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# Two kinds of Wedge Christie - Page 27

Quote:
Originally Posted by The Engineer

Ghost was weighing in, and he has enough technical knowledge and background to contribute, though he's kind of been asleep at the wheel.  I believe it's impossible to convince you no matter what the facts, but I thought I could convince Ghost.  But, now I believe you are both so entrenched it's not going anywhere.  I've noticed an open window for Ghost at times where it can be a give and take conversation, but I've seen that window shut too.

Thank you

Window is not shut at all.  See my last post.  It is not in jest.  I am offering you an opportunity to explain exactly how you cause the ski to be rotated in the plane of the slope, without exerting undue torque on the knee.  I can see two mechanisms at work.  One being a force applied by the skis tip, acting at some distance from the boot rotating the the ski about an axis perpendicular to the plane of the slope, acting in concert with an applied force from the boot acting at an angle to the plane of the slope due to the ski being tipped while the boot is pushing it perpendicular to the plane of the ski.  Tipping of the ski needs the femur to rotate due to skeletal geometry, and furthermore the ski rotating about an axis perpendicular to the plane of the slope requires the femur to rotate with.

What say you?

Quote:
Originally Posted by yogaman

But can you rotate your right leg while standing on it with your left leg off the floor with out stabilizing the rest of the body?  YM

Quote:
Originally Posted by yogaman

YAWN!!!!  YM

Yes it's boring.  All this effort for such a stupid question.  Stand on one foot and rotate the foot, and you'll see you can do it.  It's accomplished by counter rotation.

Quote:
Originally Posted by Ghost

Thank you

Window is not shut at all.  See my last post.  It is not in jest.  I am offering you an opportunity to explain exactly how you cause the ski to be rotated in the plane of the slope, without exerting undue torque on the knee.  I can see two mechanisms at work.  One being a force applied by the skis tip, acting at some distance from the boot rotating the the ski about an axis perpendicular to the plane of the slope, acting in concert with an applied force from the boot acting at an angle to the plane of the slope due to the ski being tipped while the boot is pushing it perpendicular to the plane of the ski.  Tipping of the ski needs the femur to rotate due to skeletal geometry, and furthermore the ski rotating about an axis perpendicular to the plane of the slope requires the femur to rotate with.

What say you?

I haven't taken the time to consider all the reasons why rolling the knee is more powerful than standing and rotating, but my intuition says it's about leverage.  Standing and trying to twist the ski has never been very successful in my experience and bothers my knees.  Rolling the knee whips them around with no knee discomfort whatsoever.  I don't feel like digging into the issues.  I know those skiers and other skiers roll the knee to rotate the ski, because I do it intentionally.  Forward pressure is definitely a big part.  It's harder with weight back.  It's the best way to do it for rotary turns in my opinion.

Quote:
Originally Posted by The Engineer

Rotate the femur and you rotate the ski.  But, as I said before, it rotates in a way that applies more pressure to the front of the ski which helps to pivot around a fulcrum.  You know I do have things to do Ghost, and proving things beyond a reasonable doubt is not one of them.

You might want to reconsider this statement before getting on your high horses.

Quote:
Originally Posted by Jamt

You might want to reconsider this statement before getting on your high horses.

How does it go?  Anything else I said wrong?  Anything BTS said wrong?

Quote:
Originally Posted by Jamt

You might want to reconsider this statement before getting on your high horses.

Go through the conversation and list everything I said wrong and everything BTS said wrong.

Quote:
Originally Posted by The Engineer

Quote:
Originally Posted by Ghost

Thank you

Window is not shut at all.  See my last post.  It is not in jest.  I am offering you an opportunity to explain exactly how you cause the ski to be rotated in the plane of the slope, without exerting undue torque on the knee.  I can see two mechanisms at work.  One being a force applied by the skis tip, acting at some distance from the boot rotating the the ski about an axis perpendicular to the plane of the slope, acting in concert with an applied force from the boot acting at an angle to the plane of the slope due to the ski being tipped while the boot is pushing it perpendicular to the plane of the ski.  Tipping of the ski needs the femur to rotate due to skeletal geometry, and furthermore the ski rotating about an axis perpendicular to the plane of the slope requires the femur to rotate with.

What say you?

