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# Where is pressure greatest in a turn and why? How much does a knowledge of physics help a ski instructor be a better instructor?

Below are a series of quotes that I extracted from the EB's tail carving technique thread. These bring up two points that  would like to see further comment on but are so lost in that train wreck that comments from others are unlikely.

First is the pervasive myth that toward the end of one of the arcs we call turns gravity and the inertial forces of the turn combine to make the pressure greatest in that phase of the turn.

Second, just how necessary/helpful is a understanding of fundamental Newtonian physics to teaching skiing.

Here is a quote from BB  "To get your first pin, you would need to pass a fundamental technical foundations exam, covering physics, biomechanics, snow science, basic understanding of various equipment design and function, and principles of movement analysis."  This is what Bob would like to see for a level 1 pin

I'm going to hold off making further comment until I see if there is anyone else interested in commenting.

fom

Quote:
Originally Posted by EdgeByter

... you must shift pressure to your tails at the end of the turn when you are fighting both centrifugal force and gravity combined to keep the tails from washing out.  ...

Quote:
Originally Posted by fatoldman

EB,

Gravity and the inertial forces of the turn don't combine at the end of the turn. My guess is that you assume that gravity is pulling you down the hill, it isn't, Dr Maxwell's demons are pushing you down the hill. Inertial forces are also lessened toward the end of the turn because the skier is making less change in the motion of his mass. If you feel the the pressure under your feet is greatest in the last third of the turn then you are probably braking at the end of each turn.

If the above doesn't make sense to you get a basic physics text and research how gravity works (hint, it only pulls us straight toward the center of the earth), and what ground reaction force is. A better understanding of the laws of motion would probably help also.

fom

PS JAMT, hope I got the physics part of that right. If not please set me right my understanding of the subject is that of a well read generalist as apposed to someone who has seriously studied the subject.

Quote:
Originally Posted by EdgeByter

Since we ski on a slope gravity will move us down the fall line of the slope unless it is being completely countered by a reactive force, such as the friction from standing on your edges. The steeper the slope the greater percentage of the gravitational force is combined with the centrifugal "force" in the last half of the turn. If we start on a zero degree traverse and start to carve a turn a component of gravity is pulling down slope and the horizontal component of the, so called, centrifugal "force" is working up the hill.

[Note: centrifugal "force" is not really a force, but it feels like it to us because we are turning while our momentum is trying to go in a straight line (this makes it feel like we are being acted on by a force because our body is being diverted and energy is being used to do so)]. So at the top of the turn the horizontal component of the centrifugal "force" is acting in the direction up the slope. The gravitational force is pulling us towards the center of the earth. At this point the horizontal components of these two forces are in opposition. If we don't lean enough we don't compensate for the centrifugal force and if we lean too much gravity will pull us over the other way. This is why initiating a carved turn from a shallow traverse is difficult, especially at slow speed. Once we have gone through 90 degrees and are facing straight down the hill the centrifugal "force" is still acting towards our feet but the down slope horizontal component of the gravitational force is acting at 90 degrees to the horizontal component of the centrifugal "force" so the combined forces are not opposing or combining with each other. As you continue on this same arc into the lower quadrant the centrifugal "force" and the gravitational force start acting more and more in concert and to a greater and greater degree until the the maximum combined "force" is reached at the bottom of the turn. This is why it is harder to hold an edge in the lower quadrant near the end of a turn, and why it helps to weight the tails more then so they don't wash out and result in a skid. The steeper the slope the stronger the gravitational component down slope and the stronger the combined forces (if you kept skiing in the same constant arc at the same speed). Since your speed probably increased when going more directly down the slope as well, with a constant arc the centrifugal "force" will also have increased at the same time it is working in concert with the gravitational force. I hope that is clear and I didn't make too many mistakes that can be niggled over.

Quote:
Originally Posted by fatoldman

Eb,

This whole paragraph just illustrates that you have no understanding of fundamental Newtonian physics. Without that it is impossible to carry on a discussion. Start by understanding this. There is no such thing as a 'horizontal component of gravity'.  Gravity is a simple force that only acts in one way, an attraction between the com of the earth and our body, it acts only in one direction. Would you understand the idea of the vector of the ground reaction force and how it relates to all this. Further, the inertial forces we feel (what you refer to as centrifugal force) are also diminishing in the last third of the turn because we want to make less change in our inertia at that time.

What you wrote was clear enough it was just flat out wrong.

You have complained that others are calling you out without backing up their statements. I'm backing mine up with several hundred years of facts that are called physics.

If I have made any errors in what I have said I'm sure JAMT or one of the other physics gurus here will call me on it (not absolutely sure I'm using the term inertial forces correctly). What I am sure of is that what I have written is much closer to reality than the junk science you are coming up with.

fom

PS Unless you show the ability to accept facts and learn from them I'm done here.

Quote:
Originally Posted by EdgeByter

Quote:
Originally Posted by fatoldman

Eb,

Likely my last response. My skis are not more likely to skid out in the bottom of the turn even though I am reducing the edge angle progressively through the last third of the turn. Why no skid? Because the forces are naturally diminishing in the bottom of the and I am using the skills I have developed to diminish them even further. I in fact often diminish them to the point that I float through the transition.

It also appears that you seem to think that turns are of constant radius. They aren't. Rather than being the half circle most people think of them as they are closer to a parabolic shape. I'm sure that a half circle turn is possible but it would be the ultimate in a park and ride turn and even then the pressure felt under my feet would be greatest at the apex of the turn and would diminish through the final third.

I engaged here because I thought that perhaps you might actually be interested in learning but it turns out that you are after all just another species of troll and I just don't have a billy goat handy.

fom

Quote:

The physics stuff is getting ridiculous. I don't see the relevance and a lot of it is wrong.

