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Sequential leg movements

post #1 of 28
Thread Starter 
I have been looking at photos on Ron Lemaster's site (, where I came across this sequence of photos of Hermann Maier:

Note action of inside/outside legs/feet/skis in frames 4-6. The toe pieces/tips in frames 5/6 are tell-tale.

Are Maier's large muscle groups rotating his legs or is the rotation secondary to the engagement of the outside edge of the inside ski? Could you guys help clarify this picture for me?

[ June 14, 2002, 09:25 PM: Message edited by: nolo ]
post #2 of 28
Where's the phantom move?
Where's the equal weighting of skis?
Why is he rotating so much?
And look at that pole plant, it's before he's done with the previous turn. Isn't that out of sequence?

[ June 14, 2002, 05:45 PM: Message edited by: NordtheBarbarian ]
post #3 of 28
(note, if you look at this sequence, it goes down the first column then down the second column.)

I'm not sure how representative as this sequence involves a "recovery" from a small bump (frame 5 you can see the tips of the skis are off the snow) and he has to make a very quick move to get his skis back on the snow.

But, even so, the amount of steering the skis downhill via femural rotary is quite evident. Even the phantom move is evident if you look at how much more angled the inside ski is versus the outside ski in frame 6. That indicates to me that from 5 to 6 he was really using rotary skills. So, in answer to Nolo's question, I believe he is actively using his large motor muscles.

Of interest also is the pole plant in frame 4. Note how far down the hill he plants. This has the affect of getting his upper body moving down hill in preparation of the edge change. This is counter to the conventional wisdom that it happens from the feet upward. He definately shows it from the top, then down to the feet. I'm curious if others see this too. Actually, now that I think of it, I'm wondering if he is making his move downhill with his upper body and just happens to be planting along the way. Either way, he is moving his upper body downhill before releasing the skis.

Great sequence. Thanks.


[ June 15, 2002, 07:41 AM: Message edited by: WVSkier ]
post #4 of 28
I agree with you that the photos show a sequence that occurs fromn the top down rather than from the feet. However I don't think it's a "phantom move" when the hips have already lead into the turn. Isn't the phantom move supposed to start the turn. I think think turn is already started by photo 6.

Cool things about the sequence. IMHO
1) Great similiar edge angles/ parallel boot shafts. (simultaneous knees?)
2) Cool inside-outside sequential ski weighting (kind of like a cross over turn on hockey skates).
3) Extension/retraction seqence is really evident with skis retracted off the snow in photo 6 and extended onto the snow in photo 8.
4) A beautiful end the turn pole plant, rather than a start the turn pole plant. The pole plant helps stabilize moving the hips down the hill.
post #5 of 28
All I see is a functional turn. There is nothing pretty or stylish about this turn. Herman Mayer seems to get a significant rebound in frames 5-6 and then he does his best to re-make contact with the snow and turn (pivot) the skis in the process. I see no phantom move per say, as frames 6-7 are hardly the definition of phantom move. :
post #6 of 28
Great sequence, Nolo! These photos illustrate very clearly a lot of what we've been discussing in several recent threads.

Clearly, in frames 5 and 6 especially, Maier turns his skis with active, muscular rotary movements of his feet and legs. They work against each other (when they are in contact with the snow), and against the upper body, stabilized by wide arms and--perhaps--some pole engagement as well. There is no "rotation" in the classic sense of upper body rotating first into the turn, then pulling the lower body and skis around after it. Instead, the feet and legs (and skis) turn beneath the pelvis and upper body, resulting in the slight "counter" evident in every frame (the 6th frame is close to "neutral" as the feet turn from left to right, passing through "square.")

In the final frame, the skis are firmly engaged and carving, so some of the turning of the feet and legs has become passive. The skis turn the feet here, rather than the feet turning the skis. I believe that this is what you are referring to as "secondary rotation," Nolo. This is a great sequence, showing both active and passive rotary movement.

It also shows an obvious--call it sequential, if you like--lead of the INSIDE ski turning first. That's why the skis diverge in the 5th, 6th, and 7th frames.

The so-called "phantom move" is especially evident in the 5th and 6th frames. It is the active edging/tipping movement of the INSIDE ski rolling toward its "little toe" side, and it is the reason Maier looks bowlegged, especially in frame 5.

