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# Get off those edges. - Page 8

RicB

Moving the body to the center of the turn will only result in a change of direction if you have skis on your feet. If there were no snow and no skis just your body moving down the hill then moving your body to one side or the other would not result in any change of direction. Accelerating a body requires a force acting on the body. Once the force stops acting the body ceases its acceleration. When skiing only the skis and gravity can supply these constant forces.

Also, I don't use muscular actions to move my body from one side of the skis to the other. When I release the edges of my skis my body then begins to follow a simple balistic path and crosses the path that my feet are taking. The only muscular action required is rolling the feet from one side to the other.

This could all be semantics or just how we differently interpret the sensations we experience but I find it interesting how different people performing the same activity at similar high levels of performance can have such diverse explainations of just what is going on.

yd
Well semantics and the problem of using simple physics to describe something so complex. I understand your points and agree with the notion of gravity being the constant that gives us acceleration. The skis allow this because there is very little friction bewteen the base and snow. Put on the wrong wax or put skis on rocks and we have the same situation as just our feet. In this sense don't the skis simply allow and not create?

I can change direction moving down the hill on foot. It requires muscle effort just like in skiing. The question is do the physics change? Just in fun and thought. Later, Ric.
yd,

I don't think you just roll your feet from one turn to the next. I believe you must use muscular action to accomplish this. That is how you release you edges (retract your legs) and let your skis come under you or cross your body over your skis to begin the next turn on the other side. Also to stay in balance fore and aft as your skis accelerate thru the arc of the turn you use your muscles to stay in balance and adjust where and how much pressure is applied to the ski as well as how much pressure is applied to which ski. Does this make sense?

A-man
Ric,

There are many aspects to what we are discussing here let me touch on one here. In my skiing I can produce changes of velocity that exceed a net of 60 mph in a second along the lateral axis of my path. Gravity can't account for that acceleration. Freefall acceleration of gravity will only account for a change of about 20.25 mph per second and that acceleration is lessened because we are on a slope and are only able to access a fraction of the gravity force vector. I observe that my body is accelerating in a particular way so I conclude that there must be a force that is acting to accelerate it. I conclude that the ski is generating this force because I can feel the ski pushing up on the soles of my feet and I can control just how hard the ski pushes on my feet by controlling the degree that the ski is tipped aginst the snow. This gives me a force vector that is directed more or less at my CoM and easily accounts for all the accelerations I observe in a ski turn including those that allow me to stop or even reverse the down the hill component of my motion. Ain't no way you can shear the gravity vector to produce motion back up the hill.

Atomicman,

I did overstate things didn't I. There are of course a lot of muscular actions that take place in the course of a ski turn, from those that release the edges to those that keep us in balance over the skis to those that allow us to transmit efficently the force generated by the skis to our CoM. What I wanted to say was that very subtle movements such as tipping the feet or relaxing the muscles of the leg are all that is required to allow the body and feet to swap sides. As a recreational skier I want to ski with as little muscular effort as possible. A racing mindset is to ski as efficently as possible and use my muscular effort to help me go faster and deal with the greater forces encountered and generated at the greater speeds and dictated turns.

Hope this brings our thinking closer together.

yd
Right On!
Quote:
 Originally Posted by ydnar Ain't no way you can shear the gravity vector to produce motion back up the hill. yd
Nor would you be able to ski uphill if you weren't moving in the first place. So the energy that produces motion back up the hill is the kinetic energy stored in your mass and velocity. Originally, that energy was imparted to you by gravity, or to go back further, the electricity that powered the chairlift that you rode up on, or the wind / coal / nuclear reaction that generated that electricity....

I would not however go as far as to say that gravity is the only force acting upon a skier, just the most significant. For example, a cross-country skier can accelerate along a totally flat track by skating, and so, in theory, could an alpine skier. But nowadays, the skating step is rarely seen in alpine technique, (as pointed out by Fastman recently in another thread).

Admittedly, energy can also be stored in the flex of a ski, but ultimately that energy is also generated by gravity in the first place.
Martin,

I'm pointing straight down the fall line going thirty miles an hour. At that moment my body indeed has a given amount of kenetic energy but the vector of that energy is straight down the hill, conservation of momentum says that vector will stay with me both in magnitude and direction. Only an outside force acting on my body can change my direction of travel and only a force acting in the opposite direction to that of both my kenitic energy and gravity can stop my downward progression and allow me to ski across the hill and even back up the hill. Any change of direction requires acceleration of the body and any acceleration requires a force to account for that acceleration. When I cross the fall line at 30 mph my kenitic energy will be just the same in magnitude but he direction of the force vector will be turned at 90 degrees to the force vector when I was going down the fall line. I have accelerated from 30 to 0 along one axis and accelerated from 0 to 30 along the other axis.

