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A simple self-assessment test - Page 12

Quote:
Originally Posted by fatoldman

This 'bunch of ski instructors' includes, at least one phd in physics, several who are high level engineers in their real lives, and many who have gone to great lengths to make sure  that they understand what happening from a physics stand point not just 'how it feels' or to 'have an opinion'. The workings of Newtonian physics are fairly well understood and agreed on and when people make statements that go contrary to several centuries of understanding they are likely to get called on it.

fom

According to some of these guys I should hit the hardest bump run I can find near the end of the day to suck some energy back into my legs.

Quote:
Originally Posted by jack97

have them look at the vid i posted of angular momentum specifically the classic experiment of the spinning ice skater. That should convince everyone versed in newtonian physics that velocity can increase or decrease by moving the com.

just my opinion... most posters can't even get to this point.

Wrong again. The com of the ice skater doesn't move, it stays spinning in the center. Bringing the hands in lowers the amount of energy used in moving them. This energy then goes into creating a faster spin. I believe that this has to do with the way lever arms work.

fom

Quote:
Originally Posted by jack97

BTS may have acknowledge the works of Lind in which he proves thru angular momentum, moving the com does increase the velocity.

Per definition, increasing the velocity is acceleration. Just because something is moving does not mean it is accelerating.

Remember that velocity includes direction. If I am going straight down a flat slope and I have reached terminal velocity. If I then extend my legs the velocity increases as you say. However, that is because I have added a new component to the velocity. The speed towards the bottom of the hill has not changed.

The argument about angular momentum is about conserving energy. If I push I have added energy to the moving system so the system now contains more energy, in the form of rotation and linear velocity.

How do you as a skier convert the angular momentum into linear velocity?

Do you even think it is possible to increase/decrease the velocity without angular momentum. What about skating, where is the angular momentum?

jack

Angular Momentum does not really apply to a skier in a ski turn the way you are postulating.  The reason is because the radial sphere you are thinking about (the turn shape), is not a single rotating body; its not a closed system.

At any given instant a skier actually has momentum that is a straight vector; but is harvesting centripetal forces to send them on a curved path, with a radius that is actually constantly changing, but this larger radial sphere is not a single body that is rotating.  Its somewhat interesting to calculate angular velocity in terms of radians per second or whatever, but there is no rotating object with angular momentum, unless the skier is spinning as a single body.

Now if a skier begins to rotate themselves...then angular momentum is present because the skier body represents a closed system that now is rotating without being acted on by external forces.

For example, an ice skater making a turn does not really have any significant angular momentum.  If that ice skater goes into a spin, then the straight line momentum they have becomes converted into angular momentum and they are then standing in place spinning.  At that point they could move their hands in and out to slow down and speed up their angular velocity, but angular momentum is actually unchanged...it is preserved.

A skier in a ski turn is not a rotating body.  If a skier takes a jump and starts rotating in the air, then angular momentum will be present.  If a skier takes a crash and is cartwheeling, they'd have some angular momentum.  But the ski turn shape does not represent a closed system, or single rotating object with angular momentum.

Quote:
Originally Posted by fatoldman

Wrong again. The com of the ice skater doesn't move, it stays spinning in the center. Bringing the hands in lowers the amount of energy used in moving them. This energy then goes into creating a faster spin.

fom

wrong on the "wrong again".... in the spinning ice skater , the skater is the center of rotation, the prof was holding out the weights as the com. By bringing them closer or farther away he is emulating the movement of the com. He furthers shows how the angular velocity is increase or decreasing. And as I point out, angular velocity in the limit is equal to velocity.

btw..... you have proven my point.

Quote:
Originally Posted by borntoski683

jack

Angular Momentum does not really apply to a skier in a ski turn the way you are postulating.  The reason is because the radial sphere you are thinking about (the turn shape), is not a single rotating body; its not a closed system.

At any given instant a skier actually has momentum that is a straight vector; but is harvesting centripetal forces to send them on a curved path, with a radius that is actually constantly changing, but this larger radial sphere is not a single body that is rotating.  Its somewhat interesting to calculate angular velocity in terms of radians per second or whatever, but there is no rotating object with angular momentum, unless the skier is spinning as a single body.

Now if a skier begins to rotate themselves...then angular momentum is present because the skier body represents a closed system that now is rotating without being acted on by external forces.

For example, an ice skater making a turn does not really have any significant angular momentum.  If that ice skater goes into a spin, then the straight line momentum they have becomes converted into angular momentum and they are then standing in place spinning.  At that point they could move their hands in and out to slow down and speed up their angular velocity, but angular momentum is actually unchanged...it is preserved.