This combination of active tipping and active turning of the inside ski into the turn--pulling the tip in, rather than pushing the tail of the outside ski OUT--is precisely what I've tried to describe in the "perfect turn" thread. Maier's is a picture-perfect turn and worth, of course, a thousand words and then some!

Best regards,
Bob Barnes
post #7 of 28
Bob, Nolo, and others,

Your comments on this are illuminating. However, I must say that, from my perspective, the use of the term "rotary" in so many ways poses a very great barrier to the understanding of both MA and teaching methods. I don't know if there's anything to be done given the common practice (here on Epic and elsewhere) of this but I, for one, would like to advocate for a change in this regard.
post #8 of 28
I don't entirely disagree, Si, but what's done is done! "Rotary," which really is descriptive, and words like "rotation," "pivoting," "turning," and "steering" have been around for a very long time, and are deeply embedded in the language, history, and indeed, the spirit, of skiing!

Even if we could agree, like physicians, on some clinical, parched-dry, exact, never-ambiguous terminology, would that REALLY serve us better? I think we would lose far more than we gained. The problem exists not only in skiing, but throughout language, especially English, and we seem to survive. I for one would have it no other way!

And who are "we" to institute a change in the way language is used, anyway? No one has the "authority" to do that, not to mention the ability! How others use language is certainly not in our control. Unless you're the King, and wield the authority to ban certain uses of language and dictate others--and are willing to risk the consequences of such impositions, then language is not something "we" can change!

It only ADDS to the problem, too, when someone insists on coming up with a new, proprietary term (like "phantom move") for something that can be described easily in the words we already have. Let's not forget that the "new" term MUST be described in normal words anyway, before it means anything in the first place! That's not to say it should never be done--common words have to start somewhere, and new words do find their way into the dictionary and the vernacular sometimes. But the urge to create a new term where an old one will do should be strongly governed, and usually squelched!

Indeed, a great deal of understanding often arises from exploring the very ambiguity of terms. What does this REALLY mean? What COULD it mean? What ELSE could it mean? By exploring a concept, we may never agree on a definition or use of a word. But is that the point? To me, better understanding of the concept is a far loftier, and more satisfying goal. And understanding how you might use a word differently than I do simply helps me understand YOU. What's wrong with that?

Learning to live with ambiguity is part of learning to live.

Best regards,
Bob Barnes

[ June 17, 2002, 01:01 PM: Message edited by: Bob Barnes/Colorado ]
post #9 of 28
Thanks for the response, Bob. Guess the point I'd still like to get across is that it can be (and observably has been) a barrier to student learning, instructor training, and effective discussion.
post #10 of 28
Thread Starter 
Learning to live with ambiguity is part of learning to live.
How true! Horst would perhaps replace ambiguity with paradox and Weems would say polarity and I might say uncertainty...

But what's the diff, right Si? Nuances. To Flaubert they were EVERYTHING. I figure 80% of human communication is nonverbal anyway, so let's not spend 80% of our effort on le mot juste.

That's why I posted the pics. Put a picture up and the words suddenly are tethered to a common display. The conversation gains relevance. Communication can happen.

(I thought you'd appreciate the assist, Bob.)
post #11 of 28
Is that really a phantom move that we are seeing in Frame 5? I thought the phantom move was supposed to start the turn. Herman has been moving his hips into the turn for a couple frames, by the time we see the "phantom move".

To a race coach simplicity is elegance.
Nothing really extraneous is going on in te photo sequence. Just real good skiing.
post #12 of 28
I thought the phantom move was supposed to start the turn. Herman has been moving his hips into the turn for a couple frames, by the time we see the "phantom move"
Good point, Nord. That simple observation actually brings out a surprising number of issues. Perhaps part of the solution lies in defining what exactly we mean by "start of the turn"--a point that we've debated well here at EpicSki in the past, and a question with many reasonable answers. In linked turns, skiing movements are cyclic and continuous. Where they "begin" or "end" is an arbitrary matter of definition. Furthermore, we have many different parts that all need to change direction, and may not all change at the same time, combined with PhysicsMan's valid point that "turning" involves either or both changing the direction something MOVES and changing the direction something POINTS (skis can "turn" without changing the direction of the skier's motion, as in a hockey stop).(Sorry, Si!) Even when both occur (most turns), these actions may not happen at the same time.