Again, observation of how a skier moves about on the hill they are decending demands that there is a force acting on the body that acts in ways that gravity can't account for, a force that sometimes appears to be greater than that of gravity in that it can produce accelerations greater than that produced by gravity. The ski moving forward along a curved path will generate that force vector and the skier can shear it to move their body where they want it to go.

The energy stored by flexing the ski is very minimal and can only be accessed by allowing the ski to return to a less flexed condition, it is fleeting. The energy that a ski can generate if used properly is ongoing and under my complete control.

yd

ps. You are right in that you wouldn't be able to ski back up hill if you weren't moving because you need that forward motion for the ski to generate the force to change your direction.
Quote:
 Originally Posted by ydnar ...Freefall acceleration of gravity will only account for a change of about 20.25 mph per second...
A very minor numerical detail, Yd:

1 g = 9.807 meters per sec per sec = 32.174 feet per sec per sec = 21.922 miles per hour per second (not 20.25)

Tom / PM
Tom,

Thanks, I need to get a better calculator.

yd
ydnar

if g in Denver is only 9.796, then maybe some of the higher skiing maybe nearer your figure and perhaps PM could have met you halfway (besides he comes from a city where the Monument's height fluctuates by some 2 ft , so maybe he was doing his calculations under a full moon)

Your point about accelerations greater than gravity is interesting. When Newton's apple falling under g hits hard ground what smashes it is its rapid almost instantaneous deceleration, numerically far greater than g. In the deflection of your momentum when you are skiing you too can experience forces greater than g, but not necessarily because some additional force is at work.

As I see it, ignoring muscular contractions, the only force acting initially is gravity. We then play with that force and shape its effect to our purposes in the way we deflect it off the snow and in that sense it is our force.
Yd

I was wrong to say 'the only force acting initially is gravity.' Of course there is also the normal reaction force from the ground which under static conditions keeps us standing about in equilibrium. Thinking about this pair of forces in the very different conditions of a steep and slippery snowfield allows us to consider both gravity as a motivating force and the earth's (snow's) reaction as a shaping force producing the resultant we call skiing when these two forces are unbalanced.

When the skier charges down a mountain she feels she has something the skier trudging accross flat tundra doesn't have. The forces acting on her are basically the same, but she can resolve them differently and this resultant force can seem to her like something she creates (although really she is only reshaping what is there).
Quote:
 Originally Posted by daslider ydnar - if g in Denver is only 9.796, then maybe some of the higher skiing maybe nearer your figure and perhaps PM could have met you halfway (besides he comes from a city where the Monument's height fluctuates by some 2 ft , so maybe he was doing his calculations under a full moon) ...
Sorry, daSlider, but since gravity decreases by approximately 0.3086 x 10-3 cm/s^2 per meter of altitude above mean sea level (MSL), to get G all the way down to 20.25 miles per hour per second, one would have to go all the way up to 150.5 miles above MSL, a good bit higher than even the highest mountains around Denver.

The value of G that you quoted for Denver is indeed correct, but unfortunately, it appears that when performing the simple conversion to the units that Ydnar used, either (a) you simply did not do the conversion at all, or (b), you tried but could not do it correctly. If you had done it correctly, you would have seen that the Denver number converts to 21.911 mi per hr per sec, a *very* long way from the number under consideration, 20.25.

I think it does people on Epic a disservice to compose what appears to be serious correction to someone else's post (ie, "...maybe he was doing his calculations under a full moon..") , but then put a seemingly innocuous little winkie at the end of it. This ambiguous suggestion that your comment might actually be a joke is, IMHO, little more than a preemptive request for wiggle room by should your off-the-wall mis-statement ever be caught.

Tom / PM
Yd, I'm a minimalist too. The least effort to accomplish the task.

We all have a view of how we ski, and really we are just trying to find the science behind skiing as it relates to our views aren't we? At least that's what I think I'm tryng to do. Me, I like to think of the ski as an extention of my body, and view the forces from the inside out. I see the only way to control anything is through our body movements. It's the human effort that dictates everything in skiing. How do we use our body power (weight) to our advantage? How do we keep our weight driving down onto the right place on our ski so that it will multiply that force into something much greater? One thing we all can agree is that our body has mass and inertia when moving, so is it such a leap to think that when we start manipulating the direction our body (mass) takes with our movements, that these movements lie within the realm of the forces acting on our mass and on the skis? So I put out that interesting paragraph, as Rick called it, to see. Maybe it's nonsense, but no more nonsense that saying that we ski effortlessly, or edges don't matter, even though i do believe we should strive to ski as effortlessly as we can, and there is more to skiing skills than just edging. It's been a good discussion so far, don't you think?