A skier in a ski turn is not a rotating body.  If a skier takes a jump and starts rotating in the air, then angular momentum will be present.  If a skier takes a crash and is cartwheeling, they'd have some angular momentum.  But the ski turn shape does not represent a closed system, or single rotating object with angular momentum.

it comes into play under some scenerios as Lind points out like the long S curves. And it does come into play on the mogul feilds where the troughs are deep.  In each case, you have to re orient where the center of rotation to see the physical principles.

BTW, think of how a cosine function unwraps itself as time increases. If time was not part of the coordinate system , the function would look like a circle wrapping onto itself. So by using this analogy, if you track along distance with a bumper you would see the bumper going up and down in a circular fashion. Bob B had a nice cartoon showing this but he used it show case the backpedaling concept.

Edited by jack97 - 9/5/13 at 9:17pm
Quote:
Originally Posted by jack97

wrong on the "wrong again".... in the spinning ice skater , the skater is the center of rotation, the prof was holding out the weights as the com. By bringing them closer or farther away he is emulating the movement of the com. He furthers shows how the angular velocity is increase or decreasing. And as I point out, angular velocity in the limit is equal to velocity.

btw..... you have proven my point.

This post demonstrates that you don't even understand what com (center of mass) means/is. When your misunderstanding is this fundamental problems will arise.

fom

PS If I remember correctly there was no mention of com in that video.

Edited by fatoldman - 9/5/13 at 9:21pm
Quote:
Originally Posted by jack97

it comes into play under some scenerios as Lind points out like the long S curves. And it does come into play on the mogul feilds where the troughs are deep.  In each case, you have to re orient where the center of rotation to see the physical principles.

I think you have missed the point of what I just said.  You can't reorient the center of rotation away from the actual object.  In that case, you can measure angular velocity as more of a curiosity, but there is no real angular momentum.  There is only linear momentum that is being constantly steered in a new direction that happens to represent a curve.  There is no rotating body.

Again, if a body is spinning, then it has angular momentum.  You can bring your hands in and out to make the spinning go faster or slower but the actual angular momentum would be preserved.  if you had to way to convert that angular momentum into linear momentum, the object would take off running like a slingshot.

Quote:
Originally Posted by jack97

wrong on the "wrong again".... in the spinning ice skater , the skater is the center of rotation, the prof was holding out the weights as the com. By bringing them closer or farther away he is emulating the movement of the com. He furthers shows how the angular velocity is increase or decreasing. And as I point out, angular velocity in the limit is equal to velocity.

btw..... you have proven my point.

The reason the skater rotates faster is twofold.

1. He changes the moment of inertia

2. He adds energy to the system by pulling his hands in. This is very minor.

Quote:
Originally Posted by Jamt

How do you as a skier convert the angular momentum into linear velocity?

Do you even think it is possible to increase/decrease the velocity without angular momentum. What about skating, where is the angular momentum?

I read a post long ago in this forum about a race coach where he was pumping turns and he beat out a student that was running it flat and straight. I have never seen it but the physic tells me it can be done, the pump bike track proves it. Even a physic prof who use to post here many years ago say it can be done (he's the guy who made calculations for proper dimension base on wieght to float the ski).

As for skating, it would hard to get circular motion so I doubt it can be done effectively.

Quote:
Originally Posted by fatoldman

This post demonstrates that you don't even understand what com (center of mass) means/is. When your misunderstanding is this fundamental problems will arise.

fom

PS If I remember correctly there was no mention of com in that video.

Its implied by the M=mass, hence the weight was what he was holding out.

Quote:
Originally Posted by Jamt

The reason the skater rotates faster is twofold.

1. He changes the moment of inertia

2. He adds energy to the system by pulling his hands in. This is very minor.

not quite the way it would be taught in universities.

Quote:
Originally Posted by jack97

I read a post long ago in this forum about a race coach where he was pumping turns and he beat out a student that was running it flat and straight.

He probably waxed his sidewalls with some fluoro paste .....

zenny

Quote:
Originally Posted by borntoski683

I think you have missed the point of what I just said.  You can't reorient the center of rotation away from the actual object.  In that case, you can measure angular velocity as more of a curiosity, but there is no real angular momentum.  There is only linear momentum that is being constantly steered in a new direction that happens to represent a curve.  There is no rotating body.

Again, if a body is spinning, then it has angular momentum.  You can bring your hands in and out to make the spinning go faster or slower but the actual angular momentum would be preserved.  if you had to way to convert that angular momentum into linear momentum, the object would take off running like a slingshot.