With that in mind, it is reasonable to think that a "turn" begins when a force pushes us from the side, and ends when that force ends. On skis, especially in the lower half of the turn, the force that keeps us turning results from the engagement of the edge(s) of the skis in the snow, creating a "reaction force" that causes the turn. Thus, the turn "ends" when the edge engagement ends--when the ski becomes flat enough on the snow to release its grip. And, of course, the next turn begins at the same moment (when turns are linked), as gravity can now pull the skier down the hill.

So, by this definition, the turn ends/begins when the edge of the downhill (usually) ski releases, releasing the pressure that is, literally, the force that causes the turn. But the tipping MOVEMENT that eventually results in edge release begins long before that point, just as a pole plant at the beginning of a turn represents the end of a movement that begins earlier in the previous turn as a swing. What you have observed in Maier's turns as movements toward the next turn of the hips and shoulders (obvious in frames 3 and 4) takes place clearly PRIOR to the edge release of his downhill (right) ski, although equally clearly, the edge angle of that ski decreases in these frames too. The downhill ski is still very much pressured, bent, and carving a left turn in the fourth frame, even as Maier moves out of that turn toward the next.

In the fifth frame (top of the second column), Maier's downhill leg makes an obvious move downhill, causing him to appear bowlegged, finally completely releasing the edge of the downhill ski, which he immediately steers downhill into the next turn (causing the tip divergence). That is the clearest image of the "phantom move" in this sequence. And it represents, by the definition above, the actual moment the left turn ends and the right turn begins.

HOWEVER--the "phantom move" does NOT occur only at the initiation. While it typically gets emphasized as a move to initiate a turn, it actually takes place throughout the turn. It refers to the notion that, while it is often the OUTSIDE ski that does the work--that bends under pressure and carves the turn, and that we balance on, and that must tip on edge and steer--it is more effective to focus our attention on tipping movements of the other, non-weighted ("phantom") ski. These movements initiate an often unconscious chain of movements that results in highly effective edging of the outside ski.

So in the first two frames of the sequence, Maier actively tips and steers his inside (left, uphill) ski. If he focused only on tipping the downhill ski, which bears most of his weight and does most (not all, here) of the carving, we would see a more-pronounced knock-kneed "a-frame," and his inside leg would interfere with the tipping of the outside leg. (And if he focused only on TURNING the outside ski, he would have to twist its tail out into a skid.) By the third frame, Maier has clearly started to reduce the edge angle of his skis, lead by the downhill foot and leg. The move that causes the obvious bowleggedness of frame 5, and presents the clearest image of the "phantom move"), actually BEGINS in frame 3.

To summarize, while Maier probably wasn't consciously thinking about ANY of these things, if WE want to try to duplicate his technique (at least as far as edging movements are concerned), we would focus on tipping the left ski left (toward its "little toe edge") in frames 1 and 2, then focus on tipping the right ski right (toward ITS little toe edge) in frames 3-8. That is "phantom edging" throughout the turn!

Please remember that "phantom move" is NOT a description of all the complex movements that take place, but of a simple THOUGHT that can trigger and sustain these movements! BOTH skis tip, and both skis turn, as a result of focusing on the inside ("phantom," if you must) ski.

In this great sequence, Hermann Maier clearly demonstrates the blend of both tipping and steering of the inside ski that I've referred to in the "perfect turn" thread. Whether you focus on the tipping or on the turning of that inside ski, both are involved. There is an inseparable biomechanical link between the two--tipping the lower leg (knee angulation) involves rotation of the femur. And you can't TURN the left tip left without first releasing its edge. So the very simple reminder to EITHER turn OR tip the inside ski into and through the turn can often be all it takes to make that "perfect turn."

Wow--lots of analysis to arrive where we started--the ultra simple advice to "turn the left tip left to go left, and the right tip right to go right." It works!

Best regards,
Bob Barnes
post #13 of 28
Originally posted by Bob Barnes/Colorado:
... And, of course, the next turn begins at the same moment (when turns are linked), as gravity can now pull the skier down the hill. ...
I would modify this sentence slightly to say:

"The next turn begins at the same moment (when turns are linked) when the skier does something to reverse his/her motion across the hill."