Maybe he'll come out and talk about it, cause I don't get it, but Jamt has talked about we don't accelerate at the end of a turn.

Also, forces are often the greatest at the bottom of a turn because of the direction change across the slope. Even if one ends up on the heels at the very end, just before one is often the most forward- flexed into the boots, of the whole turn.

It's not required to know the physics of what one is doing. Does an ice skater know all about conservation of angular momentum and whatever else is involved in a routine? Does someone doing tricks off jumps know the physics of the spins? Does one need to know the physics of a frisbee to make a good throw? (Is it even understood how it flies?)

More interesting is the physiological part of being either too back or too forward.

The aft technique we see with the posted montage of Ligety in slalom is possible only because he's able to pivot the skis quickly and have them switch sides. You couldn't do that in a gs type turn because there's too much distance to the next gate and you're going too fast.

Quote:
Originally Posted by fatoldman

Tog,

"Also, forces are often the greatest at the bottom of a turn because of the direction change across the slope.'

At the bottom of the turn we have already made most of our direction change (unless your goal is to ski back up the hill) so the forces are less than they were a split second before in the pressure/shaping phase of the turn.

Sorry for harping on this but it is one of my pet peeves. I hear this repeated every season and it just isn't true, both the physics of the situation and modern ski technique combine to make the last third of a normal turn a period of diminishing pressure. The first third has increasing pressure the middle third is max pressure (for that particular turn) because that is where the greatest change of direction occurs. These comments are in respect to a transition to transition turn model.

As to the question of the relevance of understanding the physics behind skiing. It's true that you don't need to know the physics to learn to ski or to teach it. But, understanding the physics of the situation has helped me and others I know to make sense of why we do what we do on skis and often why my students are not progressing to their goals. Also, a flawed understanding can easily lead to flawed technique.

fom

Quote:
Originally Posted by Tog

It totally depends on the shape of the turn.

If you're intent on showing people this then you should post some diagrams and photos. Otherwise "end of turn", "bottom of turn", "direction change" are all vague and subject to interpretation. If a gate is in the middle of a turn that's quite different than if one turns down at the gate.

Sorry, but I'm not buying what you said just on the words. There's too many types of turns. You're going to have to show me.

In slalom, usually there isn't much of a carved arc to the turn, it happens late and down near the gate. So just in that example, what you've said is not totally correct.

Exhibit A. Most force at bottom of turn. edit: I should clarify in that I consider the "turn" starting at image 3 or just after really. In the previous turn, prob max force is  before image 2. At 2 she's starting to come out of the turn.

Photo: Ron LeMaster                                                    Mikaela Shiffrin                                                Loveland SL 2011

Quote:
Originally Posted by fatoldman

Tog,

I'll use your photo to try to make my point. Notice that in images 1,5,6 there is substantial spray coming off the skis, the pressure is so great the the snow surface is breaking down. In image 2 the arced ski is holding a carve with little or no snow spray, this indicates to me that the pressure has diminished and the snow surface can now support the load without breaking down. In my way of looking at a transition to transition turn images 1,5,6 all occur in the pressure/shaping phase, image 2 would be the last third of the turn and the unseen image 4 would be the first third.

Turn shape, intent and other factors certainly effect where pressure occurs in relation to the slope/fall line, but from my frame of reference every turn/arc shows the increasing pressure, max pressure, decreasing pressure profile.

One last point. Breaking the turn into thirds like this can be misleading because in respect to time the thirds may not be equal. In a slalom turn the first and last thirds will be of very short duration in respect to the pressure phase, in a Super G turn they may be of greater duration than the pressure phase.

fom

Quote:
Originally Posted by Tog

Well I call 1+ (between 1 and 2) and 6 the bottom of the turn. So we agree! (prob not...)

If I break the turn into thirds it's distance, not time. I'm not confused about that.  However, the distance is the arc they've moved on since transition. There may be almost no pressure for the first third or even half.

But this is all vague. Let's go get some diagrams.

Different turns, different max forces in different parts:

Look at the middle one. Now, I'd call that max pressure at "the bottom" but maybe you wouldn't and maybe it's not exactly. If you take the most extended position, the go four back, that's probably where max force is. That would be 18 images from the bottom. Do we agree on that? (maybe it's 18.5?) Now if you call that the middle third, and I call it the bottom third, there's the problem.

Slope pitch and turn shape make a big difference.

#1

Originally Posted by Bob Barnes  "A Tale of Three Turns"

That's the point. People were doing just fine falling off bridges before Newton. You don't need to know the gravitational formulas. It's a safe bet that 90% of the population does not. But they're doing ok.

Now if they want to fly to the moon it's a different story if they're going to do the project. If someone else does it, no problem.

Besides, what is gravity? Beyond the formulas, how would you explain it? Are you going to use Relativity?

Quote:

I'm saying it's fine to know about those forces, but it has little to do with saying someone's technique is "wrong". Because, probably you got the physics wrong in saying it for one thing. (I wouldn't say that to Jamt...)

For another, in a way it doesn't matter all that much.

Someone does something. Does it work or not? How are you going to say it works? That's the question.

Using Physics to "prove" it doesn't work, when they are already doing it, is weird.

If someone's technique is "faster", then race. If it's a style, then there's really nothing you can say to prove it's no good.

Look, feet glued together pushing the tails out, butt wagging makes some people happy going down the hill. They think it's great. Maybe they're stuck in the past, or just don't care and really like it.

Are you going to use physics to "prove" it's not good? I'd love to see that one.

Of course you might say well in such n such a situation you can't do x, like a high speed carve. That might be true, but they don't care. Now what?