I think the problem is you like many are not seeing how to frame the scenerio where angular momentum is applied.

I have to get some sleep and do my real work... later guys.

Jamt,

Thank you. Moment of inertia is the concept that I was trying to explain but may have done a poor job of doing it nor could I recall the term hence my comment about thinking it had to do with how lever arms work. I didn't mean to imply that bringing the hands in would add energy to the system just that the energy in the system would be used to create a faster spin.

fom

Quote:
Originally Posted by jack97

I read a post long ago in this forum about a race coach where he was pumping turns and he beat out a student that was running it flat and straight. I have never seen it but the physic tells me it can be done, the pump bike track proves it. Even a physic prof who use to post here many years ago say it can be done (he's the guy who made calculations for proper dimension base on wieght to float the ski).

As for skating, it would hard to get circular motion so I doubt it can be done effectively.

Yes it can be done, it is not that difficult. I have not claimed otherwise. What I usually say is that you can pump speed on flat parts of the course but when it is steeper you need grip and turning ability, which you lose when pumping.

My comment about skating was to hint that there might be more similarities between skating and pumping for speed than a figure skater preserving angular momentum.

Quote:
Originally Posted by jack97

not quite the way it would be taught in universities.

Really, and how would it be taught?

This is just one of many pages you can find if you google: http://btc.montana.edu/olympics/physbio/biomechanics/cam02.html

Quote:
Originally Posted by borntoski683

He is very clear about stating the change is increase:

the name of the chapter is "Pumping to increase velocity"

He says the following things at various points:

Note he does not say anywhere that is possible to convert that kinetic energy back into the human reservoir of potential energy.

No word on slowing down, only about propulsion...like the pump track.

Pushes

When going through bumps and pumping them to speed up, an example scenario is stated with equations to back it up:

Last stuff in the section:

Nowhere in this chapter is slowing down mentioned, only increasing speed.  Its very clear that kinetic energy is added to create accelerations.  No mention of sucking kinetic energy back into your body somehow.

We can say that you can, however, create accelerations in the opposite direction to slow yourself down but that will require some kind of work output from your muscles to convert their stored potential energy into kinetic energy that accelerates in the opposite direction.   Pumping allows you to work against centrifugal forces to create that kinetic energy in the right direction for increasing speed.

Think this through some more and think about how your body can create more work to slow down by pushing against something.  The author did not discuss slowing down at all here.

Lots of info on kinetic, potential energy, lots of  info on muscle physiology. Some great physics, wish I had this stuff to read in college :)

Where does the elastic energy in our physiology fit in?

Quote:
Originally Posted by Jamt

The reason the skater rotates faster is twofold.

1. He changes the moment of inertia

2. He adds energy to the system by pulling his hands in. This is very minor.

Quote:
Originally Posted by jack97

not quite the way it would be taught in universities.

Quote:
Originally Posted by Jamt

Really, and how would it be taught?

This is just one of many pages you can find if you google: http://btc.montana.edu/olympics/physbio/biomechanics/cam02.html

I specifically state "not quite" b/c the movement of the hand is not minor, it is major since this the mass that has moved with respect to the center of rotation. Now if you meant "this is minor" with respect to the vid I posted where the prof show the same experiment b/c it shares the same concept then I would agree with you.

Edited by jack97 - 9/6/13 at 5:13am
Quote:

Lots of info on kinetic, potential energy, lots of  info on muscle physiology. Some great physics, wish I had this stuff to read in college :)

Where does the elastic energy in our physiology fit in?

Lind does not consider the physiology aspect of this, he clearly states that the analysis is based on physical principles. I believe that was done purposely so the latter can be studied independently by not having the other things muddy the analysis.

Quote:
Originally Posted by Jamt

.... and secondly as BTS already stated velocity does not change by moving the CoM. The velociy changes by accelerating the CoM.

Quote:
Originally Posted by Jamt

Yes it can be done, it is not that difficult. I have not claimed otherwise. What I usually say is that you can pump speed on flat parts of the course but when it is steeper you need grip and turning ability, which you lose when pumping.

Hmm... pumping to increase speed is still an increase in speed.

Quote:
Originally Posted by Jamt

My comment about skating was to hint that there might be more similarities between skating and pumping for speed than a figure skater preserving angular momentum.

I don't see it that way.

When a figure skater pulls her hands in, are they moving faster, or is she spinning faster because the hands are covering the same distance in the same amount of time?