On the subject of the acceleration, isn't this really a matter of movement in the direction of least resistance? The more resistance created in one direction, the more movement or force that is allowed or created in the direction of least resistance? Like squezing a dry bean between two greasy fingers. If we give it a direction of least resistance it will accelerate out of there. I guess we can debate forever whether the body movements manipulating our mass over the ski is responsible for the resistance or the ski by itself. Later, Ric.

Ps, I'll leave the math to the rest of you.
PM

Yd's point is not really about precise numbers and in trying to address that point about apparently experiencing g+ forces I was merely prefacing my point with a little mitigation (the numbers were not quite so far out). The reference to the monument/gravity/tides was not serious and no one on Epic need to get too excited about it. Yd's experiences would seem to be about both acceleration and deceleration (what we perceive as the heavy-g bottom of turn is deceleration in the y-axis), but I really can't see another force at work.

How would you comment on the nature of the accelerations Yd describes?
Ric,

This indeed has been an interesting discussion. One of the most interesting things is that effective skiing and ski instruction can be based on such a diverse selection of interpretations of the forces we feel acting on us as we make our way down the mountain.

One thing that we agree on is that skiing is dependent on movements of our bodies and the use of a specialized tool. I want to ramble a bit here about tools.

I want to drive a nail into a piece of wood. If I try to do that with my body alone then I will do serious damage to my hand. I can pound the nail with a rock and make better progress but odds are that the rock will breakup after a bit of pounding. A lump of iron works better but my arm still gets very tired pounding the nail, my hand gets sore from gripping the lump of iron and it takes quite a while to pound the nail in. Shape that lump of iron a little and put it on the end of a stick and I can drive that nail into the wood very quickly with far less fatigue to my arm and pain to my hand. Now a hammer is a very simple tool but it lets me do something easily that would be harder with less sophisticated tools and impossible with no tool. Now, if I jump off a cliff I can't fly no matter what movements I make with my body. But, if I jump off that cliff with a special tool called a hang glider I can fly. The tool interacts with the fluid of the air in such a way as to produce a force that counters the pull of gravity and allows me to accomplish something that would be impossible without the tool. I control the tool and therefore the forces acting on me by movements of my body just as I controlled the hammer by motions of my body. Both are just extentions of my body, one a simple extention that makes my arm and hand longer and harder the other a more sophisticated extention that gives me wings like a vulture's. The modern ski is a a tool closer to the hang glider in sophistication than to the hammer. The ski, acting as an extention of my body, allows me to accelerate my body to a degree and in directions that would be impossible if I only relied on the muscles of my body and gravity. One group of tools; hammers, pry bars, etc. allow me to multiply the power of my muscles. another group of tools; hang gliders, skis, etc. allow me to create and control forces that my body acting alone would be incapable of creating.

Only by looking on the ski as belonging to the more sophisticated group of force generating, as opposed to force multiplying, tools can I account for the accelerations I observe in skiing.

Hope this ramble makes my position a little clearer,

yd
Yd

are you saying there is another force acting on you in addition to gravity which accounts for the g+ forces you experience?

If you skiid into a brickwall you would experience a massive deceleration of many g's which is the wall's reaction to your hitting it. Similarly the snow reacts to your gravity-fuelled momentum and can deflect you, pretty severely if you want, again experiencing sideways accelerations and proportional decelerations in the other axis (downward movement converted to sideways movement). Isn't the hanglider doing much the same thing in converting air resistance into lift?

Aren't the really heavy g forces you feel at the bottom of a turn actually decelerations as g is diverted sideways?

I can see the ski using the snow reaction force, but I can't see it "belonging to the more sophisticated group of force generating" tools.
Quote:
 Originally Posted by RicB I see the only way to control anything is through our body movements. It's the human effort that dictates everything in skiing.
Conveying our thoughts/beliefs clearly and comprehensively through the written word is extremely challenging. In any pronouncement of black or white opponents can usually find gray and thereby deem the position invalid.

Ric, your post above is case in point. Though as presented it pleads to be challenged, I doubt if in reality your beliefs are this totalitarian. Such is the difficult responsibility assigned the reader; distinguishing a true lack of understanding on the part of the poster from a composition shortcoming, and treating it accordingly. Those who are here to learn, or help others to, will make that distinction. Those that are here simply to "WIN" will gleefully look at both as an opportunity to attack.