There are a bunch of reasons I would make this seemingly small change in wording:

1) Accelerating down the hill under the influence of gravity is not central to the issue of turning. Beginning to reverse the skier's sideways motion across the hill *IS* central. For example, a skier could be making nice carved S's on the flats, in which case, the mechanism of turning could remain exactly the same (eg, carving), but gravity plays only an indirect role.

2) It is important to emphasize that the skier absolutely MUST DO SOMETHING to begin to reduce and then (hopefully) change the sign of his across-the-hill (transverse) speed. If he doesn't, he might as well be standing on two lunch trays. On the flats, he will keep on heading across the hill in a straight line. On a slope, his transverse velocity will also be unchanged, but he will start to accelerate downhill, so his path will be (approximately) parabolic, and he will never start heading the other way across the hill as needed to make linked turns.

The "something" (mentioned above) that the skier does to reduce and then redirect his transverse velocity is (in my mind) what distinguishes a "better" turn from other turns (excuse my value judgement here).

In a poor turn, the skier might do any of several things. He/she might twist his skis, or up-stem, or even add in a push of his tails to the outside of the turn. All of these actions result in the ski(s) trying to plow snow towards the side of the trail. Obviously, the snow will push back and start to quickly reduce the skier's transverse velocity. However, there are some big BUT's to doing things this way:

(a) These methods reduce the skier's transverse velocity to zero (so he is now heading purely downhill), but do not do directly accomplish getting the skier heading back the other way across the hill. They merely get rid of *some* of the problem (ie, his initial transverse velocity), and put the skier in a position to use some method (eg, a snowplow, or finishing the turn in a carve, etc.) to start heading back the other way.

(b) These methods generate abrupt changes in velocity, and result in unsightly beginner's "Z-turns" (lets keep old school slalom hard edge sets out of this, pls.).

(c) These methods (as mentioned elsewhere) don't work very well in difficult snow.

OTOH, in a "better" turn, the skier will take actions to not only reduce his transverse velocity to zero, but then change its sign. He/she will likely flatten his skis and then, start to engage the inside edges in such a way that the amount of time the ski is plowing and/or skittering across the snow is minimized. In this case, snow is compacted to the outside of the turn by the edged ski, and THIS supplies the force pointing across the hill in the opposite direction to his current direction that is necessary to gradually slow and then completely reverse the skier's path.

I think the easiest way to think of the difference between the two types of turns is whether you are getting the needed force by compacting snow to the outside of the turn (ie, forming a little shelf in harder snow) or displacing/scraping snow to the outside of the turn. More of the former and you have a "better", smoother, more robust turn, so you do everything to maximize the former and minimize the latter.

Basically, all of my above comments are little more than restatements of some of the points in BobB's very articulate post but from more of a physics point of view, elaborating on statements that he made such as: "With that in mind, it is reasonable to think that a turn begins when a force pushes us from the side, and ends when that force ends."

Just my $0.02,

Tom / PM

PS (in edit) - Without getting involved in discussions of semantics, I believe that the modified phrasing that I suggested above, as well as my description of the (desirable) actions are also fairly well in line with BobB's apt reminders that one should take actions in skiing to "DO SOMETHING" (ie, take action to reverse your direction), not "stop doing something" (ie, stop traversing in a given direction).

[ June 18, 2002, 11:39 AM: Message edited by: PhysicsMan ]
post #14 of 28
PhysicsMan -what about the stored energy inthe flexed ski ?
post #15 of 28
First, lets get some basics out of the way:

1) Flexed ski(s), much like the compacted snow underneath them, can only push the things that are attached to them (ie, your feet and boots) in a direction perpendicular to the plane of the topsheet under your bindings.

2) Often, the force with which the flexed skis are pushing up on your boots is exactly equal to the force with which you are pushing down on the boots (through your bones) due to your weight or centrifugal force. In this "equilibrium" case, the skis may be flexed, but aren't doing anything very interesting (ie, dynamic). I doubt this is what you are interested in, but people bring up this situation often enough that it should be mentioned.

