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# Accelerating out of a turn - Page 7

Quote:
Originally Posted by majortato

I didn't say same speed did I?  I just said same line.  Gravity works the same on both skiers...it doesn't realize one has a longer ski and will therefore pull that person harder.  I think we both agree that the skier with the bigger sidecut needs to go faster somehow to make those turns.  Gravity provides the same forces on both skiers...this means the skier with bigger sidecut skis is adding energy into the system with the use of muscle movements.

Ok...stop here.  Explain your logic here.  That is quiet a leap you just made.

My version of reality is this:

Replacing your bold with:     this means the two skiers will travel at the same speed, meaning they will need to inclinate the same amount...however when the skier on the smaller sidecut skis does this, he will naturally be on a different arc to the skier on the longer sidecut skis.  Hence they cannot ski the same line.

The only way they could ski the same line, was if the skier on the smaller sidecut skis was artificially slowed some how, like perhaps with a small parachute.

Quote:
Originally Posted by majortato

If you've read my previous posts..I've said multiple times that this force might not accelerate the skier out of the turn.  It might just be accelerating the skis out of the turn and across the hill while the skier is unweighted.

The skis are part of the mass, if the skis are accelerating you are affecting the total mass as well. You have also said " The ski rebounds with energy coming out of the turn.  How you use that energy determined by your skill level and ultimate goal.  You could use it to send your skis forward with more speed, or send your COM forward and down the hill to setup the next turn, or dissipate it in an up move, or absorb it in a retraction move."

Which is why fischer put a big hole in their racing skis at the tip?  And no other company has followed suit after 3 years.  I don't buy that the ski tip does anything substantial other than help prevent the skis from digging into the snow.  I think the ski tip is just a remnant of classic ski design.  It doesn't hurt to have it there..so why remove it?  Racing course might be complete ice, but the stuff around it is probably fluff.  Imagine if you skied out of the course and hit the soft stuff, then flip over your skis because you have no tip and your skis dig in....pretty dangerous situation.

We are not talking about the same thing. Of course I am not talking about the tip that extends significantly above the snow. I am talking about the front part of the ski. This is what tip rocker is all about. This means that the front part of the ski is not in contact with a flat surface when the ski is not edged. When it is edged enough it engages. The height of the tip is mandated by FIS equipment rules, just like many other things. Fischer has IPR for the hole, that is why you don¨t see it in other skis.

I never claimed you push muscularly after the fall line...please re-read what I wrote.  I agree that we apply force above the fall line...it is that application of the force above the fall line that allows us to accelerate after the fall line.

No, the added acceleration happens when you push, i.e. before the fall line. Just like when you are skating the acceleration happens when you push, not when you are gliding away on the other ski.

Why does the total energy have to be the same?  There's no reason for this restriction.  IF the total energy is the same..then your argument makes sense.  But in the case of the ski with higher potential energy, the skier needs to work harder to bend that ski.  This means the total energy in the system is higher for the ski with higher potential energy.

You are assuming that the forces from the skier, weight+ centripetal are not enough to press the "spring" to the hard ground. That if you press more you load the skis with more energy. This is not the case, you are just pressing harder into the ground, unless you have a ski with really, really strong camber. This is why I assumed that the skier used the same pushing. If you did the carpet experiment described earlier you would see that it does not take very much force. I did this experiment with my race department GS skis that were on the europa cup last year, and it did not take that much force.

I pretty much agree with this completely except "energy stored makes the average speed slower."  The storing of energy is the result of a lot of things at play.  Reducing average speed to build this energy is not one of them.  So the last statement...a "certain" amount of increased speed is possible at the end of the turn.  I think this is NOT negligible.

Edited by Jamt - 2/19/13 at 2:52pm
Quote:
No, the added acceleration happens when you push, i.e. before the fall line. Just like when you are skating the acceleration happens when you push, not when you are gliding away on the other ski.