Energy is conserved unless work is done on the system to increase or decrease it's energy.:

1/2 m V^2 + I w^2 + mgh = constant   (or  including work....delta( 1/2mV^2 + I w^2 + mgh) - Work done on the system = 0  )

If you increase angular velocity of a skier, you decrease his linear velocity.  To increase energy you must work, i.e. push on something with motion in the direction of the push.  Pushing in the opposite direction of the motion decreases total energy (and velocity), pushing in the same direction increases energy (and velocity).

I hope that's simple enough.

Oh, and btw at least one of that bunch of ski instructors isn't a ski instructor, but he is a professional engineer with a Ph.D. in civil engineering, and a B.Ed. with aphysics specialty.

Quote:
Originally Posted by MrGolfAnalogy

In the skate park, using flexion and extension of your legs to create or lose speed in transitions is a proven reality. The same techniques also work on the flats when turning. Most of the time it is used to gain speed, but can be used to dump it as well.

Do the same laws of physics change at the ski resort?

The huge benefit of pumping the transitions in the skate park is the way it seems to stabilize your balance. The very act seems to put you on rails with an almost gyroscopic effect. I am interested to know if these same techniques and benefits can be used on the slopes? It doesn't seem like many of the posters here even believe in the process, let alone understand it or utilize it.

It can be used in the ski resorts if you believe crud and bumps are part of the resorts. What most believe as absorbing to stabilize balance lose sight of the fact that speed is decrease by performing this action.

Quote:
Originally Posted by Ghost

When a figure skater pulls her hands in, are they moving faster, or is she spinning faster because the hands are covering the same distance in the same amount of time?

Energy is conserved unless work is done on the system to increase or decrease it's energy.:

1/2 m V^2 + I w^2 + mgh = constant   (or  including work....delta( 1/2mV^2 + I w^2 + mgh) - Work done on the system = 0  )

If you increase angular velocity of a skier, you decrease his linear velocity.  To increase energy you must work, i.e. push on something with motion in the direction of the push.  Pushing in the opposite direction of the motion decreases total energy (and velocity), pushing in the same direction increases energy (and velocity).

I hope that's simple enough.

Oh, and btw at least one of that bunch of ski instructors isn't a ski instructor, but he is a professional engineer with a Ph.D. in civil engineering, and a B.Ed. with aphysics specialty.

does prof engineer with phd in CE agree with concept of increasing or decreasing speed by moving com?

Edited by jack97 - 9/6/13 at 5:14am
Quote:
Originally Posted by MrGolfAnalogy

In the skate park, using flexion and extension of your legs to create or lose speed in transitions is a proven reality. The same techniques also work on the flats when turning. Most of the time it is used to gain speed, but can be used to dump it as well.

Do the same laws of physics change at the ski resort?

The huge benefit of pumping the transitions in the skate park is the way it seems to stabilize your balance. The very act seems to put you on rails with an almost gyroscopic effect. I am interested to know if these same techniques and benefits can be used on the slopes? It doesn't seem like many of the posters here even believe in the process, let alone understand it or utilize it.

Quote:
Originally Posted by jack97

It can be used in the ski resorts if you believe crud and bumps are part of the resorts. What most believe as absorbing to stabilize balance lose sight of the fact that speed is decrease by performing this action.

With the combo of getting the treatment from the posters here and trying to get of out the house.... I forgot other ski resort applications; if the groom trail has a small hill or small trough area, you can pump to launch yourself across it, much akin to how bumpers get "pop" at the kickers and how a ski jumper springs out of the tuck to launch themselves onto the bottom part of the course.

As mentioned in other post pumping to increase speed in the flat parts can be done, how it rates to running them straight is dependent on how flat is flat.

Quote:
Originally Posted by jack97

What most believe as absorbing to stabilize balance lose sight of the fact that speed is decrease by performing this action.

I really can't tell if you pumpers are just pulling our legs or seriously are that misguided about the physics.  Its been explained to you now half a dozen times in very thorough and clear language and there are plenty of places for you to research some more and see that perhaps your understanding of physics has been off.  Pretty crazy.  Is all of the bike pumping and skating community also believing this nonsense?

But anyway, nobody has questioned whether its possible to propel yourself with pumping action, ie, work, I don't know why you keep coming back to that as if nobody believes you.

But slowing yourself with anti-pumping is what led us to this nutty debate which is kind of going in circles.  Your basic premis for believing that is possible is not based on real physics, thus you are not quite understanding what is going on if and when you are able to slow yourself on a pump track.  Hey does it really matter if you don't understand the correct physics exactly?  Probably not, whatever works.  Unless you try to translate your false physics to other situations such as in a ski turn or on a flat catrack, or while instructing someone, wow.  The above statement is where you are off.