This thread (now over 200 posts) has shown examples of both. I've enjoyed the higher level segment of our interaction here. I have no tolerance for the rest.

Quote:
 Originally Posted by RicB I like to think of the ski as an extention of my body, and view the forces from the inside out. How do we use our body power (weight) to our advantage?

Ric, your description of your view of skiing (this one, combined with others), and the nature of the forces that influence it, prompts me to share an analogy that conveys mine.

For me, carving precise turns can be related to driving a car down a curvey road. We ride in the car as we ride on our skis.

On the road we're powered by the engine of the car. On the slope we're powered by gravity. In our car we have control over the power, we can alter it at will. On our skis the pitch of the slope we happen to be on controls the power exerted on us, we merely react to and direct it.

On pavement we turn by rotating the steering wheel and allowing the mechanics of the car, and its interaction with the road, to produce a change of direction. On snow we turn by tipping our feet and allowing the mechanics of the ski, and its interaction with the snow, to produce a change of direction.

How much physical input is required to direct our car to turn the desired amount depends on the mechanics of our car. An old clunker with an antiquated power steering system requires a large amount of steering wheel rotation compared to a sports car with tight rack and pinion steering. Likewise, the old clunker straight ski requires much more tipping to direct it to produce a given turn than the (sports car like) shaped slalom ski.

The car makes the turns, we merely ride and steer. So too with a ski. Ydnar, I tend to concur with your ski as a tool position.

Quote:
 Originally Posted by RicB It's been a good discussion so far, don't you think?
Indeed.

FASTMAN
Quote:
 Originally Posted by daslider "…you too can experience forces greater than g, but not necessarily because some additional force is at work. … the only force acting initially is gravity. We then play with that force and shape its effect to our purposes in the way we deflect it off the snow and in that sense it is our force…"

And, from another of his posts:

> “…Aren't the really heavy g forces you feel at the bottom of a turn actually decelerations as g is diverted sideways? …”

As in many of his previous posts, the above statements by daSlider once again show a dreadful and complete lack of understanding of some of the most elementary principles of high school physics.

Gravity is not, and can not be turned off, re-directed, deflected, sheared, or manipulated in any way at any time, including during such encounters. If anyone knew a way to do this, not only would they win the next Nobel prize, but they would become a millionaire from the practical applications of the technique. Statements to the contrary are physics gibberish.

From the equation which is the foundation of all classical mechanics, F = m * a, if there is an acceleration, then there is a force acting on the body to cause that acceleration. Acceleration is a change in either the direction or magnitude of the velocity, so both the apple hitting the wall and the skier making a turn are both experiencing acceleration, and hence must be experiencing a net force of sufficient magnitude and in the correct direction to cause the observed change in velocity.

Again, from daSlider:

> “…I can see the ski using the snow reaction force, but I can't see it "belonging to the more sophisticated group of force generating" tools. …”

From a quick read of the past couple of days of posts, Ydnar’s explanations are much closer to correct. The skis are indeed a specialized tool designed to interact with the snow in such a way to generate the forces desired by the skier to change his velocity in ways he wants. Think of the brakes on a car. They are an analogous “specialized tool” designed to cause the car to interact with the road in such a way as to generate the forces desired by the driver. If someone runs out in front of me while driving, using minimal effort, I can cause the brakes to generate decelerations (ie, forces) larger than the accelerations used to get up to speed. In fact, with a properly equipped car, I can even generate decelerations somewhat larger than G.

I no longer suspect that daSlider’s incredibly blatant erroneous comments are a troll, but an attempt by him/her to understand the fundamentals of physics by debate / discourse. This has led him/her to come up with various ill-defined concepts (eg, “deflecting gravity”) that have absolutely no predictive power, and which are at variance with hundreds of years of successful experience with Newtonian mechanics as well as all more modern physics. I would suggest that learning physics by debate is an extremely time-inefficient approach, and that a night or two curled up with a high school level physics text would be a much more efficient way to achieve this goal. OTOH, if I am wrong and your source of enjoyment is really wordy debates and not getting to the truth of the matter being debated, then, of course, you are using the correct approach (for you). Personally, I detest recreating endless dialogs which I know were resolved in Newton’s day (or very shortly thereafter).