I haven't taken the time to consider all the reasons why rolling the knee is more powerful than standing and rotating, but my intuition says it's about leverage.  Standing and trying to twist the ski has never been very successful in my experience and bothers my knees.  Rolling the knee whips them around with no knee discomfort whatsoever.  I don't feel like digging into the issues.  I know those skiers and other skiers roll the knee to rotate the ski, because I do it intentionally.  Forward pressure is definitely a big part.  It's harder with weight back.  It's the best way to do it for rotary turns in my opinion.

This makes me believe that we share the opinion of how to get skis to come around and come back around the other way while skiing moguls.  We just haven't described it well enough.  I also suspect that the anit-rotary people also just don't understand the explanation, in particular since everybody seems to share the idea that standing and trying to twist the ski is bad (that twisting the ski while standing and torquing the knee, imho is the "move" they object to, but is not the move you mean to imply when you "use rotary").

Quote:
Originally Posted by Ghost

This makes me believe that we share the opinion of how to get skis to come around and come back around the other way while skiing moguls.  We just haven't described it well enough.  I also suspect that the anit-rotary people also just don't understand the explanation, in particular since everybody seems to share the idea that standing and trying to twist the ski is bad (that twisting the ski while standing and torquing the knee, imho is the "move" they object to, but is not the move you mean to imply when you "use rotary").

Fair enough.

Just to be clear, I said it before and I will say it again...I am not opposed to using rotary in the bumps.  I'm just saying...you can't twist the ski without either another anchor point or quite a bit of counter-rotation.

TE has already stated clearly he also believes this to be the case within the past few posts, so I really don't understand all the carrying on that he has made about it.

TE if you had simply said from the beginning, that yes its possible to stand on one leg and twist it while counter-rotating significantly...I don't think anyone would have disagreed with you.

The essence of YM's question, however, was that you can't twist the ski without pushing or twisting something else the other way.

Quote:
Originally Posted by The Engineer

There's counter rotation in her movements.  That's why she is able to rotate her only weighted ski.  There has to be.  But looking to the shoulders is not the entire equation.  You have to balance all her movements taking into account each mass, distance, and angle.  As I replied to LiquidFeet, there are countless video examples of rotary in the air with a seemingly stable upper body.  When I look at Aiko, I see counter rotation that enables her to rotate the ski she stands on.  It doesn't mean that her shoulders rotate the opposite way relative to the earth in every situation.  It can mean that the shoulders just don't rotate as much as they would have if she already had angular momentum.  Standing on a frictionless surface is not much different than rotating while in the air.  Standing and rotating your ski in mud would be quite difficult, but easy on ice.  Allot of beginner skiers that rely heavily on rotary hate the slop, because it's hard to twist those skis, but easy on cold hard pack.  Carving can be done in slop or hard pack, so it's a nice go to move.  But, once there are bumps rotary can again be used easily in any condition, because the rotation happens when you are weightless with very little friction.  So, I would add that using leg rotary to steer while standing on one leg is not going to take the same amount of effort or look the same as the surface conditions change.

Quote:
Originally Posted by The Engineer

Look at Brassard mogul skiing or most fast direct line mogul skiers.  They apply a tremendous amount of rotary motion in the air from one bump to the next, but the upper body is fairly stable.  There does need to be counter rotation for rotating skis in the air, but when balancing the masses times distances of all parts of the skier, the amount of twisting of the shoulders ends up being quite small.

Quote:
Originally Posted by borntoski683

Just to be clear, I said it before and I will say it again...I am not opposed to using rotary in the bumps.  I'm just saying...you can't twist the ski without either another anchor point or quite a bit of counter-rotation.

TE has already stated clearly he also believes this to be the case within the past few posts, so I really don't understand all the carrying on that he has made about it.

TE if you had simply said from the beginning, that yes its possible to stand on one leg and twist it while counter-rotating significantly...I don't think anyone would have disagreed with you.

The essence of YM's question, however, was that you can't twist the ski without pushing or twisting something else the other way.

I did say it from the beginning several times, at least twice.  I said it to you, and I said it to Liquid Feet.  Quotes above.  YM's question was posed several times in different ways.  I saw two of the questions that did not exclude counter rotation.  I didn't see the time he posed the question with counter rotation, but I don't care about the question.  What I thought was interesting was the emphasis of rotary motion in high C from mogul skiers and wanted to share that in this conversation about leg rotary.  When I gave my answer you treated me like an idiot, and I will not be bullied.  In your defense, I gave a low blow to you when I entered this thread which was a result of my frustration of thread after thread of BTS arguments with one person after another. Those videos show rotation of the ski powered by the leg while standing on one leg and it's made possible by counter rotation.  That was my story in the beginning and that's my story now.