When you push a ball against a spring, does the ball accelerate (in regards to speed) when you push, or when the spring unloads and sends the ball flying?

Quote:
You are assuming that the forces from the skier, weight+ centripetal are not enough to press the "spring" to the hard ground. That if you press more you load the skis with more energy. This is not the case, you are just pressing harder into the ground, unless you have a ski with really, really strong camber. This is why I assumed that the skier used the same pushing. If you did the carpet experiment described earlier you would see that it does not take very much force. I did this experiment with my race department GS skis that were on the europa cup last year, and it did not take that much force.

This might be case of semantics.  I agree the skier, weight, and centripetal force is enough to compress the "spring".  "pressing" is really just bracing against the forces and pressing into the ground to brace against the centrifugal forces generated.  Higher edge angles also means "pressing" harder to bend the ski into a smaller arc.

The carpet test shows that at low edge angles, it doesn't take much to get it to touch the ground.  At high edge angles, I have to bend the ski quite a bit to get the full edge to engage....it takes a decent amount of muscle.  Note the rebound force of the ski is not linear.  Every cm of extra bend adds exponentially to the energy stored until it hits a ceiling.

I want to clarify a bit, because I feel like you think that I believe a skier can "push" on the ski harder to generate speed.  I don't really believe this.  Most of the "pushing" that our legs do is to dictate the turn shape and direction of travel and to stay in balance.  The ski will bend as a result - a side effect of the turning movements.  Pushing harder at higher edge angles will result in a tighter turn radius and could result in more energy loaded into the ski...but this is not the reason we "push"....we push because we have to tighten the turn.  I believe that a good skier skiing with proper dynamics will be able to manage the natural rebound of the ski in such a way so that the rebound energy can be used positively rather than negatively or wasted.  And I believe that this rebound energy is not negligible.

A skier is not a passive lump along for the ride.

The skier can choose to add energy to the system by extending under load.

This adds energy to what is a non conservative system (the skier is losing energy from drag and gaining energy from the grade).

It's pretty complicated to analyze.

I remember hearing of a demo where Killy got to the bottom of a hill faster in gates than a schusser.

This discussion has gotten muddled.  Let's clean the slate and simplify.  Let's divide up the skier's velocity to its horizontal and vertical components.  Horizontal being across the hill and vertical being down the fall line.  Now let's forget about the vertical component and focus on horizontal.  How does it look?

Well the skier starts off going in the positive x direction.  He decelerates to 0, then accelerates in the negative x direction until crossover point...then he starts decelerating again until 0 before switching directions to the positive x direction again.

Can we agree that the faster he can go from positive, to negative, back to positive again, then faster he will be overall?

So how can he be faster?  If he decelerates and accelerates faster, he can reduce the amount of time where his speed is slow and increase the maximum speed he reaches at transition.  Most of this needs to be achieved through leg strength.

Does the ski play a role?  At fall line, the skis rebound direction is on the x axis.  If this energy is useless or not used, then the acceleration of the skier is completely dependent on leg strength.  If this energy IS used and not negligible, then it will help accelerate the skier in the x direction.

Another way to look at this is a person bouncing back and forth between two trampoline walls.  The trampoline has a spring effect.  The person can either choose to take advantage of this trampoline effect, or let it go to waste by absorbing the trampoline effect.  When jumping on a trampoline..I can either go higher with each jump, or reduce height with each jump.  It's all up to me and how I divert that spring energy.

Majortato...I once argued that allowing one's Bos to move further ahead of the CoM than usual whilst exiting a turn and moving towards transition (finishing further aft) would increase as skiers speed. Can you guess what my reasoning was?

No peeking at the thread in question, either...

zenny

Edited by zentune - 2/19/13 at 7:10pm

you said "once" so my guess is you don't believe this anymore.  Then my guess is you figured the ski would pull the body forward if it shot forward.  Of course, this all depends on how the ski went forward in the first place.  If it went forward because you moved your COM back, then you have done nothing to increase speed.  Newton's 3rd law.