Now if you tell them that physiologically, if they constantly weight their quads they have less balance response etc... That could make a difference. That's more a "proof" then any physics you could throw at it.

However, if they're doing it all day and are still fine with it, then whatever, they're ok. It might drive you nuts, but they like it so there's nothing to do.

Some runners that win have inefficient techniques but they're used to it. You could take it away, change it, and they might get better. They might get worse. They might not want to bother.

Take baseball hitting. Some styles are so bizarre you wonder how they hit the ball. They do, and they're good at it. You wouldn't teach someone that, but there it is, it works for them. They're in the major leagues and doing well.

Quote:
Originally Posted by Tog

How about settling the forces in a turn business? It's got physics in it....

Quote:
Originally Posted by fatoldman

Tog,

I'll respond this evening when I have a little more time but for now one question. What criteria are you using to identify the 18th image as the one with max pressure? I ask because the better I understand your thinking the better I can frame my response to you.

fom

Here is a quote from BB  "To get your first pin, you would need to pass a fundamental technical foundations exam, covering physics, biomechanics, snow science, basic understanding of various equipment design and function, and principles of movement analysis."  This is what Bob would like to see for a level 1 pin! This is from the Pass rates on level 3 exam thread.

Let's hope that Jamt comes out for this one!

Well I think that's a great idea to have a technical foundation for a level 1 pin. I'm certainly not against knowledge and knowing this stuff. Would this settle the whole centrifigal force issue?

I'm certainly not opposed to having a knowledge of physics and what happens. I just think it's ridiculous to "prove" someone's technique is wrong when it's clearly a style.

It's one thing to discuss physics and figure out what's happening in a specific turn. It's another to use it to say, "see, that won't work". Yet the person is already doing it! Sure, it may be inefficient etc., but there you are.

Also, judging by the physics discussions we've had here, there is mass confusion. That means at least half the answers are blatantly wrong. I doubt that most of the high level skiers in whatever discipline could not carry on a physics discussion about what happens in a turn. They have intuitive sense of how things work and the forces that happen.

Exhibit C:

This was posted a while back and there was a discussion as to what happens if Hermann suddenly flexes both legs in the turn thus taking out his base of support.

Does he go flying with the blue arrow? The green arrow? Down into the ground? The answers were all over the place. Now it would be great to have the issue settled, and have a reference physics department here. My point is that most of the people arguing could ski just fine and coach people's skiing just fine. I agree that it would be much better to have this stuff nailed down, but usually intuitive experience works. The physics discussions get complicated real quickly. Often the simple explanations are inaccurate by people who do not have a high level of understanding.

(This was part of a whopper 39 page thread in 2007)

Originally Posted by michaelA

Ron LeMaster                                         Hermann Maier

The apple fell and hit Newton on the head. (not really) At that time there were no formulas to express gravitational force. The apple still fell, he still knew that if it hit him in the head it would hurt.

Quote Tog:   "This was posted a while back and there was a discussion as to what happens if Hermann suddenly flexes both legs in the turn thus taking out his base of support."

Perhaps the only way to definitively know the answer to that is for one of us to try it.!!
I agree with your point however...FWIW, I bet Jamt's on his sailboat...

zenny

Gravity is a force acting on the skier that points straight down, but the slope is at an angle to it.  We handle this difference in orientation by setting up a coordinate system.  The easiest one for most folks is an x-y Cartesian one with x lined up with the slope and y perpendicular to it.  However, because the skier is in contact with the sloped surface of the hill, a normal force is acting on the skier.  This normal force is perpendicular to the slope and since the skier isn't accelerating into or out of the hill, N is equal in magnitude to the perpendicular component of the gravity force, the into-the hill component of the gravity vector, negating its effect and leaving an along-the slope component of the gravity vector (g) which acts to accelerate the skier in the along-the-slope direction.  Centrifugal (means fleeing from the centre), however you want too look at them (do a search if you want to get into frames of references and reality), act towards the outside of the turn.  So if your outside of the turn lines up with the along-slope direction say the bottom of the typical turn (imagined but not defined by most ski instructors in most discussions),  the along-the-slope component of the gravity force and the centrifugal forces also pull in the same direction, while at the top of that same turn gravity is helping outworking against the centrifugal force.  It might be tempting to say that it is the along-slope component of the normal force that is pushing you along, but thee is no along slope component of the normal force.

Yes, it is weird how a force that we know points in one direction can be split up the sum of two other vectors that point in two other directions.  If it helps, think of the along slope direction being a little bit down and therefore having a little bit of gravity; if it doesn't don't

EDIT: another way to think of it is to line up the X-Y axes so x is horizontal.  Then you will have an X- component of the normal force pushing you in the X direction as gravity pulls you down.  Adding the x component of the normal force to gravity results in a force in the along-the-slope component.  This resultant force, which has a component of gravity in it, also aligns with the centrifugal force in the bottom part of that mythical ski instructor turn.

Edited by Ghost - 6/8/13 at 1:38pm

I like where this is going but I'm afraid it's above my level of physics.
Like I said to Tog in the other thread putting a complex 3D system into a 2D drawing only works for basic understanding.
What happens when the COM moves? Does it creates different moments acting on the fulcrum? Is the fulcrum at the boots?
That's too much for me!
But I'll keep reading here if it progress into a nice discussion!

PS talking with my dad the other day he suggested that extension at the bottom creates a torque that moves you into the next turn, never thought of that but I found it very interesting...

Quote:
putting a complex 3D system into a 2D drawing only works for basic understanding

...and perhaps not even there. Sometimes "simple" is simply wrong!