As far as I am concerned, one useful thing has come out of this thread, and it is something that several participants (including daSlider) have repeatedly pointed out. Ydnar put a positive spin on it:

> “… One of the most interesting things is that effective skiing and ski instruction can be based on such a diverse selection of interpretations of the forces we feel acting on us as we make our way down the mountain. …”

My interpretation is that it is now extremely obvious to me that there are lots of people in the world who, like daSlider, have a very primitive understanding of mechanics, and hence have invented for themselves some very strange concepts to explain mechanical interactions in the world. As ski instructors, we have to be aware of this, and as I have said before, not even try to venture into this area students unless they bring it up, and even if they insist on discussion, caution them that the complexity of a technical approach to the subject can take huge ammts of time that are almost certainly better spent on actually skiing.

Tom / PM

PS – Getting back to one of the early issues raised in this thread, “Do you ski on your bases or your edges?”, would anyone like me to post a little fill-in-the-blanks spreadsheet (like the one for sidecut radius) that will tell you exactly how much force the snow is exerting in a carved turn on the base surface and on the sidewalls (and side of the metal edge) of a ski? The user fills in the slope angle, the angle of the base off the snow (ie, 0 deg = flat ski), and the centrifugal/centripetal acceleration in G's. The program returns the two forces (base and side) in units of multiples of the skier’s weight.
Quote:
 Originally Posted by PhysicsMan would anyone like me to post a little fill-in-the-blanks spreadsheet (like the one for sidecut radius) that will tell you exactly how much force the snow is exerting in a carved turn on the base surface and on the sidewalls (and side of the metal edge) of a ski?

Yikes!! Tom, when I first read this I misread it. Thought you were asking if one of us would like to create and post the spreadsheet. Before discovering my mistake I had already begun the process of considering the relevant elements that would have to be accounted for in the formula. I'm glad you've already done it!!

Let me take a stab though:

* My thinking is that during a turn the ratio of base pressure to sidewall pressure is dependent on the relationship between the edge angle (perpendicular rise line from the ski base) and the resultant force vector (RFV) angle; RFV being the resultant effect of gravity and centrifugal force (or momentum).

* The more RFV angle diverges from the edge angle (toward vertical) the greater the sidewall pressure in relation to base pressure.

* In calculating RSV angle one would have to know skier velocity, (as determined by slope angle, coefficient of friction for a particular snow, wax and base material, and turn shape), and turn radius (as dictated by the particular edge angle, ski sidecut, skier mass, snow hardness, and ski flex).

* Different sidcuts producing similar turn shapes at similar speeds will produce different base/sidewall pressure ratios because, while the RFV angle remains constant, the base rise line does not.

That's as far as I got in my thinking on the subject. Would like to see your formula. Thanks for all the time you donate to providing us with your knowledge of this aspect of the sport, Tom, it's greatly appreciated by all here who truly are motivated to expand their knowledge. As far as your frustration with those who degrade the potential of this forum by treating it as source of distorted self gratification at the expense of accuracy,,,well,,,, I strongly agree.

FASTMAN
It is a shame Physicsman if you allow your self-appointed expertise to stray into personal abuse, surely as a teacher you should have learnt that there is no place for such attempted ridicule. By all means contribute your authoritive comment, but try doing this even-handedly; some of what you attribute to my "incredibly blatant erroneous comments" were actually contributed by others.

In a discussion on reaching an acceptable vernacular for describing skiing, probing and loose terminology will abound as people try to find ways of discussing their own feelings and communicating them. The answer is not to avoid all scientific explanation, but to try and find a suitable language other than the liberal use of complex notions that this thread has shown to bear very little examination. It can be part of a learning process for all of us and all orthodoxies are questionable.

Yd interestingly appeared to be suggesting there was another outside force at work other than gravity and the snow's inertia.

Your analogy of the car's brakes is imo inadequate in support of that position as the braking is entirely dependent on harnessing that inertia when the free running tyre then grabs at the road surface. The braked tyre does what the ski in a hockey stop does, it tries to maximise the road/snow inertia rather than minimise it as it does in free running.

You rubbish the notion of the skier deflecting his momentum and the gravitational force that propells him by using the reaction force of the snow, what I have been calling a countervailing force. Of course it is this reaction force his ski can use but that itself arises out of the influence of gravity, without which skiing would not happen. I think you are point scoring.
[quote=Rick] It's like ydnar says, we sheer the gravity line.

Fastman,

perhaps in the light of PM's latest pronouncement you need to reconsider this statement.