Much more interesting is the case in which the skis have been flexed by more than this amount, and now capable of momentarily pushing back with more than usual force.

One relatively uninteresting thing you can do with such a pulse of force is to simply "swallow it". In other words, let it push your feet but allow your knee and hip joint to flex and not resist it. In this case, there will be almost no transmission of this force to your torso, and hence, no change in the motion of your CM. This is essentially the same thing that bump skiers do when they "absorb" a bump.

Things get much more interesting if you don't allow your knee and hip joints to move quite as freely, and hence, this pulse of force actually is able to perturb the motion of your CM.

Since this force can only be in a direction perpendicular to the ski's topsheet, it does different things in different parts of turns, and in different situations. For example, if you are balanced over your edged skis on a steady, straight-line diagonal traverse, forces due to ski flexing (and de-flexing) can only be vertical, and hence, can only generate momentary periods of unweighting and not do anything (directly) to the component of the motion of your CM that is in the horizontal plane.

I think the case that you were suggesting in your question is what happens with the ski flex when you are in the middle of a high speed turn, say, at the moment your skis are pointing exactly downhill.

If you are making smooth, relatively slow linked turns (ie, and not hard edge sets), your skis will always be in (or close to) instantaneous force equilibrium, so, in the plane of the hill, they will be pushing back on your feet with exactly the correct force to needed to match the centrifugal force you would calculate from the curved path of your CM. For a given ski, the exact shape of the flex pattern will depend on how soft the snow is, but basically, if its a pure carve, the skis are merely reflecting the force distribution with which the compacted snow is pushing on their bases. Put differently, you won't feel much "rebound" from the skis in this case.

OTOH, if you speed things up, say by doing a older slalom-like turn in softer snow (hard edge set and rebound), you will flex the ski(s) by more than the amount you would expect in a comparable smooth turn, and then, there will be "extra" force available during the pulse (the rebound) that is exactly in the correct direction to reverse the direction of motion of your CM across the hill.

Hope this helped. Gotta run,

Tom / PM
post #16 of 28
Tom, you are forgetting about the torsional flex and rebound of the bent ski. When released, this flattens the skis on the snow.
post #17 of 28
Hi MilesB - Thanks for pointing this out, and while in principle, these properties could enter, in practice, I don't think you will find that they are very important to this discussion (ie, Sequential Leg Movements).

Torsional stiffness of skis has increased dramatically over the decades. Skis are now so torsionally stiff that the only situation in which any remaining small amount of torsional compliance might be important is on extremely hard snow where a difference of a degree in edging between the tip (or tail) and center of the ski (due to twist) might mean that one portion of the ski has an acceptable "critical edge angle" and the tip or tail does not. The solution is this case is to simply angulate by an extra degree.

In snow sufficiently soft that the entire base is in contact with the snow when edged, torsional stiffness is completely irrelevant because the snow pressing upwards on the inside edge exactly cancles the twisting torque from the snow pressing up on the outside edge, and hence there is no net torque around the long axis of the ski that would (say) tend to twist the tip relative to the boot area.

I am aware of various advertising claims relating to torsional properties. For example, Rossi mentions "a shaped metal layer in the tail", said to promote smooth release. I can assure you that it won't do a blessed thing in soft snow, and that personally, I prefer to do my own "releasing" on hardpack rather than have someone intentionally decrease the torsional rigidity of my skis.

Torsional dynamics (rebound) are of even less concern than torsional static properties for this discussion. The reason is that the torsional resonant frequencies of skis are in the 10's of milliseconds. This means that the torsional response times will be very fast, and hence will never get out of mechanical equilibrium due to skier input. Skiers just can't provide inputs this fast.

I should note, however, that there is one aspect of ski performance where torsional dynamics can make a difference, and that is with respect to chattering on ice.

Tom / PM
post #18 of 28
Physics quiz for physicsman

1) How much force does the skier apply to the skis when carving?
2) How much force does it take to bend a ski into the shape it assumes when carving?
3) What is the reaction force of the snow on the ski(s)?
4) Is the impulse of the force calculated in question 2 sufficient to reverse the direction of the skiers momentum?
5) Is the impulse of the force calculated in question 2 sufficient to totally unweight the skiers skis?