I'm sorry to hear that you feel down trodden TE..  To be fair, this quote below was your first post in this thread....where you specifically called me out without me having said anything to you whatsoever on this topic...  so while you feel attacked somehow, it is actually you that attacked me first totally out of the blue with zero provocation on my part...

Quote:

Originally Posted by The Engineer

This video was shown to me in this forum, which was the single most valuable moment making pages of angst worthwhile.  I knew how to do on the lower C, but did not know to do on the upper C.  Weight shift and knee roll on upper C whips the skis around without a mogul.  When there's a bump, the weightlessness makes it easy to bring the skis around, but in irregular moguls, it's hard to bring them around in time to keep the rhythm even though the bump isn't there, and this weight shift, knee roll, to rotate starting in upper C is the answer.

If I may borrow a move from BTS's playbook and say this is the mark of the truly expert skier, then I can offend everyone that skis great in different ways and create 200 pages of argument.  There's a name for that kind of internet behavior, and with these statements, I plead guilty.  But applying these techniques you can ski irregular moguls like this video below at 6:18.  Though I realize that some want to and some don't

This was my first subsequent response to you.  Can you please tell me how I was making you feel like an idiot with this?

Quote:
Originally Posted by borntoski683

TE, glad to hear you have discovered high C engagement. However that didn't answer YM's question.

And for the record, the demos at 4:23 of this video would not pass a wedge demo exam.

everything that happened from there on out was simply talking about skiing and exposing numerous fallacious arguments you attempted to use.   I will always do that, you should know by now.  Don't use fallacious arguments.

Personally I think some good came out of the discussion.  You shouldn't take it so personally, I for one am not out to get you.

Quote:
Originally Posted by borntoski683

I'm sorry to hear that you feel down trodden TE..  To be fair, this quote below was your first post in this thread....where you specifically called me out without me having said anything to you whatsoever on this topic...  so while you feel attacked somehow, it is actually you that attacked me first totally out of the blue with zero provocation on my part...

Yes I just admitted this in my last post.  Read my posts you idiot.  Just kidding.  Let's not argue.

Quote:
Originally Posted by The Engineer

I haven't taken the time to consider all the reasons why rolling the knee is more powerful than standing and rotating, but my intuition says it's about leverage.  Standing and trying to twist the ski has never been very successful in my experience and bothers my knees.  Rolling the knee whips them around with no knee discomfort whatsoever.  I don't feel like digging into the issues.  I know those skiers and other skiers roll the knee to rotate the ski, because I do it intentionally.  Forward pressure is definitely a big part.  It's harder with weight back.  It's the best way to do it for rotary turns in my opinion.

I'll respect your desire not to to go into this topic & it is off topic.... it's not my intent to prolong this discussion. Its just something I ponder for a while, IMO, while in transition and toward the middle of the turn that forward pressure does a couple of things. Makes that tip bite into the snow...that's the start of the turn, ok nothing new. The more important item is the forward pressure does bring the COM close (or more forward) to the ski. Not sure if I should bring this up again..... but by doing so, that is moving the axis of rotation such that the distance between the COM to the pivot point of the ski is shorter. By conservation of angular momentum, the ski has more angular speed so it should whip around.

I know there are other factors in play but all being equal.... edge angle, snow condition, ski construction and forces normal to the snow. With respect to it's plane or space, forward pressure or getting forward does  move the axis of rotation even in a minute way.

It's interesting what you say jack. If the CoM is moved forward during a transition where the skis then pivot, that forwardness instantly becomes outsideness. A bit.

not sure about the angular momentum idea, have to ponder that some more, that sounds like a pump track argument about to explode. ;-) where does the angular momentum come from though they you feel moving forward would effect in some way?
Quote:
Originally Posted by jack97

I'll respect your desire not to to go into this topic & it is off topic.... it's not my intent to prolong this discussion. Its just something I ponder for a while, IMO, while in transition and toward the middle of the turn that forward pressure does a couple of things. Makes that tip bite into the snow...that's the start of the turn, ok nothing new. The more important item is the forward pressure does bring the COM close (or more forward) to the ski. Not sure if I should bring this up again..... but by doing so, that is moving the axis of rotation such that the distance between the COM to the pivot point of the ski is shorter. By conservation of angular momentum, the ski has more angular speed so it should whip around.