I actually agree with what you are trying to say here and reject the orthodoxy that would dismiss it as bad science unworthy of highschool etc. We are dependent on using gravity both in its action and reaction at the snow. Transfering this idea into digestible ski-speak seems like a worthwhile enquiry and because of this your last para has to be taken with considerable quantities of salt -

"As far as your frustration with those who degrade the potential of this forum by treating it as source of distorted self gratification at the expense of accuracy,,,well,,,, I strongly agree."

if you have a comment to make, please do so. Your conspiratorial nuances are rather pointless.
[quote=daslider]

Quote:
 Originally Posted by Rick It's like ydnar says, we sheer the gravity line. Fastman, perhaps in the light of PM's latest pronouncement you need to reconsider this statement.
By "sheer the gravity line" I refer to the concept of utilizing some alternate force (IE: ground reaction force) to change the direction gravity propels us. The force of Gravity is a constant, we don't alter its direction or magnitude. But we can alter its effect on our speed and direction of travel.

By my definition any time we are skiing down a mountain we're sheering the gravity line (even when skiing the falline) because our direction of travel is not parallel with the direction of gravity. Tom was not debating the validity of this concept as I've stated it here. What he's saying is that we can't in any manner alter the gravity line itself.

His response was directed toward your assertion that the direction of gravity can be changed, as you implied in the following guestion:

Quote:
 Originally Posted by daslider “…Aren't the really heavy g forces you feel at the bottom of a turn actually decelerations as g is diverted sideways? …”
His response to that was an attempt to point out that the force of gravity can't be diverted into a new direction, or for that matter altered at all. See below.

Quote:
 Originally Posted by PhysicsMan Gravity is not, and can not be turned off, re-directed, deflected, sheared, or manipulated in any way at any time, including during such encounters
I think the point of his statement is quite clear.

Quote:
 Originally Posted by daslider if you have a comment to make, please do so. Your conspiratorial nuances are rather pointless.
I've said it to you already in an earlier post, daslider. If I make a general statement it just that. It aggravates me when threads degrade because posters succumb to the ego trap. When the motive becomes winning at the expense of learning (or teaching) then confusion and misinformation can spread like a virus, and everyone loses. That is the antithesis of what this forum should promote.

I've seen you do this, daslider, and I've spoke out about it. I've also seen you conduct yourself better, and I've responded accordingly. That will continue to be my policy. My general statements are directed to all as an indication of the nature of the interaction on this forum I desire. You don't hold the patent on the behavior I dislike.

FASTMAN
Yd said: "Ric,

This indeed has been an interesting discussion. One of the most interesting things is that effective skiing and ski instruction can be based on such a diverse selection of interpretations of the forces we feel acting on us as we make our way down the mountain."

RicB: Isn't that because we are skiing for the sensory joy and not to scientificaly explore the forces. As to how many diverse view points can be effective at teaching the same thing, well,,,isn't this because we teach how to move and where to stand on our skis, and personalize the interpretation of feelings to the student? Doesn't it all boil down to having the right moves?

Yd: "One thing that we agree on is that skiing is dependent on movements of our bodies and the use of a specialized tool. I want to ramble a bit here about tools."

RicB: I understand this point of view. It's valid so to speak, I would only ask, is there only one way to think of this tool we use to ski?

Fastman: Originally Posted by RicB

I see the only way to control anything is through our body movements. It's the human effort that dictates everything in skiing.

Conveying our thoughts/beliefs clearly and comprehensively through the written word is extremely challenging. In any pronouncement of black or white opponents can usually find gray and thereby deem the position invalid.

Ric, your post above is case in point. Though as presented it pleads to be challenged, I doubt if in reality your beliefs are this totalitarian. Such is the difficult responsibility assigned the reader; distinguishing a true lack of understanding on the part of the poster from a composition shortcoming, and treating it accordingly. Those who are here to learn, or help others to, will make that distinction. Those that are here simply to "WIN" will gleefully look at both as an opportunity to attack.

This thread (now over 200 posts) has shown examples of both. I've enjoyed the higher level segment of our interaction here. I have no tolerance for the rest.

RicB: Rick, no one would ever accuse me being the best at communicating through my fingers. It's not my strong suit. Putting this aside, skiing really does boil down to how we move our body doesn't it? How many layers of complexity we choose to put on skiing should serve only to further our understanding and comunication. Here in a public disscussion forum, open free communicaton is what makes it work. If you read something that begs you to differ with, then please do it. My guess is you could get six skiing physicists, and still get six personal interpretations of skiing, or how we ski. Not on the natural laws governing us all, but on how we untile them with body movements.

