for sure. Results are even better when "'it" works for lots of folks.

Tilt to turn or twist to turn? I'll go with tilting and balancing movements.

Please, tell me (us) in what way is PSIA stymied, bio-mechanically speaking.
Quote:
 Originally Posted by michaelA Just came home a little bit ago to find things had traveled far (but along which Arrow seems to be in question). Gee - and I thought I might get into trouble saying what I said... Ghost, Kinda surprised that you didn't jump on my bandwagon here but considering your posted statement that the reference arrows are moving with the skier rather than a snapshot in time maybe I'll have to think about it a bit more... ...OK, all thunk - nope sorry, still doesn't fly. Thought maybe we could somehow pin down the CM's trajectory along that Green path as the skier moves forward while turning since a line moving with the skier would essentially rotate with the skier... But in the case of a full double-retraction the skier will not be turning any longer! (And besides - if the arrow set is always moving with the skier - how could the skier's path ever follow any of those lines? Whichever way the skier moves, the lines would keep their relationship to the skier by moving with him!)
Michael,
To use BKs grading system, you get an A-. You have clearly stated that if you remove the base of support the skier will continue in more or less his forward direction while having his direction changed by gravity (the only remaining force acting on him) and will hit the ground, which is correct.

The fly in the ointment is that the diagram in the photo does not show all the forces. It only shows the forces which must be transferred to the snow and omits the reaction force of the snow on the skier (equal and opposite). The frame of reference in the photo is moving at constant speed in a curve of fixed radius. It is "moving with the skier" only as long as the skier is in the curve. The force vectors as shown exist only in that frame of reference and exist as long as that frame of reference continues it's motion. We know this is the frame of reference because of the centrifugal (yellow?) force vector, V^2/(MR). As long as the skier is turning he is applying a reaction force (not shown) so that the net force includes a centripetal component such that this net force is zero and his acceleration with respect to this frame of reference is zero. Once the skier removes his support, then the centrifugal force is no longer resisted and it accelerates the skier with respect to the origin of this revolving frame of reference. The skier only accelerates centrifugally in the frame of reference that is still going around the curve.

Quote:
 Originally Posted by michaelA BK There's no need to be hostile when you're right! Slamming only causes us grasshoppers to hunker down and defend ourselves at all cost. You came to the same conclusions I did, and probably in the same way. If Ghost has another point of view then he likely has a reason for it. If we can figure out where he and others are coming from, we can decide if they're on to something - or just on something. Ghost your car example includes a continuation of the Turn. Lift all four tires off the road, then take your finger off the ball bearing. Now which way does the bearing travel?
If you take all four wheels off the road, then the frame of reference is no longer moving in a curve and hence there is no V^2/(MR) centrifugal force term in that NEW frame of reference. The ball bearing and the car both travel the same path and the ball bearing has zero acceleration with respect to the car or the NEW frame of reference. We now have three frames of reference, on pinned to the Earth, one moving at constant speed in a curve of set radius, and another moving along at constant speed in a straight line (ignoring gravity for the moment in the car example).

Quote:
 Originally Posted by michaelA --- *ALL* - be sure to look again at the *diagrams colors* before stating your point! Methinks people are posting correct thoughts but mentioning the wrong arrow (possibly) and causing some of the frustration. --- Sounds like RicB is on this particular bandwagon also. Anyone else? The best case statement so far is "Take away the centripetal force (Blue) and you're left with Gravity (Yellow) so the CM moves straight ahead." I would revise this to say "...so the CM initially moves perpendicular to BOTH the Yellow and (former) Blue arrows, then accelerates downward in the Plane of the Yellow arrow." The Green arrow is history and no longer relevant the moment our feet leave the snow because our lost foot-contact removes the centripetal force, and thereby the centrifugal force. --- Yes - this does eventually get back to ILE and OLR relevance. But first the idea being debated must be pinned down. Then, we gotta pin down the 'Flexing causes Crossover' idea. Only then can we meaningfully examine some new variations on ILE and OLR ideas. --- I'm beat after going up today. If I'm not lured back up there tomorrow I'll try to put some JPG's together that might provide better insight than words seem to be doing here. .ma
Ric IS on to something. He is looking at it in terms of being proactive and looking at the forces he needs to provide. It is also EASIER to view things in a simple frame of reference pinned to the ground, with no centrifugal forces, just applied centripetal force, weight resisting force, and gravity.
Please do draw in the arrows in this frame of reference (Gravity down as before, a reaction force consisting of a centripetal component towards the centre and a perpendicular component.
Quote:
 Tilt to turn or twist to turn? I'll go with tilting and balancing movements.
I'll go with all three effectively blended together to serve my intent.