Bonus question
What is the pole vault effect?
post #19 of 28
Nord - In dimensionless units suitably normalized to the correct value, the answers to questions #1 - 3 is unity. The answer to Q4 & Q5 is "sometimes". The answer to the bonus question is, "Its the reason that residents of Warsaw feel cold when they visit their safe deposit boxes."

Any more questions?

[img]tongue.gif[/img] [img]tongue.gif[/img] [img]tongue.gif[/img] [img]tongue.gif[/img]

Tom / PM

[ June 19, 2002, 10:24 PM: Message edited by: PhysicsMan ]
post #20 of 28
On the chance that your questions weren't a joke/troll, you may not have liked my previous answers, so maybe you will like these more:

1) How much force does the skier apply to the skis when carving?

The force equals the vector sum of the skier's weight plus M times his instantaneous acceleration. If the skier isn't doing anything to induce vertical acceleration (eg, unweighting moves, terrain rolls, etc.) and is maintaining a constant downhill velocity, then the only component of acceleration left will be the instantaneous centrifugal acceleration due to the turn he is making.

2) How much force does it take to bend a ski into the shape it assumes when carving?

This is obviously somewhat of a trick question, mostly because you haven't given any specific conditions like longitudinal stiffness, hard or soft snow, sidecut shape, edging angle, etc. So, the best reply I can give is just the tautology, "It takes whatever force it takes to make it carve with the specified shape."

3) What is the reaction force of the snow on the ski(s)?

If the ski is not jumping off the snow or being driven into it (ie, is not accelerating in a direction normal to its running surface), the reaction force of the snow is exactly equal (but opposite in direction) to the answer I gave to question #1.

4) Is the impulse of the force calculated in question 2 sufficient to reverse the direction of the skiers momentum?

Again, you didn't give any specifics, but I will say that the impulse probably won't be enough in most cases, but may contribute significantly to the change in direction in the case of old-school slalom hard edgeset / rebound turns.

5) Is the impulse of the force calculated in question 2 sufficient to totally unweight the skiers skis?

Again, there are many, many variables that enter this, and you didn't give any specifics whatsoever, so I can't really give you a proper, technically sound answer. About the best I can do would be along the lines of "if the skis are really loaded up and this is suddenly released (say with the help of a bit of fast leg retraction), sure, the skis will come off the snow. Don't load them up as much, do things slower, and don't help out with active leg retraction, and they won't." (Not that you didn't already know this.)

Bonus question: What is the pole vault effect?

While you may think it has something to do with storing a runner's kinetic energy energy in a long bent rod and then releasing it in the form potential energy (ie, height) as the pole straightens, I still maintain that it has to do with people in Warsaw, but will grant that it is vaguely similar to a flexed ski.

Tom / PM
post #21 of 28
It seems to me that you all are making this a lot more complicated than it has to be. In skiing we have a body in motion (literally a body in this case) and we want to move that body around the slope to where we want it to go. Now my buddy Newton tells me that the only way to do that is to apply a force to the body to push it in the new direction. I derive this force from the interaction of my skis with the surface of the snow. By controling this interaction I can generate a lot of force to make a quick change of direction/speed or a little force to make a more gradual change of direction/speed. I can even lift the skis off the snow to allow my body to follow a strictly balistic course. Why make it more complicated than that with all this talk of centrifugal force (which I don't believe in anyway) and reactive force (isn't that what makes a balloon fly around a room and a rocket fly through space). I think I'll stick with my buddy Occam and KISS this whole thing off.


PS P-man, will you please explain to me how inert substances like snow, the ground or the sheet rock of a wall push back when I push on them. This makes no sense to me.
post #22 of 28
Not a Troll. I was trying to see whether you would derive this solution. I just didn't want to do the math myself.

"When a skier sets his edges, the snow exerts a force on the skier similar to the one exerted on the pole vaulter.The resulting upward redirection of the skier's momentum results in an up-unweighting, which the skier feels as rebound. Contrary to common belief the ski themselves do not contribute appreciably to rebound. Compared to a skier's weight, they are simply not stiff enoughto provide much of an effect."
Quote by Ron LeMaster from his excellent book "the Skier's Edge". (page 76)
post #23 of 28
Ydnar - I'm guilty as charged. I know that I type much too fast for my own good. I really liked your explanation and using your buddy Occam to get to the meat of things.