I know there are other factors in play but all being equal.... edge angle, snow condition, ski construction and forces normal to the snow. With respect to it's plane or space, forward pressure or getting forward does  move the axis of rotation even in a minute way.

That makes sense to me. For a given torque you will get more angular acceleration with the center of mass on a shorter radius.
Quote:
Originally Posted by jack97

I'll respect your desire not to to go into this topic & it is off topic.... it's not my intent to prolong this discussion. Its just something I ponder for a while, IMO, while in transition and toward the middle of the turn that forward pressure does a couple of things. Makes that tip bite into the snow...that's the start of the turn, ok nothing new. The more important item is the forward pressure does bring the COM close (or more forward) to the ski. Not sure if I should bring this up again..... but by doing so, that is moving the axis of rotation such that the distance between the COM to the pivot point of the ski is shorter. By conservation of angular momentum, the ski has more angular speed so it should whip around.

I know there are other factors in play but all being equal.... edge angle, snow condition, ski construction and forces normal to the snow. With respect to it's plane or space, forward pressure or getting forward does  move the axis of rotation even in a minute way.

The different angular accelerations you get vs. radius boils down to the same fundamentals as conservation of momentum, but because we're applying a torque there doesn't need to be an initial angular momentum.  Even if the skis have no angular momentum you can apply a torque to rotate it, and if the mass is on a shorter radius the acceleration will be greater so they will whip around faster.  So, I would add that this concept you are considering would apply to the beginning of the turn as well as the middle of the turn.

So jack and TE, here is a question for you. Take a skier making a carved turn. Does the skier as a whole have any angular momentum while completing the carved turn?

I say no.

I say a skier doing that has linear momentum which is continuously being accelerated in a new direction by centripetal forces under the ski. But at any given instant if the edge were to release they would continue on a straight line with linear momentum conserved and angular momentum non existent.

In the case of steering the skis with twisting and counter rotation ( right TE? ) there is no net angular momentum created. Momentum would remain linear.

in the case of oversteering or tails washing out, the reactionary force under the tips can potentially cause some of the linear momentum to be converted to angular, like spinning a top, but generally even this is extremely minimal except in a wipeout where the skier literally spins in some way. Just because a skier is turned to face the other way through a series of accelerating changes of direction does not mean that angular momentum has been created.

How would you know? If the object is able to sustain rotational movement without being operated on by external forces, then it has angular momentum. Like a spinning top for example. Skiers making most all normal ski turns do not possess this angular momentum. we change direction we are facing by a series of snow reaction forces that keep accelerating us in a new direction. When we are done turning one way we release the ski and discontinue those accelerations so that we can engage the other edges and start accelerating the other way. There is no angular momentum at all unless you are in the process of spinning out or wiping out

First, I don't wrap my mind around a carved turn to the details around here. What I stated and assume is that the tip has bit into the snow and that the front part of the ski has engage and most likely the tail has washed out.... some have called this a micro-carve.  Within that time interval, the ski is moving in a radial fashion, so angular momentum does apply. And I do agree with TE that even with the transfer of linear momentum to angular momentum can applied at the beginning of the turn. A short turn can be made by small intervals of radial movements.

I just didn't want to get into this b/c it is off topic and some times I have to get off the grid b/c of work.

Quote:
Originally Posted by borntoski683

So jack and TE, here is a question for you. Take a skier making a carved turn. Does the skier as a whole have any angular momentum while completing the carved turn?
If something is rotating it has angular momentum, except the cat righting reflex considerations.

I say a skier doing that has linear momentum which is continuously being accelerated in a new direction by centripetal forces under the ski.
This is not a bad description of angular momentum.  We often look at the tangential components of momentum, force, velocity, which are instantaneously linear to understand and determine the rotational parameters.
But at any given instant if the edge were to release they would continue on a straight line with linear momentum conserved and angular momentum non existent.
Right.  The moment there is no centripetal force there is no angular momentum.