To add even more needed complexity, it is a FOUR-dimensional system that is squeezed into a 2D drawing. That image of The Herminator only shows a snapshot of the situation--a moment in time--a frozen slice of a whole bunch of different movements happening simultaneously, at different rates, and in different directions. Furthermore, the question asked (which way will Maier move?) lacks a specified reference frame, and therefore can have many correct--but contradictory--answers. In other words, we need to ask which way he will move relative to something. The two most common reference points would be the snow (the earth's surface), or his feet. But there are infinitely more points or objects relative to which "he" could move--or not move.

Attempts to simplify these things to make them more easily understandable are usually well-intended. But they almost invariably result in oversimplification to the point of inaccuracy.

As Tog has suggested, if "the physics" don't appear to work out, sometimes--even for a bona fide physicist--it is a good idea to step back and think a little from the neck down. Like Tog points out, people were falling off bridges for a long time before Newton invented gravity!

Sometimes, the more you know, the easier it becomes to mistake your knowledge for facts. I suspect that that is what Einstein alluded to with his famous statement, "If the facts don't fit the theory, change the facts." If a study of physics leads you to conclude that something that is happening cannot happen (based on the "laws of physics"), you need to reconsider your understanding of the physics!

Best regards,
Bob
Quote:
Originally Posted by Tog

Originally Posted by michaelA

Ron LeMaster                                         Hermann Maier

The apple fell and hit Newton on the head. (not really) At that time there were no formulas to express gravitational force. The apple still fell, he still knew that if it hit him in the head it would hurt.

Shouldn't there be an additional line that coincides with the fall line?

How about "the pressure is greatest where the edge angle is highest"? It's not always true, but understanding the exceptions will help an instructor a lot more than understanding physics will. A knowledge of physics can make an instructor better or worse.

Last year I picked up that book, "The Physics of Skiing", by David Lind and Scott P. Sanders.

It become obvious fairly quickly that the actual physics involved are quite complex, way beyond any discussions I have ever seen on this forum, as nutty as they have gotten at times, the complete picture with all the equations and calculus, is just way beyond what most of us can handle.  I know there are a few physics oriented brainy nerds on here that can handle it just fine, but they are by far the minority.

To go even further, I had an additional question which was not really answered in the book so I wrote the author an email about it and he actually responded that what I wanted to find out would be so complex to determine, pretty much forget it.

Most of the physics discussions that have taken place on this forum have been related to claims that this or that person made about how we are supposed to ski.  Someone else thought they were wrong, so a debate ensued, with claims about physics to back it up on one side or the other, followed by lengthy attempts to debate the actual physics.  And as others have said, I think a huge percentage of information has been wrong here in many cases.  Also as those discussions get deeper and deeper into it, it quickly becomes a discussion that only a few people can understand anyway, and in my opinion, some of the best skiers and instructors at that point can't even follow the physics discussion anyway, so where is the practical confirmation?  Its not there anymore, its entirely academic and perhaps innacurate.  The innaccuracies happen because in order to really analyze skiing it becomes obvious that there are a lot of moving parts and its just way more complicated to analyze then most people, including the authors of that book, can really endeavor to do in an entirely complete way.

The reality is that top level skiers do know what works and doesn't work from practical experience and they just do it.  Generally they do it because someone else who skis really well told them to do just shut up and do it.

That being said, there are a lot of people out there that ski decently, making claims which are in disagreement with other decent skiers....  Someone's claim on one side or the other is false.  Note that also sometimes people say one thing and actually do something else, they are not always skiing the way they are describing their skiing.

Could these decent skiers that are making claims that are actually in opposition to physics be even better if they reached a better of understanding of the actual physics?  Maybe.  I certainly think that instruction which is based on fantasy physics can be helpful, but only if its teaching someone to do movements which makes their skiing better.  if the fantasy physics instruction leads to incorrect understanding of how skiing works, it can lead to years or decades of problems for that skier to get out of a rut.  And for instructors it can lead to years or decades of them teaching simplified concepts which are false.

So yes I think its important to make sure that concepts you are teaching are in line with physics.  I personally feel that LeMaster simplified most of all that down into basic concepts which any skier can understand.  If you understand the very simple physics he presents (especially in his first book), then you will have a good ability to sniff test what you hear coming out of other instructor's mouths.  Sometimes though the physics debates get kind of academic and helpful fantasy physics are thrown under the bus.

For example the centrifugal vs centripetal force debate that always resurfaces.  The real physicists will tell you that centrifugal force is a fantasy force, the real force present is centripetal.  Yet what i feel when I ski, I feel centripetal forces under my feet yes, but my CoM feels centrifugal forces acting on it.  Fantasy or not..i feel it and its helpful to my skiing to think about the effects of that "fantasy" force.

So yes, in some cases incorrect understanding of ski physics can lead to some nutty concepts which simply aren't true and can even lead entire national ski instructor associations to teach movements that they shouldn't be teaching.  But on the other hand, fantasy physics are also sometimes useful and helpful.

It really comes down to  practical experience and consulting with the best skiers and coaches in the world to see what works and doesn't work, keep the physics discussions as simplified as possible, but stick to those core concepts which are proven to be the best for skiing...and forget about trying to hyper analyze it...the hyper analysis leads mostly to insufficient and incorrect analysis anyway...and sometimes the fantasies are still ok.

I have always suspected ski instructors and physics do not mix.  One of my colleagues was a 'real physicist' in a previous life and he confirmed that most of the stuff the trainers come out with is absolute BS.  By implication the CSIA high command is full of it as well because that's where they all get it from but lets not go there.  It sounds impressive but simply doesn't stand up to informed scrutiny.  But lets say you do understand the physics how does that help your students?  If they don't understand it all they hear is blah blah blah.  Worse is when an instructor doesn't understand it but repeats it anyway.  Classic from a session last season was "we need to turn by deflecting the mass".  Erm OK.

Yes the physics is complex.