My only defense is that from my experience in teaching physics, I know that different people demand different levels of detail in their explanations. For example, some people make an earnest (but novice) attempt to analyze something like skiing, get all caught up in the if's, and's and but's, and then erroneously conclude that it's either way over their heads, or that it can't be analyzed. I would hope that my posts actually show that there is nothing all that complicated going on at *whatever* level of detail you need or decide is appropriate.

With respect to my previous two posts, I think Nord was engaging in a bit of good natured chain-pulling, so my last post was a bit of the tongue-in-cheek, mock serious variety.

Now, with respect to walls pushing back, probably the easiest way to think about it is something like this: Put a thin piece of paper on the wall and push on it with your hand. This piece of paper is going to be a little probe for the forces that are going on between the wall and your hand. If it moves (towards or away from the wall), you know there must be some net (ie, unbalanced) force acting on it. If it doesn't move, the forces acting on it must be exactly balanced.

Well, it didn't move did it? So, there must be some force exactly balancing the force that your hand is exerting on the paper, and the only place this can come from is the wall.

You can prove that this mysterious new force is coming from the wall by moving the piece of paper off the wall by a bit (holding it with your other hand), and pressing again. In this case, the paper immediately moves towards the wall, showing that there is an unbalanced force acting on it (ie, just the force of your hand). However, as soon as the piece of paper contacts the wall, it stops. This is because the wall is now pushing back on it.

To simplify discussions of things like this, we term the equal and opposite force with which the wall is pushing back the "reaction force".

Now, if you don't like the above explanation, simply look closely with a really good microscope and you will find an army of tiny Maxwell Demons all with their arms outstretched in your direction. Truth!

Hope this helped,

Tom / PM
post #24 of 28
Tom, does that mean when we rid ourselves of these demons we will thus be able to achieve perpetual motion?
post #25 of 28
I'm truly enjoying being a fly on this wall! Thanks, PhysicsMan, for taking the time here--your explanations are getting better and better!

I will add one thing, that may help the "how can an inert thing like a wall be a FORCE?" explanation. When we envision something pushing on us, I believe that the common image involves MOTION. In other words, if I hold up my hand and ask you to push on it, I would expect your hand to move toward mine. There's probably also some common connotation of energy being spent when a "force" applies.

Your explanation of the wall pushing with equal force on the paper is excellent, but it does not incorporate this common (although clearly unnecessary) component of the wall coming at you, or burning fuel.

Our common, everyday experience with forces involves motion often enough that it becomes part of the definition in many people's minds. Of course, we just as often, probably MORE often, deal with forces in equilibrium--as when we place a book on a table, stand on the ground, build a house, park a car, or push on a wall.

Expanding the understanding to recognize that forces act in static situations, and to recognize that EQUILIBRIUM can involve intense forces acting in opposition, brings the question to a clearer focus. Only NET forces--unbalanced forces--affect motion.

Ydnar--let's add a slight twist to Tom's paper "force probe" explanation. It's a great illustration, because the paper, being so light, will detect and respond to even the slightest net force acting on it. Tom described holding the paper against the wall. I'm pushing on the paper, yet it isn't moving, so something else MUST be pushing on it in an opposite direction. That "something" is defined, by Newton's First Law of Motion, as a "force." When you push harder, the wall has to push harder too--even though it is "inert" and unable to move.

It's clearer, perhaps, if I replace the wall! I'll press the paper, with my hand, against YOUR hand, and we'll try to keep it in one place. Clearly, if I press harder than you, the paper will move toward you, and vice-versa. To keep the paper from moving, like the wall, you will have to push just exactly as hard on it as I do, in an opposite direction. If I push harder, you will have to push harder too. The force you are applying should be very obvious, yet you are doing EXACTLY what the wall was doing.

Whether this strict definition of "force" fits with the common definition (which probably comes closer to describing "energy" or "work") or not, it IS the correct technical definition.

We cannot escape this discrepancy between common definitions and strict, technical definitions. To describe skiing, we OFTEN use words contrary to their technical definition. Even my simple use of the word "motion" above is not really correct. What I'm really referring to is "acceleration"--CHANGE in motion. But to substitute the "correct" word for the common word would be to create more confusion than it eliminates!