It is interesting now as I teach to see the changes in skier skill behavior as it relates to the new skis. Now we get students who can arc around all day long but when you take them into the steep and tight they can't get the job done. But, help them develope better upper lower body separation and blend in some focused steering and these situations no longer beat them up. I spend time teaching both ends of the spectrum to help develope more versatile skiers. Tipping skills are very important, but so are steering skills when needed. A versatile skier can play both ends of the spectrum at will, and so is able to play anywhere in between at will also.

Bolter I have no doubt that your programs are very effective. I am just now breaking into the relm where I may have some influence in shaping our programs and instructor training. Our program is strong and not hung up on any one right way though we do encourage certification, and have excellent pass rates.

Quote:
 Please, tell me (us) in what way is PSIA stymied, bio-mechanically speaking.
Stymied is your word. I said as an organization I think PSIA is behind in bringing their membership along in bio-mechanical knowledge. They give it superficial mention in the tech manual, but leave it entirely up to the individual with no real guidance or requirements from within certification or continuing ed. It is being discussed at the Dteam level now, and an effort is being made to address this field of knowledge. I was very encouraged at the fall DCE training by the presentation on bio-mechanics that Nick Herrin gave.

### What happens when the skier loses his base of support?

Quote:
 Originally Posted by Ghost Michael, As long as the skier is turning he is applying a reaction force (not shown) so that the net force includes a centripetal component such that this net force is zero and his acceleration with respect to this frame of reference is zero. Once the skier removes his support, then the centrifugal force is no longer resisted and it accelerates the skier with respect to the origin of this revolving frame of reference. The skier only accelerates centrifugally in the frame of reference that is still going around the curve.
He falls down.

BK
Quote:
 Originally Posted by RicB I'll go with all three effectively blended together to serve my intent. It is interesting now as I teach to see the changes in skier skill behavior as it relates to the new skis. Now we get students who can arc around all day long but when you take them into the steep and tight they can't get the job done. But, help them develope better upper lower body separation and blend in some focused steering and these situations no longer beat them up. I spend time teaching both ends of the spectrum to help develope more versatile skiers. Tipping skills are very important, but so are steering skills when needed. A versatile skier can play both ends of the spectrum at will, and so is able to play anywhere in between at will also. .
You mean skiers making arc turns because of the skis design and shape, despite flawed fundamentals or what?

Quote:
 Bolter I have no doubt that your programs are very effective. I am just now breaking into the relm where I may have some influence in shaping our programs and instructor training. Our program is strong and not hung up on any one right way though we do encourage certification, and have excellent pass rates.
They are lucky to have you.
I am "hung up" on what works.
The race club kids, use air carves and all sorts of rotational movements to "hang in there." BUT If I spent time coaching pivoting skills and blending them I would fire myself.
If I saw you blending skills with a private client I would not fire you but you would not be a fist call coach. Sorry.

Quote:
 Stymied is your word. I said as an organization I think PSIA is behind in bringing their membership along in bio-mechanical knowledge. They give it superficial mention in the tech manual, but leave it entirely up to the individual with no real guidance or requirements from within certification or continuing ed. It is being discussed at the Dteam level now, and an effort is being made to address this field of knowledge. I was very encouraged at the fall DCE training by the presentation on bio-mechanics that Nick Herrin gave
Being behind the eight ball...
"They (PSIA) give it superficial mention in the tech manual, but leave it entirely up to the individual with no real guidance or requirements from within certification or continuing ed. It is being discussed at the Dteam level now, and an effort is being made to address this field of knowledge."
This (your quote) is true for arc to arc skiing also.
Sorry for the delay. Slept in all day trying to heal up from being jabbed all yesterday by pointy little Force-Arrows directed into me, rather out of me.

NOTE; the diagram we're referencing has: Blue=CF, Yellow=Gravity, Green=Result. If anyone is using a different diagram with other colors, please point us to it so we can stay in context with your post.

---
Ghost,
Before I peck away at a response I need to better understand your Frame-of-Reference idea as compared to my own.