Here's a Phd Dissertation on analyzing a ski turn.

Not simple.

Dissertation for the Swiss Federal Institute of Technology, Zurich

Finite Element Simulation of a Carving Alpine Ski

Peter Andreas Federolf

2005

Body of paper approx 130 pgs

http://e-collection.library.ethz.ch/eserv/eth:28070/eth-28070-02.pdf

Did someone say the want physics!

http://e-collection.library.ethz.ch/eserv/eth:28070/eth-28070-02.pdf

I went through this two summers ago.  Someone else recently posted it to.  Did I understand all of it?  No way.  Did it open my eyes to certain things and inform me of certain things?  Absolutely.  Just 143 pages on figuring out what is going on with the ski!

just to how how complicated it can be, that paper only covers "carving skis".  If they are discussing the various physicas related to a perfectly arcing ski, and it takes 140+ pages to do so, it becomes even more complicated with a ski that is not perfectly carving/arcing.

Great thread, I missed it before. Excellent post BTS.

Generally in engineering and system estimation we follow the principle pf parsimony.

Basically it means that if you try to describe a system with too many parameters they will be more difficult to estimate.

Also if you have a lot of parameters it means you have a more complex model, and if you have a more complex model the chances are larger that it does not correspond to reality.

In skiing and discussions around skiing this means that if we can limit ourselves to some basic properties we are better off.

Regarding what happens when Herman retracts his legs that is really quite simple. A moving body never changes direction abruptly unless a large force is present. When Herman has retracted the only force present is gravity, which means that directly after the retraction he will continue in the same direction that his CoM was travelling. In this picture it seems to be along the surface and in the same direction as his outside ski is pointing. Gravity is however acting so he will start a trajectoy towards the surface and land pretty soon.

I think this is quite interesting because it gives us some basic properties how how a float/retraction must work. If you just retract your legs without creating some upwards momentum first you will simply sink to the ground just like Herman would. If you are aware of this fact you have a much better chance of having a great float. If you are not aware of this chances are you will have your CoM level in transition and support you weight in a squatted stance. This is bad for several reasons.

Regarding the original question about where the pressure is greatest. Like BB often says, there are very few absolutes in skiing and this is one example of this.

Generally we can say that the pressure is mostly created by the centripetal force and the gravity.

The gravity component is more or less constant if the slope is very flat, but the steeper it is the more it will increase the pressure after the fall line. The reason is that the centripetal force and gravity force affect the pressure in the same direction then.

The centripetal force is directly related to the turning radius and speed of the CoM. It is important to notice here that it is the turning radius of the CoM, not the skis. Particularly towards the end of the turn this is important. Your skis may have quite a tight turn radius, but since you may have started to manage the pressure by flexing the CoM is turning less than the skis, compared to the fall line where the COM may be turning more than the ski.

So the answer to the question is as is so common "it depends".

Physics cannot tell you where the maximum pressure is, because it depends on how you made the turn.

In racing it is generally desirable to have a lot of turning early and this means more pressure early. In practice this means the maximum pressure will be close to the fall line. When it is steeper it will likely be later than if it is flat because of the gravity component mentioned earlier.

Quote:
Originally Posted by Jamt

Great thread, I missed it before. Excellent post BTS.

Generally in engineering and system estimation we follow the principle pf parsimony.

Basically it means that if you try to describe a system with too many parameters they will be more difficult to estimate.

Also if you have a lot of parameters it means you have a more complex model, and if you have a more complex model the chances are larger that it does not correspond to reality.

In skiing and discussions around skiing this means that if we can limit ourselves to some basic properties we are better off.

Regarding what happens when Herman retracts his legs that is really quite simple. A moving body never changes direction abruptly unless a large force is present. When Herman has retracted the only force present is gravity, which means that directly after the retraction he will continue in the same direction that his CoM was travelling. In this picture it seems to be along the surface and in the same direction as his outside ski is pointing. Gravity is however acting so he will start a trajectoy towards the surface and land pretty soon.

I think this is quite interesting because it gives us some basic properties how how a float/retraction must work. If you just retract your legs without creating some upwards momentum first you will simply sink to the ground just like Herman would. If you are aware of this fact you have a much better chance of having a great float. If you are not aware of this chances are you will have your CoM level in transition and support you weight in a squatted stance. This is bad for several reasons.

Regarding the original question about where the pressure is greatest. Like BB often says, there are very few absolutes in skiing and this is one example of this.

Generally we can say that the pressure is mostly created by the centripetal force and the gravity.

The gravity component is more or less constant if the slope is very flat, but the steeper it is the more it will increase the pressure after the fall line. The reason is that the centripetal force and gravity force affect the pressure in the same direction then.

The centripetal force is directly related to the turning radius and speed of the CoM. It is important to notice here that it is the turning radius of the CoM, not the skis. Particularly towards the end of the turn this is important. Your skis may have quite a tight turn radius, but since you may have started to manage the pressure by flexing the CoM is turning less than the skis, compared to the fall line where the COM may be turning more than the ski.

So the answer to the question is as is so common "it depends".

Physics cannot tell you where the maximum pressure is, because it depends on how you made the turn.

In racing it is generally desirable to have a lot of turning early and this means more pressure early. In practice this means the maximum pressure will be close to the fall line. When it is steeper it will likely be later than if it is flat because of the gravity component mentioned earlier.

Great explanation, jamt.  This takes care of my questions about where pressure is greatest and why.  Physics comes to the rescue!

I'm still unclear about why the pressure might be greater after the fall line instead of at the fall line on a steeper slope, all other things being equal.

Say a skier makes the same turn on two slopes of different pitches, flexing at the same point after the fall line in both turns.