However, if the ski went forward (across the hill) due to other forces (such as ski rebound) and you failed to have your COM keep up, then it can pull your COM with it.  But really, you don't want that either...you should have anticipated it and kept your COM moving through the shortest line down the hill.

With that said, there are reasons to put your COM back at the end of the turn.  One thing is to pressure the tails of the skis hard and allow you to carve a tighter turn with the tails of the skis.

Oh I still believe it. I just meant I argued it in a thread one time only (once). Keep guessing.

zenny

Quote:
Originally Posted by majortato

This discussion has gotten muddled.  Let's clean the slate and simplify.  Let's divide up the skier's velocity to its horizontal and vertical components.  Horizontal being across the hill and vertical being down the fall line.  Now let's forget about the vertical component and focus on horizontal.  How does it look?

Well the skier starts off going in the positive x direction.  He decelerates to 0, then accelerates in the negative x direction until crossover point...then he starts decelerating again until 0 before switching directions to the positive x direction again.

Can we agree that the faster he can go from positive, to negative, back to positive again, then faster he will be overall?

So how can he be faster?  If he decelerates and accelerates faster, he can reduce the amount of time where his speed is slow and increase the maximum speed he reaches at transition.  Most of this needs to be achieved through leg strength.

Does the ski play a role?  At fall line, the skis rebound direction is on the x axis.  If this energy is useless or not used, then the acceleration of the skier is completely dependent on leg strength.  If this energy IS used and not negligible, then it will help accelerate the skier in the x direction.

Another way to look at this is a person bouncing back and forth between two trampoline walls.  The trampoline has a spring effect.  The person can either choose to take advantage of this trampoline effect, or let it go to waste by absorbing the trampoline effect.  When jumping on a trampoline..I can either go higher with each jump, or reduce height with each jump.  It's all up to me and how I divert that spring energy.

No.

You are making assumptions that are not valid.

On a trampoline we only move on say the X-axis...our speed goes from 0 to some value Y.  Correspondingly our kinetic energy goes from some value X to 0, as our potential energy does the opposite.

When we ski, we cant just look at the x-axis...becuase at the point our speed in the X direction is 0, our skis are in the fall-line, and our speed in Y-direction is now at its maximum...and correspondingly our kinetic energy is staying constant, its not getting transferred into potential as you believe....at least not 100% kentic to 100% potential like a trampoline.  Some energy is lost to bend the ski, it has to...but it comes back...so where is the net gain you claim?

As for we can "push" to create tigher turns?

We resist forces with our legs...that is about it.  Try standing on a bathroom scale - lets say you weight 200lbs...can you push with your legs so the scale reads 220lbs?  No.

Have you expained my old school mountain bike riddle yet?

majortato, the answer is quite simple, really. i'm not gonna say just yet, though (it's not a trick question either!). i DO think it has relevence for this discussion

hint: remember as we move out of the fall line and towards transition, we begin to lessen our edge angles (usually)...

zenny
Quote:
Originally Posted by majortato

This discussion has gotten muddled.  Let's clean the slate and simplify.  Let's divide up the skier's velocity to its horizontal and vertical components.  Horizontal being across the hill and vertical being down the fall line.  Now let's forget about the vertical component and focus on horizontal.  How does it look?

Well the skier starts off going in the positive x direction.  He decelerates to 0, then accelerates in the negative x direction until crossover point...then he starts decelerating again until 0 before switching directions to the positive x direction again.

Can we agree that the faster he can go from positive, to negative, back to positive again, then faster he will be overall?

So how can he be faster?  If he decelerates and accelerates faster, he can reduce the amount of time where his speed is slow and increase the maximum speed he reaches at transition.  Most of this needs to be achieved through leg strength.

Does the ski play a role?  At fall line, the skis rebound direction is on the x axis.  If this energy is useless or not used, then the acceleration of the skier is completely dependent on leg strength.  If this energy IS used and not negligible, then it will help accelerate the skier in the x direction.