It's my understanding that a Frame of Reference doesn't dissolve and get reborn whenever a Force known to that Frame is added, changes or is deleted. Your post above suggests the Frame is destroyed when something in it changes.

From the skier's Frame (internal) when we stop applying centripetal force (retract our skis) we no longer experience any centrifugal force in our Frame. Assuming our Frame didn't dissove (and I do) we are now only experiencing Gravity with no opposite force since our feet are off the snow. The only acceleration left is down (Yellow arrow).

Without the Blue arrow (CF) our Green arrow becomes parallel with the Yellow - right?

Also....
There may be another Frame-of-Reference issue to contend with. Are you looking at this only with "Rigid Bodies" in mind? That may complicte things later on.

If a skier partially releases guidance of their Upper-Body but continues to manage their feet such that the skis continue track properly in the turn... would we then need to talk about two separate Frames of Reference?

I'd prefer to stay in a Single Frame and consider the skier to be a jointed-segment amorphous entity. This works best when working with a Dynamics Library DLL too.

.ma
Quote:
 Originally Posted by michaelA Sorry for the delay. Slept in all day trying to heal up from being jabbed all yesterday by pointy little Force-Arrows directed into me, rather out of me. NOTE; the diagram we're referencing has: Blue=CF, Yellow=Gravity, Green=Result. If anyone is using a different diagram with other colors, please point us to it so we can stay in context with your post. --- Ghost, Before I peck away at a response I need to better understand your Frame-of-Reference idea as compared to my own. It's my understanding that a Frame of Reference doesn't dissolve and get reborn whenever a Force known to that Frame is added, changes or is deleted. Your post above suggests the Frame is destroyed when something in it changes. From the skier's Frame (internal) when we stop applying centripetal force (retract our skis) we no longer experience any centrifugal force in our Frame. Assuming our Frame didn't dissove (and I do) we are now only experiencing Gravity with no opposite force since our feet are off the snow. The only acceleration left is down (Yellow arrow). Without the Blue arrow (CF) our Green arrow becomes parallel with the Yellow - right? Also.... There may be another Frame-of-Reference issue to contend with. Are you looking at this only with "Rigid Bodies" in mind? That may complicte things later on. If a skier partially releases guidance of their Upper-Body but continues to manage their feet such that the skis continue track properly in the turn... would we then need to talk about two separate Frames of Reference? I'd prefer to stay in a Single Frame and consider the skier to be a jointed-segment amorphous entity. This works best when working with a Dynamics Library DLL too. .ma

Almost there. From the frame of reference in the picture (with centrifugal force), there always exists a centrifugal force. This centrifugal force exists for the sole purpose of making Newton's laws work out in this frame of reference. The centrifugal force always exists in this frame of reference; it exists because the frame of reference itself is accelerating as compared to the Earth. It is not dependent on the motion of the skier. It is the support force that is not shown in the diagram that is needed to make the skier not accelerate in this rotating frame of reference. Removing the support of the skier means that the still existing centrifugal force accelerates the skier in this frame of reference. In the Earth-pinned frame of reference the centrifugal force never existed, and the support force was accelerating the skier in the centre-seeking direction. Remove that support in the Earth-pinned frame of reference, and the skier is no longer accelerating toward the centre.

The motion of a released skier is as follows....

Earth-pinned frame of reference: no support no centre-seeking acceleration - skier doesn't accelerate (except down)

Rotating frame of reference: no support centre-fleeing force causes skier to accelerate away from the centre of this (accelerating) FOR (and gravity causes him go accelerate down).
Quote:
 Originally Posted by Ott Gangl It happened to Hermann in Nagano, he followed the blue line, I think.http://www.youtube.com/watch?v=7lhQ6PbnNzU ....Ott
Notice momentum and gravity act on Hermann. (centrifugal force is a product of momentum ever changing it's direction). Momentum keeps him moving forward in his last direction of travel prior to leaving the snow, and gravity slams him into the ground. Notice his CM never crosses over his feet (until he bounces of the ground).

Upon total removal of a skiers base of support while turning (such as, during retraction), momentum moves the skier further away from the center of the arc he was following before his base of support was removed, and gravity sends him into the snow.

Ghost, your stuff jives with me. You/we just need to simplifiy the language, concepts and applications to make it something the general non scientist recreational skier can relate to and use in his/her skiing. I can here computers all across internet land turning out as we speak.
And here's something for you to ponder Michael. (others fed up with all this physics babble please look away)

When a skier is in an arc and in balance, with loaded skis, the force picture represented by the blue/green/yellow arrows keeps the skier's CM positionally stable in relation to the feet.