I think you saying that the total pressure will still be building up afterwards on the steeper slope, but not on the shallower slope.

Is this due to the skier not being able to flex fast enough to override the increasing speed's (from gravity) effect on pressure?

I find that sometimes it is easier to realize concepts if you go to an extreme.

In this case imagine that you have a slope that is 90 degrees, i.e. vertical. Off course in reality you cannot ski this but just for the sake of an example.

Say that the turn is going completely across the fall line and back. In the beginning of this turn the gravity component (your weight) and centripetal component are opposite each other. If you manage to turn with a speed and radius so that the centripetal force is equal to the gravity force you are completely unweighted at this time. You are free falling in the fall line direction.

When you reach the fall line the gravity is the same direction as the skis so it does not affect the pressure. The only turning force is the centripetal, and if the speed and radius is still the same the the pressure "force" is now equal to your weight.

When you reach the end of the turn, still with the same radius and speed, the gravity component and centripetal component will be in the same direction and the pressure "force" will be equal to twice your weight.

This is an extreme example but the same concept applies when the slope is not vertical and the turn is not 180 degrees. That example would involve some trigonometric calculations etc though, and it would not bring any further clarity to the basic concept.

Quote:
Originally Posted by Jamt

I find that sometimes it is easier to realize concepts if you go to an extreme.

In this case imagine that you have a slope that is 90 degrees, i.e. vertical. Off course in reality you cannot ski this but just for the sake of an example.

Say that the turn is going completely across the fall line and back. In the beginning of this turn the gravity component (your weight) and centripetal component are opposite each other. If you manage to turn with a speed and radius so that the centripetal force is egual to the gravity force you are completely unweighted at this time. You are free falling in the fall line direction.

When you reach the fall line the gravity is the same direction as the skis so it does not affect the pressure. The only turning force is the centripetal, and if the speed and radius is still the same the the pressure "force" is now equal to your weight.

When you reach the end of the turn, still with the same radius and speed, the gravity component and centripetal component will be in the same direction and the pressure "force" will be equal to twice your weight.

This is an extreme example but the same concept applies when the slope is not vertical and the turn is not 180 degrees. That example would involve some trigonometric calculations etc though, and it would not bring any further clarity to the basic concept.

I think I've got this part.  (I think.)  Now add the release/flex, just after the fall line.  Why does the pressure continue to build up on this steep hill but not on a shallower hill?  In both cases you're floating after that flex move...

Quote:
Originally Posted by LiquidFeet

I think I've got this part.  (I think.)  Now add the release/flex, just after the fall line.  Why does the pressure continue to build up on this steep hill but not on a shallower hill?  In both cases you're floating after that flex move...

How do you create the float LF? It is not by flexing. You need to resist the forces in order to create the float, and while you are doing this they are increasing due to the slope effect. The moment you start to flex the forces are not growing as much, and likely starting to decrease quite soon.

Quote:
Originally Posted by LiquidFeet

Why does the pressure continue to build up on this steep hill but not on a shallower hill?  In both cases you're floating after that flex move...

The same pressures can be produced on shallower terrain as on the steeps. One just has to create the pressure and balance against the gravitational pull. Speed on the flats will create the same pressures as slower skiing on the steeps.

Seems like the fatoldman likes to stir the pot with this thread. Excellent!!

Where is pressure in the turn the greatest and why?

Lets take it from Neutral. That is the cross over or cross under or flat skis or whatever you want to call it. At this point NEUTRAL the bases of the skis are flat on the snow and the BOS should be directly under the COM. It is from this point the COM begins to fall inside the arc and the BOS moves away from the COM. Floating stage if you wish, free fall is another, CSIA phase 2. As BOS continues around the arc it moves in the direction back under the COM. Phase 3 or Loading Stage or Shaping Stage whatever you want to call it.

This Floating of the COM is caught by the BOS at the bottom of the arc just as if one were to jump off a platform and then land on even or uneven ground.                                                                 The feet are placed so you can remain in balance.

Really it is a game of catch and release.

Quote:
Originally Posted by Jamt

How do you create the float LF? It is not by flexing. You need to resist the forces in order to create the float, and while you are doing this they are increasing due to the slope effect. The moment you start to flex the forces are not growing as much, and likely starting to decrease quite soon.

This is what I think you are indicating.....

The highest pressure happens farther into the turn on steeps because you, the skier, take longer to get yourself properly set up for the float.  On the steeps, after your skis pass the fall line, your speed is higher and the resulting forces you feel are also higher.  It takes your muscles more time to do their thing, so you are farther through the turn before your release occurs.

Have I got this right?

Quote:
Originally Posted by LiquidFeet

This is what I think you are indicating.....

The highest pressure happens farther into the turn on steeps because you, the skier, take longer to get yourself properly set up for the float.  On the steeps, after your skis pass the fall line, your speed is higher and the resulting forces you feel are also higher.  It takes your muscles more time to do their thing, so you are farther through the turn before your release occurs.

Have I got this right?

Kind of. One thing that delays thing when it is steep is the inclination (not banking). Before the fall line it is really difficult to get a lot of inclination when it is steep. For example if the slope is 45 degrees and you want 45 degrees inclination. If you are totally across the slope then you need to be completely horizontal. At the end of the turn completely across you can be vertical, which is obviously a lot simpler. Therefore early engagement is difficult and things will be delayed, but when they do come they come very rapidly.

Can you also see that to support these inclnations we need a sharper turn above the fall line than after? (alternatively higher speed but that is obviously not possible in practice)

Quote:
Originally Posted by Jamt

I find that sometimes it is easier to realize concepts if you go to an extreme.

In this case imagine that you have a slope that is 90 degrees, i.e. vertical. Off course in reality you cannot ski this but just for the sake of an example.