Another way to look at this is a person bouncing back and forth between two trampoline walls.  The trampoline has a spring effect.  The person can either choose to take advantage of this trampoline effect, or let it go to waste by absorbing the trampoline effect.  When jumping on a trampoline..I can either go higher with each jump, or reduce height with each jump.  It's all up to me and how I divert that spring energy.

As I said you can increase the speed by pushing when the CoM and skis have diverging paths. Theoretically you can do this at any point in the turn. For maximum effect you can do this at the apex, only problem is you are pushing against 2-4G on one leg. You are also pushing the CoM upwards which means you are pushing to release. It was a very long time ago, in the straight ski era that something like this worked. So you are left with pushing before or after the apex. After the apex has a lot of problems for obvious reasons. So how about pushing above the apex. As I said a long time ago in this thread it works when you are going slow. If you are going faster you are pushing the CoM upwards and you will not reach high edge angles. You are rushing the turn.

If you are going to push you'd better know how and under what circumstances it is useful. A lot of recent MA is full of turn rushing leading into park and ride.

There is nothing like acceleration only in the X direction. The acceleration from the skis is perpendicular to the the skis. Some of the energy is released in the negative x direction (good) and some in the negative y direction (bad). If you release completely in the fall line you are going straight down.

I argued before that if you have the same muscular input it is slower to have a more powerful spring. The only argument you have that could do something for us is if you can pump more energy into the system when you have a stiff spring compared to when you don't. That is where you should focus if you want to convince us that it could work, and note that it needs to be significantly more since you are on the loosing side when you pump the same amount of energy into the system. To be convincing you need to describe power input, output and movements in every part of the turn, no instantaneous effects.

Actually, how about this argument. To increase speed the force should be added when the CoM and skis are diverging. In the simplified example with two springs I have before the paths were more or less identical. However in reality for a modern carving turn the paths are converging after the fall-line. Then if this theory is true, how can the speed increase when the paths are converging. Trick question eh?

You are using the trampoline example again, and my answer is the same. What if the trampoline is mounted 1 inch above snow? Also I don't like this example because there are so many more degrees of freedom in skiing.

Edited by Jamt - 2/19/13 at 11:54pm
Quote:
When we ski, we cant just look at the x-axis...becuase at the point our speed in the X direction is 0, our skis are in the fall-line, and our speed in Y-direction is now at its maximum...and correspondingly our kinetic energy is staying constant, its not getting transferred into potential as you believe....at least not 100% kentic to 100% potential like a trampoline.  Some energy is lost to bend the ski, it has to...but it comes back...so where is the net gain you claim?

I was hoping to make things simple by focusing on the x-axis.  If we can increase speed in either component, then we can increase speed overall.  With discussions around ski rebound, I wanted to focus on the x-axis because that's where most of it happens.  There's no need to bring in the Y-axis into the discussion.  I know its not 100% transfer...doesn't need to be.

Quote:

As for we can "push" to create tigher turns?

We resist forces with our legs...that is about it.  Try standing on a bathroom scale - lets say you weight 200lbs...can you push with your legs so the scale reads 220lbs?  No.

semantics.  That's why I said "push" in quotes.  I'm talking about resisting forces.  It feels like a push with your legs.  And btw, you can make the scale temporarily read higher than your weight if you pushed off it.

Quote:
Have you expained my old school mountain bike riddle yet?

A bike turns with combination of tipping and the front wheel being able to steer...it's very different from a ski.  In any case, you could get chucked anywhere in the turn depending on what you did wrong.  Not sure where you're going with it, but I think it just adds complexity to the discussion as we have to take into account complicated bike mechanics as well.

Quote:
Oh I still believe it. I just meant I argued it in a thread one time only (once). Keep guessing.

zenny

Zenny, I'm confused...I thought you're arguing that you don't believe you can accelerate out of a turn.  But you're asking why a person might put their COM behind their BOS to generate more speed?