In the case of steering the skis with twisting and counter rotation ( right TE? ) there is no net angular momentum created. Momentum would remain linear.
Part of the system has angular moment in one direction and another part has angular momentum in another direction.  The net is zero, but you can still consider just part and apply rotational equations to that part.

in the case of oversteering or tails washing out, the reactionary force under the tips can potentially cause some of the linear momentum to be converted to angular, like spinning a top, but generally even this is extremely minimal except in a wipeout where the skier literally spins in some way. Just because a skier is turned to face the other way through a series of accelerating changes of direction does not mean that angular momentum has been created.
Pushing off something to rotate will create net angular momentum.  There's that cat righting reflex discussion I believe you are familiar with which would allow rotation without any net angular momentum, but you need to see parts extending and retracting.

How would you know? If the object is able to sustain rotational movement without being operated on by external forces, then it has angular momentum. Like a spinning top for example. Skiers making most all normal ski turns do not possess this angular momentum. we change direction we are facing by a series of snow reaction forces that keep accelerating us in a new direction. When we are done turning one way we release the ski and discontinue those accelerations so that we can engage the other edges and start accelerating the other way. There is no angular momentum at all unless you are in the process of spinning out or wiping out
If you see reactionary force from the snow causing the skier to rotate than he has angular momentum.  If you see part of his body rotating then we can still discuss the rotational physics including angular momentum of that part.

But does it? Let's take an extreme example. Let's say your tails washout to the point that you are going backwards ( but no longer spinning ). That is a lot of tail wash out, perhaps the max possible; yet the skier does not continue spinning, therefor either angular momentum was never there or something had to decelerate the angular momentum and convert it back to linear again to turn into going backwards. True angular momentum would continue spinning unless acted on in some way to stop it
Quote:
Originally Posted by borntoski683

But does it? Let's take an extreme example. Let's say your tails washout to the point that you are going backwards ( but no longer spinning ). That is a lot of tail wash out, perhaps the max possible; yet the skier does not continue spinning, therefor either angular momentum was never there or something had to decelerate the angular momentum and convert it back to linear again to turn into going backwards. True angular momentum would continue spinning unless acted on in some way to stop it

On earth typically friction or drag sucks away angular momentum. With the spinning skier example, ski to snow friction keeps him from continuing to spin. Edges digging in will slow him too, but I consider edging, friction, and drag to be all similar concepts, just one is more macroscopic with material displacement and overlapping material pushing against each other.

Picture a car going north and then going around a curve until it is going west.  At any point on that curve the car has angular momentum and linear momentum, by definition.

So in the example given you believe friction in the snow is enough to decelerate to zero. Personally I don't think there is enough friction for that. Plus, the skier in that case would feel quite a bit of decelerating torque in their legs as the skis stopped spinning due to friction and their body wanted to keep spinning. The example I am imagining and have witnessed many times, I don't think this torque is experienced. What really happens is the tips of the ski stop creating torque. Like jumping out of a swing.

think when a skier continues to spin it is because they have actually achieved some angular momentum with a corresponding moment of inertia. When they don't continue spinning it is because their body as a whole has not achieved angular momentum with a corresponding moment of inertia, or even in this extreme case, it's minimal. In the example given edging would not typically be able to stop angular momentum. The skis would be flat by then, but then again they might keep spinning if so because at some point I agree some angular momentum will be created. Or perhaps they edge a little bit at the end somehow as you say to stop it.

For a carving skier, the skier gradually is rotating as they change the direction they are facing. The instant they release the edges they stop rotating. There is no period of time for friction to stop rotating them. Instead their linear momentum is allowed to continue straight rather then being continuously acted upon. I simply do not see this as angular momentum and if there are some micro amounts present, there is not enough to "whip the skis around" as someone suggested. I realize that textbooks analyze angular motion for the ball on the end of a string the same way as a spinning top, so fine...but still I draw a distinction between an object that wants to go straight unless something else continuously diverts it, versus an object that is actually spinning on its own, such as an ariel freestyler, ice skater spinning or a top spinning. When talking about ski turns we are primarily talking about linear momentum that is being continuously acted upon to change the direction the skier is moving. If an actual angular momentum were created, the skier would be spinning in some way and probably not desirable.