Say that the turn is going completely across the fall line and back. In the beginning of this turn the gravity component (your weight) and centripetal component are opposite each other. If you manage to turn with a speed and radius so that the centripetal force is equal to the gravity force you are completely unweighted at this time. You are free falling in the fall line direction.
When you reach the fall line the gravity is the same direction as the skis so it does not affect the pressure. The only turning force is the centripetal, and if the speed and radius is still the same the the pressure "force" is now equal to your weight.
When you reach the end of the turn, still with the same radius and speed, the gravity component and centripetal component will be in the same direction and the pressure "force" will be equal to twice your weight.
This is an extreme example but the same concept applies when the slope is not vertical and the turn is not 180 degrees. That example would involve some trigonometric calculations etc though, and it would not bring any further clarity to the basic concept.
You meant in the opposite directions no? Centripetal would be pointed up- vertical and gravity down.
Or am I more clueless than I thought?

Btw, while you're here why don't we talk about the 'f' word - Centrifugal force. Seems to be stumbling block # 1 in most discussions in public at least. So how do you view/ describe the terms usage? " pseudo force" would you use?
Would you say it's similar to saying "the sky is blue" when there actually is no color to the air? It certainly feels like one is pressed to the wall on those spinning amusement park cylinders. Ski turns I experience more as resisting the force from turning as it pushes on me.
Quote:
Originally Posted by Tog

You meant in the opposite directions no? Centripetal would be pointed up- vertical and gravity down.
Or am I more clueless than I thought?

Btw, while you're here why don't we talk about the 'f' word - Centrifugal force. Seems to be stumbling block # 1 in most discussions in public at least. So how do you view/ describe the terms usage? " pseudo force" would you use?
Would you say it's similar to saying "the sky is blue" when there actually is no color to the air? It certainly feels like one is pressed to the wall on those spinning amusement park cylinders. Ski turns I experience more as resisting the force from turning as it pushes on me.

You are right Tog, I tried to avoid the sign of things by being a big vague, e.g. with "gravity component" I actually mean the normal force on the ski surface, but I figured that would complicate things for people not familiar with it. (Actually normal force can be extra confusing here; are we talking about the average snow surface or the surface platform that a carving ski has built up?)

In layman's terms I don't really care what terms are used, as long as they are not added or subtracted with the wrong sign. If someone thinks it is easier to think with centrifugal force that is fine.

Gravity is always making your mass feel heavy towards the ground and the ground pushes back up at you with as much pressure as your body weight.  In terms of "pressure", the pressure force vector is straight up.  If you are in a ski turn then your CoM feels like it wants to topple outside of the ski turn radius as centripetal forces push back laterally towards the inside of the turn, ON YOUR FEET.  The two pressure force vectors add up to a diagonal resultant force vector that is pushing upwards/inside ON YOUR FEET.

Yes mostly all diagrams made for skiing will show the arrows going down and out, rather then up and in.  I believe the reason for that is that its just more intuitive to think of the directions of motion, and in a way, its what the upper half of our body feels.  Our CoM is being pulled down and out unless we redirect it otherwise with our feet.  But yes, technically speaking for the pressure discussion all the arrows would be up and in.

But I also don't care how you want to look at it, centripetal or centrifugal.  Whichever way makes sense to you.  Myself I feel its easier to think about how I am balancing my CoM and the G forces I seem to experience pulling it out and down.

Back to the question about where is pressure the greatest in a turn, as JAMT was trying to explain you can see that particularly on steeper runs the force vectors will line up with each other at the end of a turn to be closer to the same direction, which means the resultant force vector will be stronger, ie, more pressure.  UNLESS you do certain things with your body to change things and control the pressure.  Gravity pulls in a direction that is more in line with the bottom of your skis at the bottom of the turn.

Conversely, at the top of a turn its more difficult to establish pressure because gravity is pulling you in a direction that is more inline with the top of your skis, if you even managed to tip them much that soon.

But you can manage pressure and there are many ways to do that, so the question about where will pressure be, depends on what you're trying to do.  The above applies to simplest of turns.  You can create more pressure at the top of a turn, for example, by engaging the side cut and carving early, creating turn shape radius, which will create some centripetal forces for you.  As you turn into the fall line, gravity is less able to rob the pressure from you and the pressure can increase very quickly.  And that is where you need and want the most pressure.  After the fall line pressure will continue to build unless you actively work to reduce some of it, which is generally a good idea and there are numerous ways to do that including skidding, releasing the edges, flexing the legs aggressively, widening turn shape, etc.

So in a nutshell, what I can say is that if you don't do anything to manage pressure, it will be very little pressure at the top of the turn and pressure will increase throughout the rest of the turn right until the end/release.  If you seek to manage pressure, you can increase pressure at the top and reduce it at the end, kind of even it out.  Where do you need pressure the most?  I say the middle around the fall line, but your focus should be on trying to create pressure at the top, where its hard to get it.

Also a lot of recreational skiers are typically focused on speed control as much as anything.  They are not trying to win races.  If you don't develop pressure at the top of the turn, you can't control your speed.  Speed control comes from developing pressure through edging, and allowing the skiis to skid.  No pressure means no speed control.  If you have pressure and if your skis have a higher steering angle such that they are skidding, with that pressure...then you get speed control.

So an awful lot of skiers on the hill, I think the vast majority, are not getting any speed control before the fall line.  The reason is because they haven't engaged their edges and established pressure there.  That means they have to play catch up later and get all their speed control in the last half of the turn.  that is the area where there is naturally a lot of pressure building up so its very easy for a skier to just let all that pressure happen, skid like crazy and get their speed control there.  However, that is not smooth skiing.  If they develop more pressure earlier, then they can control their speed all the way through the turn from top to bottom, making it possible to ease off on the pressure on the second half of the turn a bit and even things out.