The car analogy never has worked for me. Too many differences as I see it.
have no more time now, so I gota go. Later, Ric.
daslider,

I am not suggesting that a force other than gravity exists. I am saying it out loud and with confidence. For a skier to exhibit the motion that they do the force must exist just as for the moon to exhibit the motion it does gravity must exist. As I said, you don't have to accept or understand this fact to be a good skier, instructor or coach, but it is a fact none the less.

In skiing the main forces we deal with are gravity, the forces our muscles can generate, and the force the ski generates. Gravity is constant and acts only in one direction, toward the center of the earth. Our muscles can only provide a force that will change our motion in a fleeting manner. The force generated by the skis is the major force in skiing, it produces accelerations that dwarf those we get from gravity. Again, it isn't necessary to accept or understand this to ski well. Our minds and bodies just deal with the reality of it with no need to understand or quantify it. In the same way we know where to run to to catch a fly ball from the moment it leaves the bat without having to do the calculus computations to figure out where it is going.

Tom,

We have been using the term shear in our discussion here, apparently wrongly. Is there a term that describes just using a portion of a force vector in the way we only access part of the gravity vector when we ski down a slope as opposed to being in free fall?

yd
Fastman

'You don't hold the patent on the behavior I dislike.'

LOL, I really don't know what you're on about, but am happy to ignore your remarks if it means we can continue on the substance of this thread which seems now to be about finding a suitable vernacular for technical expression of the wonder of skiing without reverting to misunderstood scientific cliche/jargon.

Your sheering stuff sounds very much like what I was saying and had PM bothered to read all my comments rather than using very selective quotations (a surprisingly unscientific methodology) he would have realised that. My lapse into a shortened form, talking about deflecting g rather than deflecting the momentum gained from g is something I acknowledged, but seen in the context of my wider argument did not deserve such vitriol. Your attempts to question my ability to ski and his to rubbish my grasp of the theory as well as my motives in posting are hardly attempts at constructive dialogue imo.

Perhaps a little more tolerance wouldn't go amiss.
Quote:
 Originally Posted by Rick … Thought you were asking if one of us would like to create and post the spreadsheet. … I'm glad you've already done it!! …

Actually, Fastman, it’s only partially ready. Currently, I have the calculation programmed in a software package called MathCAD. However, since most people on Epic won’t have this application available, I will have to re-program it in Excel, and then ask Dave Chan to host the Excel file on his server as he did with my sidecut radius calculator. This would probably only take a few days to do, but I want to try to add some more functionality before I post it.

Currently, my calculation is only for one point in a somewhat odd type of carved turn, namely the lowest point (ie, the position of momentary traversing and highest forces on the skier) in a semi-circular shaped turn, which if continued, would go back up hill. I will see how difficult it will be to extend what I already have to a more realistic sinusoidal path with smooth transitions between turns, and allow the user to request the normal component of the base and edge forces at any point along the path. This is much more complicated and might take a week or more. Stay tuned.

Quote:
 Originally Posted by daslider … surely as a teacher you should have learnt that there is no place for such attempted ridicule …

When a student begins to seriously think about a new topic and starts formulating various hypotheses about the way things work, that should be encouraged and cherished, even these initial concepts are somewhat inaccurate.

However, when the student continues down a line of reasoning that is clearly wrong, persists in doing this after strong suggestions to re-formulate his concepts, and, most seriously, persists in expending the valuable time and energy of the students and the teacher (ie, essentially disrupting the class) by trying to stand up and continually debate concepts and issues which have been settled centuries ago instead of by simply cracking a book, then I will use a very direct description of the situation to get the student’s attention. This is what I am doing with you.

As I said in my previous post, if all you want to do is debate, fine, but be prepared to take hits from people who know the subject better than you do because I have no intention of leaving nonsense concepts such as “deflecting gravity” stand unchallenged, especially when the concept may get accepted by people not at all technically knowledgeable, but who could easily be swayed by your debating and writing skills.

If you truly want to learn some physics correctly and in a reasonably efficient manner, again, as I said in my previous post, I most strongly suggest that you spend some time going through the relevant parts of almost any introductory physics text.

Finally, I would add that if, as you claim, all you want to come up with an acceptable vernacular for describing the mechanical aspects of skiing, it is absolutely essential that your own understanding of the technical content is correct. Only then you will be in a position to find the best words to describe it to others without the very likely chance of adding more confusion to field, something you say you are trying to remove.

Finally, to some technical matters –

Quote:
 Originally Posted by daslider … You rubbish the notion of the skier deflecting his momentum and the gravitational force that propels (sic) him by using the reaction force of the snow, what I have been calling a countervailing force. Of course it is this reaction force his ski can use but that itself arises out of the influence of gravity …

Don’t mix concepts. Of course, the skier uses the force that the snow exerts on his skis to deflect his momentum. This is precisely analogous to what I said early in this thread when I gave the example of the geek on the carnival ride. The cage pushes him (aka, centripetal force) towards the axis of rotation, and thereby keeps him moving in a circle instead of traveling in a straight line.