Relaxation via retraction is not an immediate full to zero pressure event. It's a process that takes a period of time, that period being dependant on how rapidly the skier relaxes the legs. As the process takes place, and while a portion of resistance still remains, the skis will still be arcing, centrifugal force will still exist, and the CM will therefore be driven DOWN along the resultant force vector (green arrow). Also, because a portion of resistance still exists during the relaxation process, centripetal force will still move the CM along with the changing direction of the skis, as it had been doing, keeping it on the green line, and keeping the green line connected to the feet. This is how we can flex and extend as we carve a turn, without falling inside or being driven over the top of our skis.

As soon as the relaxation process has completed, centripetal and centrifugal force come to an immediate end, but the skier remains on the green arrow track due to the movement momentum which developed along that vector as the relaxation process was taking place.
Ghost,
You and I are definitely in a different Frame of Reference when it comes to the idea of these Frames and their appropriate application against the idea I’m proposing.

The Frame you’ve intentionally gone with appears to be nicely Self-Supporting internally but does not describe what actually happens in the "real" world (or perhaps I should say 'Rest of the world'?) - something you clearly understand since you accurately resolved the ball-bearing’s movement on the tracing paper should the cars wheels be lifted.

Discussing FoR issues is a bit outside the scope of where I'd like to go: The idea that Flexing (in isolation) can initiate crossover when skiing.

Rather than me trying to translate your chosen Frame into what I’m trying to communicate, I’ll simply move forward and leave it to you to hopefully provide a translation later on. In particular, once you see my presentation maybe you can tell me how to get There from your Frame’s perspective.

I’ll peck away at some diagrams tonight. Haven’t tried to attach any Local images to a post before so perhaps someone can tell me how to get a jpg file on my local machine into the post…? Do I need to upload it somewhere first, then attach it?

.ma
Rick,

In the meantime, can we agree that if the retraction is full and immediate (to an imaginary degree of perfection) that the skier's CM *will* fall in the direction of the Yellow Arrow, and not be influenced at all by the Former Blue arrow?

.ma
Michael,
I only used the rotating frame of reference because that was used by the original picture. It is indeed an exercise in futility for the novice physicist to comprehend without some prior background and familiarity. It is far better to start with a new diagram, sticking to the simple "real world" as you say frame of reference.

All you need understand is Newton's three laws, or at least the first two.
1. An object will remain in a state of uniform motion (keep on doing what it's doing and not accelerate or change its speed or direction of travel) unless it is acted upon by a force.

2. The total net force (adding up all the forces acting on the figure taking direction into account) will be equal to the objects mass times its acceleratiion (f=ma)

With these two laws you can draw a force diagram for the skier:

a force of gravity pushing him down (simple enough).
an applied force acting at the ski snow interface.

The applied force can be broken down into two component forces, one aimed at the centre of the curve, and the other at a right angle to the first one.

Adding all these forces you can see that with the ski on the snow the skis are making the skier change his direction of travel and he is in the turn, turning around the curve.

As soon as you release the skis you no longer push your cm around the curve, and inertia (rule 1) causes the skier to continue go in whatever direction he was going when you released the leg....in a straight line.

Now heres the thing. You can stop pushing sideways with your skis, while maintain some ability to push yourself up off the ground. If you release your outside leg almost completely, keeping just enough weight on it to keep the edge pressed against the snow, it will continue in the curve as your cm goes straight. The skis "cross under" you; they are still in the curve as your cm goes straight. If you release the outside ski almost completely and don't want to end up lower than when you started, you must push yourself up with something. That something is the old inside ski which will become the new outside ski.