Anyway this is way off topic. I was just curious. I do think it's possible that if the CoM is moved forward as jack suggested; that this could contribute to washout in some way by virtue of the moment of inertia, in that case.... Everyone knows that leaning too forward causes more tail washout. So.... Is that because the tails lose pressure? Well yes losing pressure at the tails is needed and sure enough that could be some conversion of linear to angular momentum due to pressure at the tip of the ski. And leaning forward closer to the tips of the ski would change the moment of inertia and perhaps contribute to washing them out faster.. So there you go jack! But I say that angular momentum is only present as long as the tails are continuing to wash out into bigger and bigger steering angle. If they stop washing out, are maintained at a consistent steering angle....then the micro angular momentum has been stopped already.
Edited by borntoski683 - 11/25/15 at 2:16pm
And just to reiterate again....perhaps bringing this back to topic......the above stuff is based on oversteering of the ski.........not rotation and counter rotation, which produce zero net rotational movement on their own.

So twisting the leg does what?

Like Ghost explained with the car example, as long as a mass is turning there is angular momentum. However, in a ski turn it is quite small compared to the linear momentum. In a short turn it is smaller due to counter movements, and in a long turn it is small due to the slow rotation.

I'd advice from using the laws of conservation of angular momentum in a turn. It only holds in a closed system, so unless you include moving snow particles, chemical reactions in the muscles and the movement of the earth it is not valid. You can use it during a weightless transition though.

When we are close to the critical platform angle with normal skis, being statically forward displaces the fronts, moving from forward to aft through the engagement displaces the tails.

car's mass is somewhat intact and is nearly impossible to move its mass to shift its axis of rotation. The more appropriate analogy is a (steerable) sled going downward on a slope with a man/woman sitting on it, where that person can lean to either side to shift its COM and move the axis of rotation.

Quote:
Originally Posted by jack97

car's mass is somewhat intact and is nearly impossible to move its mass to shift its axis of rotation. The more appropriate analogy is a (steerable) sled going downward on a slope with a man/woman sitting on it, where that person can lean to either side to shift its COM and move the axis of rotation.

Correct, but

Quote:
Originally Posted by Jamt

Like Ghost explained with the car example, as long as a mass is turning there is angular momentum. However, in a ski turn it is quite small compared to the linear momentum. In a short turn it is smaller due to counter movements, and in a long turn it is small due to the slow rotation.

I'd advice from using the laws of conservation of angular momentum in a turn. It only holds in a closed system, so unless you include moving snow particles, chemical reactions in the muscles and the movement of the earth it is not valid. You can use it during a weightless transition though.

When we are close to the critical platform angle with normal skis, being statically forward displaces the fronts, moving from forward to aft through the engagement displaces the tails.

If you think about the situation, the only thing with any noteworthy angular momentum is the pair of skis, and that only has the momentum given to it by the force at the tip, which is pretty much neutralized and reversed by the next turn.  In other words no more significant than the Coriolis effect.  (Where's that rabbit hole emoticon?)

Quote:
Originally Posted by Jamt

Like Ghost explained with the car example, as long as a mass is turning there is angular momentum. However, in a ski turn it is quite small compared to the linear momentum. In a short turn it is smaller due to counter movements, and in a long turn it is small due to the slow rotation.

I'd advice from using the laws of conservation of angular momentum in a turn. It only holds in a closed system, so unless you include moving snow particles, chemical reactions in the muscles and the movement of the earth it is not valid. You can use it during a weightless transition though.

When we are close to the critical platform angle with normal skis, being statically forward displaces the fronts, moving from forward to aft through the engagement displaces the tails.

I didn't want to get into semantics, but technically what Ghost said about having angular momentum and linear momentum is wrong, and unfortunatetly Jamt ran away with it.  When you move around a circle and you want to try to determine the total energy, based on what Ghost said literally you would need to determine the rotational energy relating to the angular momentum and the kinetic energy relating to the linear momentum, add them together and unfortunately get the wrong answer.  The energy in the system can be described entirely by the angular momentum, so technically there isn't linear momentum too.  There's this equation that links linear momentum with angular momentum through the radius, but that linear momentum is a particular concept referring to the instantaneous momentum or the momentum before entering the turn.  Linear momentum is a vector with constant direction, if you change the direction it is no longer linear momentum.  When you look at the tangent to a circle and you take a small enough angle the slope of the circle is approximately flat, so for a small enough time, his momentum is approximately linear.  Then as you take the limit as the time goes to zero, the momentum is linear, so this is called the instantaneous linear momentum, which is the linear momentum only for an instant in time.  It's a construct to use tangential linear parameters to help understand rotation and relate it to parameters before and after the turn.  Once you are moving in a circle around a fixed point, you do not have any linear momentum.  All the energy in the system is described entirely by the angular momentum.  A skier turning around a circle can have angular momentum around the center of the turning circle or around the COM.  In Jack's example, the ski and lower leg pivot around a center point, so it has angular momentum, and applying a force to the mass closer to the pivot point will result in faster angular acceleration for the ski and lower leg.