It's very interesting how this discussion parallels some of the recent discussions about the confusion caused by using ambiguous terminology. Here, we're using the absolutely non-ambiguous term "force"--which has a clear and distinct definition, yet there is no less confusion! It's a good illustration of how strictly defining terms solves very little in the real world. That a term HAS a strict, unambiguous definition is no help to anyone who does not understand that definition. The solution requires education, either way, and definitions alone do not equal--or create--understanding.

Communication still requires asking, and answering, the question, "what do you MEAN by that?" and checking for understanding to see that OUR intended meaning comes across!

Best regards,
Bob Barnes

[ June 20, 2002, 12:27 PM: Message edited by: Bob Barnes/Colorado ]
post #26 of 28
Originally posted by NordtheBarbarian:
...Quote by Ron LeMaster from his excellent book "the Skier's Edge". (page 76)
Hi Nord -

I totally agree with you about the quality of this book. However, I think that in the particular section that you quoted, when he says that "the skis themselves do not contribute appreciably to rebound", he is not putting in all the "if's, and's, and but's" and that can be misleading.

In particular, I'm quite sure that he is only thinking about the case of relatively hard snow when he made that statement. In a turn on such snow, the snow doesn't compact very much. The skis stop flexing as soon as their waists touch the surface and start digging in. At reasonable edge angles, this might only amount to less than an inch of flex. Flex the skis this amount by hand and you will see that they only push back with a few pounds of force, certainly not enough to reverse your direction in a turn. The most important "spring" in a situation like this is your legs, not the skis. Thus, LeMaster is absolutely right in this case.

OTOH, in contrast to the case of hard snow, think about going straight down a moderate slope in a foot or two of soft snow. Imagine that you quickly extend your legs and abruptly push downward on your skis. What do you feel? It feels like you are standing on springs, and they immediately push back. Soft snow has no "sproing" (only "squish"), so what you are feeling must be coming from the flex of your skis. The energy stored in the flex of the skis in this case will clearly assist any active retraction that you might employ to get your skis to unweight / rise. Put differently, the flex does assist rebound in this case.

To prove that the effects of flex of the skis are at least noticeable, if not important in the case of softer snow, repeat this experiment on a pair of skis that have more or less the same width and length, but are much stiffer (eg, Stockli Asteroids vs XXX's). The stiffer skis feel "dead" because they hardly flex at all, and all you feel is the "squish" of compacting snow, not the "sproing" of a spring.

This is why I couldn't (and didn't) give you a clear-cut "flex is important" or "flex is unimportant" answer in my previous post.

Generalization certainly helps with exposition and teaching at the introductory and intermediate level, and is certainly appropriate for a book such as his, but doesn't give all the "if's, and's and but's" needed for a full explanation of the phenomena involved. On the other hand, this is exactly what makes scientists (like me) dull and Ron LeMaster able to keep people awake and sell books.

Tom / PM

[ June 20, 2002, 11:52 AM: Message edited by: PhysicsMan ]
post #27 of 28
Originally posted by Si:
Tom, does that mean when we rid ourselves of these demons we will thus be able to achieve perpetual motion?
Sheesh, Si - do I have to explain everything? To get perpetual motion, you can't get rid of them. Rather, you have to keep a bunch of them around, but instead of having them all pushing back on you in parallel, you line a bunch of them up in a circle with each one pushing on the guy ahead. Pretty soon, they'll be going so fast you wouldn't believe it.

Tom / PM

PS - I understand that stationing one at a door between two chambers works pretty well, too.
post #28 of 28
Originally posted by Bob Barnes/Colorado:
...I will add one thing, that may help the "how can an inert thing like a wall be a FORCE?" explanation. When we envision something pushing on us, I believe that the common image involves MOTION. ...
Bob -

Thanks for your comments. Those were excellent points, both about the inappropriate image of motion often found in such discussions, and the similarities of this discussion to our previous discussion of the various definitions of the verb, "to turn", and the imprecision in language that this can lead to.

I agree fully that clear communication at whatever level of detail that is appropriate is necessary to ensure complete understanding.

Hope you are having a good day,

Tom / PM
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