Or if you prefer, you can almost completely release both legs, keeping just enough force on the skis to keep them pressed into the snow and following the curved path, as your cm crosses over them falling down the hill in a straight line until you are inside the new turn and extending into the new turn.
Quote:
 Originally Posted by michaelA Rick, I'll certainly ponder your posts. In the meantime, can we agree that if the retraction is full and immediate (to an imaginary degree of perfection) that the skier's CM *will* fall in the direction of the Yellow Arrow, and not be influenced at all by the Former Blue arrow? .ma
I'd certainly be more receptive to the imaginary idea of that, than the idea that it causes the CM to be launched over the top of the skis.
Quote:
 Originally Posted by michaelA Rick, I'll certainly ponder your posts. In the meantime, can we agree that if the retraction is full and immediate (to an imaginary degree of perfection) that the skier's CM *will* fall in the direction of the Yellow Arrow, and not be influenced at all by the Former Blue arrow? .ma
The skier in the picture is coming out of the page towards you at the instant the picture is taken. If he does not retract, both he and his skis will continue to turn. Upon retraction when the skier stops pushing himself towards the centre of the curve, the skier's inertia will cause him to come straight towards you, and unless he pushes himself up with his skis he will fall in the direction of the yellow arrow.

The skier comes straight out of the page as he starts to accelerate down the yellow arrow. The skier's skis however continue to travel in an arc. His cm ends up on the side of his skis indicated by the blue arrow due to the skis continueing to cross under him as he goes straight out of the page.
Quote:
 Originally Posted by Ghost Or if you prefer, you can almost completely release both legs, keeping just enough force on the skis to keep them pressed into the snow and following the curved path, as your cm crosses over them falling down the hill in a straight line until you are inside the new turn and extending into the new turn.
Here's the thing, Ghost. Yes, you can do this. But you can also relax the legs as you suggest, and settle into a deeper leg flexed position, with the CM still balanced over the arcing skis, and continue carving the same turn. I know this to be true, as I can easily execute either alternative.

Now the question becomes; what are the movement differences that cause differing outcomes from the same mid-turn flexion? I've explained in an earlier post my take on the means by which we manually deflect our CM across our skis, and suggested that without this manual input the CM will remain turning with the skis in the manner I explained above.

Is it as I stated? That a pure force driven outcome results in a continued turn, and to cross over requires additional manual input? Or, could it be the manual adjustment is needed to keep the CM traveling with the skis, and a cross-over is the natural force driven outcome? Or is perhaps a little manual adjustment required to accomplish both separate outcomes?

One thing we know for sure; something different is happening in the two executions to create the two separate outcomes.
Ghost
What you say above makes perfect sense to me. We can't solve a three dimensional problem in two. That's something I've been trying to get across for almost 25 years now.
Quote:
 Originally Posted by Rick Is it as I stated?
As I see it yes.

I'm getting tired, but it is a matter of degree and adjustment.

To see what causes the different outcomes you have to consider rotation and torque. If you have the inclination, draw a diagram and place the gravity force at the cm, and the two ski/snow forces at the skis. The magnitude direction and distance from the cm determine torque the forces apply to rotate the skier about his cm. Balance torque about any point of the skier (using the cm or one edge eliminates that force from the torque equation). The net torque determines which way the skier rotates which changes where the cm is in relation to the skis which makes the difference in outcomes between a pressure controlled continued turn or cross under.

We adjust how much sideways to upwards force to a small degree (if we like our knees), and the amount of force carried on each ski to affect our rate of rotation, which makes the difference between catching ourselves or crossing over or under.
Uh Oh... Uh Oh...
Quote:
 Originally Posted by Rick Is it as I stated? That a pure force driven outcome results in a continued turn, and to cross over requires additional manual input? Or, could it be the manual adjustment is needed to keep the CM traveling with the skis, and a cross-over is the natural force driven outcome? Or is perhaps a little manual adjustment required to accomplish both separate outcomes?
This is exactly what I'm trying to draw right now. And yes - I've been trying to do it in 3D since it's so dang confusing when done in 2D. Still - Rick & Ghost just gave me an idea how I might do something easy in 2D.

---
Food for thought: We *think* we know what we ask of our bodies - but our old mammalian brain may recacluate what we asked and do something else to deliver the desired outcome. We input an Intent and the brain figures out how to deliver on it.

When we Flex directly down the Green Vector (RFV) during a turn - does our Intent cause us to *unconsciously* make balance adjustments that permit maintaining the same rate of curviture? If it does, we'd have no perceivable indication that this is happening. Our only way to figure out what's really going on is to model the situation in as simple a form as possible.

SO: What if a skier were riding on a Pogo-Stick attached to a Mono-ski? If during a perfectly balanced park & ride turn some mechanism inside the Pogo retracts the base-peg of the stick (lowering the skier) what would happen? Would the skier hold the turn? Would a lateral imbalance now occur that causes a crossover/under? I say Yes.

Just gotta figure out a way to document it!

.ma
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