Quote:
Originally Posted by jack97

car's mass is somewhat intact and is nearly impossible to move its mass to shift its axis of rotation. The more appropriate analogy is a (steerable) sled going downward on a slope with a man/woman sitting on it, where that person can lean to either side to shift its COM and move the axis of rotation.

That's the thing jack, you are not moving the axis of rotation, you are moving some of the mass at the radius, which could effect the moment of inertia. In a typical ski turn, the axis of rotation is a point in the snow at the center of the turn radius.

The ball and string anology to moving your CoM would be changing the length of the string.

It gets more complicated though because in skiing there is no string. At each instant our skis interact with the snow and provide external forces in myriad of ways depending upon how we manipulate them, which continuously diverts the direction of travel, much like the string does to a ball on string, but we are continuously able to manipulate that interaction.

If your skis are carving the axis of rotation is pretty far away from the body. If your tails washout the axis of rotation is probably right around the tips of the skis.
If you actually start to spin out of control the axis of rotation could be within the body of the skier itself.

Perhaps this is the distinction I am trying to convey. when the axis of rotation is within the skier's CoM then an actual moment of inertia will be formed within the skier itself, which equates to the skier spinning. When the axis of rotation is outside the skier, then the skier no longer has their own moment of inertia that can perpetuate spinning, the skier becomes like a ball on the end of a string which is only rotating and turning by virtue of continuous external forces making it happen. By this analysis makes the skier just a lump of mass in a larger rotational system, that is rotating around some theoretical center point by virtue of the ski to snow external forces.

I don't know what language will explain this distinction, but there is a distinction and it matters a lot in skiing. Maintaining yourself as a ball on the end of a string rather then becoming yourself the center of the circle is the difference between turning and spinning. And since there is no string, it's manipulated in real time by our edges. Turn off the edges and a non spinning skier will continue straight. if the axis is within the skier and they are spinning then turn off the edges and the skier keeps spinning. That is the distinction. I am not sure what is the correct physics terminology at this point according to you PhDs to note this distinction.
Quote:
Originally Posted by The Engineer

Quote:
Originally Posted by Jamt

Like Ghost explained with the car example, as long as a mass is turning there is angular momentum. However, in a ski turn it is quite small compared to the linear momentum. In a short turn it is smaller due to counter movements, and in a long turn it is small due to the slow rotation.

I'd advice from using the laws of conservation of angular momentum in a turn. It only holds in a closed system, so unless you include moving snow particles, chemical reactions in the muscles and the movement of the earth it is not valid. You can use it during a weightless transition though.

When we are close to the critical platform angle with normal skis, being statically forward displaces the fronts, moving from forward to aft through the engagement displaces the tails.

I didn't want to get into semantics, but technically what Ghost said about having angular momentum and linear momentum is wrong, and unfortunately Jamt ran away with it.

If a thing is rotating it has angular momentum.  If a thing has liner velocity it has linear momentum.  If it is both displacing and rotating it has both linear and angular momentum.  I hope you were not involved in any motor vehicular reconstructions.  Simple high school physics experiments show that conservation of energy must include both linear and angular velocity.    A car travel down a road at 70 mph in a normal linear fashion has less energy than a car spinning at one rotation per second while traveling down the road at 70 mph.  As Jamt correctly stated it is not that it is not there in a long or short curve, it is that it is minimal (for example car rotates from facing north to facing west in a long time so slow rotation).

It is correct that the angular momentum conservation equation involves applied torque to calculate angular accelerations and the linear momentum equation involves forces to calculate accelerations and both equations are separate vector equations not to be added to a common sum.   However, the Energy equation must include all energy.

Some people deserve a refund from their schools, I however have gotten my money's worth from my education.

Sorry, but I will not stand by when someone claims I am "wrong" and is either not fully cognizant of high school physics  or attempts to mislead with half-truths.

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