Quote:

Lots of info on kinetic, potential energy, lots of  info on muscle physiology. Some great physics, wish I had this stuff to read in college :)

Where does the elastic energy in our physiology fit in?

The elastic properties of your body structure are very very minor in comparison to the momentum, speeds and forces we are generating while skiing.  You are correct in stating that some relatively tiny amount of kinetic energy will be converted to potential energy in a stretched out ligament or bone that is perhaps bending to the point of not yet breaking; which will immediately return back to neutral soon after, releasing that potential energy again...but these aspects of our physiology are very minor compared to the greater picture of making ski turns....and furthermore if you ski around using the elastic properties of your body as primary means of speed control, I can assure you that you are going to end up with an injury sooner or later.

Quote:
Originally Posted by jack97

With the combo of getting the treatment from the posters here and trying to get of out the house.... I forgot other ski resort applications; if the groom trail has a small hill or small trough area, you can pump to launch yourself across it...

Gullies and ditches, inbounds, can also be good places to use this, both on the walls in general and then on little terrain variations within them.  Some resorts have more of these than others, but Snowbasin and Jackson, to take two examples, have terrain in abundance where this is very relevant...and where taking some of the advice on here that's basically 180 degrees off, such as "Coming in prewound and extending on the 'uphill' side slows you down ..."  can put you in situations you really don't want to be in.

I think, Jack, that you're still trying to communicate on a technical level, and it's clear to me that unless a poster who is part of a social "inner circle" on here makes those points, they won't be well-received.  Look for people on here to start talking about sucking up speed in maybe 2015 or so, though, and talking about how it's been routine and accepted for a long time.  For passive readers, the point should be very careful about where you get your bump advice, and also add absorption and pumping to a ticklist of good things to be able to do.

Slarving is another good one more in line with the original skidding/drifting bike discussion, btw.

Quote:
Originally Posted by CTKook

.and where taking some of the advice on here that's basically 180 degrees off, such as "Coming in prewound and extending on the 'uphill' side slows you down ..."  can put you in situations you really don't want to be in.

That's a bit of a mis-representation CTKook.  You're locked into the notion that the only way to resist is to extend and that flexing always means releasing.  We have stated numerous times that flexing to absorb an obstacle, while using eccentric resisting to create pressure, is a way to slow yourself down on the tops of bumps, and that would apply to any other fun 3D terrain you can find.

Quote:
Originally Posted by borntoski683

That's a bit of a mis-representation CTKook.  You're locked into the notion that the only way to resist is to extend and that flexing always means releasing.  We have stated numerous times that flexing to absorb an obstacle, while using eccentric resisting to create pressure, is a way to slow yourself down on the tops of bumps, and that would apply to any other fun 3D terrain you can find.

It was an exact quote, using the cut and paste function, from one of the posters going on about how I don't understand physics.  And he was clearly stating that coming in pre-wound and then extending AFTER the trough slows you down.  It is hard to see how that could be a misrepresentation.

But, to not just pick on one poster, another's just said that it's not POSSIBLE to come  in flexed and then extend, "In practice it is not possible to start flexed and extend into the face of the bump, because you are not strong enough. The maximum force you can produce is then eccentric resistance when you hit the bump. It may look like sucking up, but it is not."  Another demonstrably false statement, even if you qualify it to say you mean skiing at higher speeds, which he did then do.

There is a reason why these things don't hold up, are all over the place, and often at odds with one another.

That's even leaving out the fact that all this focus on "pressure" slowing you down ignores the fact that the physical motions of pumping involve more than just absorption and extension, and when you add all the driven types of pumping or reverse pumping, getting distracted by pressure makes even less sense.

there have been a number of hypothetical situations presented in this thread, including the ones you are quoting, for the sake of making points about physics.  But still you are entirely misrepresenting the opposing view of your own.  Nobody in here seriously suggested that the best way to ski bumps is to extend into the face of the bump to slow down.   What I just said in my last post is the position of just about everyone opposing you.  What do you have to say about that?

What this really comes down to is that you feel you can slow yourself down by changing your moment of inertia in terms of angular momentum.  Your opposers feel that you need to embrace external forces, through pressure, to slow yourself down.  Your opposers also say that you have an incorrect understanding of angular momentum and how it exists or doesn't exist in skiing.  we have tried numerous times to explain it to you and you have responded with various insults and red herring arguments such as the one above.  Maybe we can give it a rest now?

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