However, your statement that the force that the snow is exerting on the skier “arises out the influence of gravity” is one of these logically mushy statements that can easily mislead people. Of course, gravity caused the skier to have a non-zero velocity and thus be able to use his skis to get the snow to push on him in appropriate directions. What is logically mushy about it is that gravity is not required for this to happen. The skier could have just as easily been given a non-zero velocity on the flats by skating, by holding onto a towrope, or by rockets on his back. Gravity is not required for the interaction with the snow (other than the trivial reason of keeping the skier in contact with the ground) and gravity most certainly is not deflected by the skier. All that is required is a non-zero velocity and contact with the surface.

Would you claim that gravity “was required” for the apple to feel the force of the board that it crashes into? No, just like in the case of the skier, the apple could have been launched upwards at the underside of a board and still gotten smushed into it. Velocity and proximity is required, gravity isn’t.

Tom / PM
Quote:
 Originally Posted by PhysicsMan The skier could have just as easily been given a non-zero velocity on the flats by skating, by holding onto a towrope, or by rockets on his back. Gravity is not required for the interaction with the snow (other than the trivial reason of keeping the skier in contact with the ground) and gravity most certainly is not deflected by the skier. All that is required is a non-zero velocity and contact with the surface.Tom / PM
Out of interest, PM, are these velocity-generated forces of interaction with the snow dependent on the skier's kinetic energy or his/her momentum? My vague recollections of high school physics (George Watsons, daslider) tell me there are different formulae for those.
Look forward to seeing the forces table. Any World Cup ski tech can tell you that considerable friction takes place at the section of the ski base that is adjacent to the metal edge, causing the dreaded "edge burn".
pM

ok, I should not presume that gravity is what makes all this work, rockets and skating can provide the momentum to make a ski turn. I've even been blown uphill on a lively Glenshee day. However in downhill skiing is it so absurd to point out gravity's role as prime mover?

I suppose what I am trying to construct here is an explanation which keeps the skier in the driving seat rather than the centrifugal diagram which imo makes out the skier is having to react to a force being imposed on him; I want to see him as proactive. His momentum is largely attributable to the inability of a slippery mountainside to resolve the force of gravity through his slippery skis without a resultant acceleration downhill. I then see him using that momentum and to shape turns by using ski design/technique against the snow. In this sense he is converting the gravitational force into skiing action (though I admit some of my shorthand has been guilty of ommission). The idea of the skis themselves generating forces is correct in that they do deflect this momentum, but it seems back to front; I prefer to think of the skier doing this in a more holistic model.

Maybe this is hairsplitting, but I know early skiers can find the whole thing daunting and the more they can feel in control of events, the better. I accept Martin Bell's point about racers having to endure hazardous forces as they push their limits, but for most recreational skiers it is the other way about, aren't they are shaping the forces to fit their own capacity to handle them?
Quote:
 Originally Posted by daslider pM I suppose what I am trying to construct here is an explanation which keeps the skier in the driving seat rather than the centrifugal diagram which imo makes out the skier is having to react to a force being imposed on him
The skier can be in control, but not of both the elements you refer to simultaneously. He must choose one or the other. If one chooses to control the path of travel down the slope then the magnitude and the resultant angular orientation of those forces is out of his control, he must react to them and manage them. If he desires to exercise an element of control over the forces generated while he skis then the line of travel becomes subservient.

Understanding the resultant force vector concept and/or managing its influence on us while skiing is fundamental to mastering this sport Daslider. You can certainly continue to butt heads with it, and those who attempt to explain it, if that's what floats your boat, but I don't see much payoff in it.

FASTMAN
I used the most elementary way trying to see what happens to a shaped ski edge when in contact with the ground – I cut an arch on one side of a name card.

My observations:

1. When the edging angle is small, one does not need much de-camber, and the full length of the edge is in contact with the flat surface. At such an angle, if one press further (more de-camber), the tip and tail is off the ground.

2. When the edging angle is increased, one needs to de-camber more to enable the whole length of the edge to be in contact with the surface. In doing so, the radius is decreased.

3. So, it seems that the designed side cut of a ski, say 12 m, means minimum edging to bring the whole ski edge in contact with the snow to result in a 12 m turning radius.

Am I correct in all the above observations?
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