There is also the job of actually redirecting the skier mass in a new direction, whether carved or skidded.  Its one thing to change the direction the skis are pointing, and its something else to actually change the direction the entire skier mass is moving.    That is acheived from less skidding, and yes also from pressure.  No pressure means no change of directional movement.  So the more you establish pressure and reduce skidding, the quicker you will change the direction you are moving.  Again a lot of skiers on the hill are currently doing this in the bottom half of their turns because that is where gravity gives them plenty of pressure to work with.  As they skid and slow down eventually their skis reduce skidding and eventually they change the direction they are moving.

if you want to change directions faster then create pressure earlier and reduce the skidding sooner.

If you can get strong pressure with minimal skidding establishing before the fall line you are doing very well...

Pressure is greatest where our mass is being interupted the most - whether it be slowing, or redirecting or some combination of both.  Period.

Quote:
Gravity is always making your mass feel heavy towards the ground and the ground pushes back up at you with as much pressure as your body weight.  In terms of "pressure", the pressure force vector is straight up.  If you are in a ski turn then your CoM feels like it wants to topple outside of the ski turn radius as centripetal forces push back laterally towards the inside of the turn, ON YOUR FEET.  The two pressure force vectors add up to a diagonal resultant force vector that is pushing upwards/inside ON YOUR FEET.

Almost every sentence above is wrong.

Gravity pulls us "down" - ie to the earths center.

A "normal" force pushes us at 90 degrees to its surface. - Only the vertical component of that force counters gravity the component of the force parallel to the surface is what "moves us".  The steeper the run - the greater this component.  I think BB linked a diagram in this thread above.

Pressure is felt in the direction equal to the resultant force (ie the green line in the diagrams below).

In a ski turn, we feel our momentum want to go straight and resist interuption or turning (this is known as centrifugal force - its not really a "force" per se, it is momentum and is very real), yet we have our skis pushing on our feet, and our feet on our mass turning us.  So we can either look at the forces acting on the skier to the snow- gravity and centrifugal force the resultant is the diagonal going from our COM to our BOS.  This creates this familiar diagram.

Conversly we can look at the snow acting on the skier and get (simplified to assume 100% on outside ski):

(arrows of course now pointing up/to the right respectivley)

Forward motion caused of course by the component of the normal force parallel to the slope - I think BB posted a diagram showing that in this thread.

ski dude, think about it some more.

Newtons's third law, when one body is exerting force on another body, the other body exerts back the same force.

If gravity is pulling you into the ground, there is pressure on the bottom of your foot.  Is the pressure upwards or downwards?  Its both!  Equally.

Same thing goes for centripetal forces created laterally under your foot.

There is absolutely nothing wrong with describing it that way.  What is wrong is to confuse them and have one of them backwards.

As a matter of fact, the skier mass would not make a turn shape at all without some kind of force pushing him inward.

Quote:

Pressure is felt in the direction equal to the resultant force (ie the green line in the diagrams below).

In regards to this statement, I would like to ask you to think about is some more, if your ego will allow that.  How exactly does your leg or foot feel the force going in an outward/down direction?

Conversely, what you "feel" is the opposite force pushing back at the foot....which is the ground pushing back at you and centripetal forces...in the up/inward direction.  That is actually the force you "feel" as "pressure",

and muscles in your leg will react biomechanically to push back or resist against that inward and upward resultant force vector as well.

But this is exactly the type of disruptive physics argument which detracts from any meaningful discussion about skiing while we get distracted and side-railed onto the topic of "what exactly is momentum?"

Edited by borntoski683 - 6/13/13 at 3:47pm
Quote:
Originally Posted by borntoski683

ski dude, think about it some more.

Newtons's third law, when one body is exerting force on another body, the other body exerts back the same force.

If gravity is pulling you into the ground, there is pressure on the bottom of your foot.  Is the pressure upwards or downwards?  Its both!  Equally.

Same thing goes for centripetal forces created laterally under your foot.

There is absolutely nothing wrong with describing it that way.  What is wrong is to confuse them and have one of them backwards.

As a matter of fact, the skier mass would not make a turn shape at all without some kind of force pushing him inward.

One of use needs to think of it some more - definatley.

Bold - only true if both bodies remain in a static state.  Skiers move.  What enables the movement?  BB posted it earlier - the ground only acts perpendicular to its surface thus inclined surfaces do not counteract gravity 100%.  This is called the "normal force".  JamT wrote of this also.  What the ground doesnt counter-act - is "left over" and what moves us.  The steeper the slope - more is "left over in the plane parallel to the slope", thus there is more force on a steeper slope propelling us, then a flatter slope...and a perfectley horizontal slope, gravity is 100% counteracted by the normal force, thus we dont move at all.

Pressure thou acts in the direction of the resultant - green.  Is it down and left or up and right?  Well correct, its just a matter of perspective - symentantics - and not really useful to understanding skiing.

You need to re-read what I wrote.  Look up "normal force" and look at the BB diagram (sorry I thought the BB diagram was in this thread - looking for it).

Edited by Skidude72 - 6/13/13 at 4:00pm

well I thought the discussion was about pressure yes?

Yes you are correct in stating that motion down from gravity comes from relieving pressure in the right ways as you suggested,

but also we can manipulate centripetal pressures through turn shaping, which also contribute to the resultant force vector and contribute to the direction we will go.  Momentum is created from gravity and those offsets of pressure you mentioned, but the direction we go on the snow is a little more complex, and also contributes to creating jetting effects where we can suddenly accelerate due to turn shape and harnessing those forces into the right direction.

Edited by borntoski683 - 6/13/13 at 4:08pm
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