Quote:
There is nothing like acceleration only in the X direction. The acceleration from the skis is perpendicular to the the skis. Some of the energy is released in the negative x direction (good) and some in the negative y direction (bad). If you release completely in the fall line you are going straight down.

I know...I'm not talking about accelerating only in the x-direction...I'm talking about looking only at the x component of the acceleration force.

ok lets try the turn phases and power inputs/outputs....still somewhat simplified because listing everything would be incredibly complicated.

Before we begin though, I just want to make sure everyone understands exactly what centripetal force is (I know at least Jamt does), but in case you missed out on HS physics class.  Centripetal force is a force that acts perpendicular to the direction of travel, always inwards towards the center of a circle.  Think earth going around the sun...the sun's gravity is the centripetal force that's always towards the sun.  The earth orbits but never falls in because the earth has momentum, even though the force is always directly in the direction towards the sun.  It is a force that turns us without slowing us down or speeding us up in a turn.

• phase 1 - transition to right before fall line:  movements: legs extend to cause divergence of COM and skis.  The COM is also pushed in a direction down the fall line as well (upside down position).  power inputs: leg muscles, gravity.  power output: skis dig into snow and centripetal force is created to turn the skis.  Skis start bending as a result.  Gravity contributes to some speed increase and helping accelerate the COM down the hill.
• phase 2 - fall line: movements: drop COM inside of turn to stay balanced against forces generated as a result of the turn.  power input: leg muscles, gravity.  power output: skis against snow creates centripetal forces to turn the ski/skier.  Skis reach max bend as a result.  Gravity has most effect on forward momentum of skis and skier.
• phase 3 - after fall line to transition:  movements: legs relax to allow convergence of COM and skis. The retraction move isn't really a muscle move...it's the result of allowing the convergence of COM and ski to happen as the COM passes over the BOS.  Power inputs: gravity, ski rebound.  Power output:  Skis still on edge creates centripetal force that keeps the ski turning.  Meanwhile, the leg muscles have relaxed so the centripetal force acting on the COM goes away.  This allows the COM to exit the old turn and enter the new turn.  However! there's the case of the ski rebound.  It is in the direction of the centripetal force (perpendicular to direction of travel).  This force can be used to help the skis finish the turn while the COM starts moving into the new turn.

What's funny is after this exercise, it's even more clear that the direction of the rebound force being perpendicular to the ski is actually exactly what we need.  We need it to be a centripetal force so that it can continue turning the skis even after we have reduced the centripetal force created by the leg muscles.  It doesn't actually matter where in the turn this release happens..even if it's across the hill.  Additionally, it actually doesn't matter if this force is released quickly or slowly because it generally adds on to the centripetal force to turn the ski after the leg muscles have stopped pushing the skis against the snow.

So why is it faster? cause it allows us to project the COM into the new turn slightly earlier than we would be able to otherwise.  I don't need to wait the extra split second for the skis to finish its turn before projecting my COM..I can start projecting the COM and let the rebound of the ski help create the centripetal force to finish the ski's turn.

So I'm moving away from how much it speeds up the ski...rather it helps turn the ski and allows convergence to be quicker.  The end result is faster!

majortato, i suspect the reason for your confusion regarding my question to you stems from your belief that one can store energy in the ski and then later release it...thereby making one faster can. since I have actually been contending thus whole time that any type of "rebound" (a word you're still using...did you read the VB thread??) energy us negligible, so when i mentioned a speed increase, you naturally assume ive contradicted myself.

have you ever heard the phrase "turn shape for speed control", the idea (roughly) that if a skier finishes more perpendicular to the fall line, the slower they'll be moving than if they finish more towards the fall line?

the answer to my question involves the latter. if we lessen tip pressure sooner whle rolling flat (skis) we "reduce the skis self steering effect" as Skidude so aptly said to me once...there by moving with the skis more towards the fall line than usual, and thereby being more subject to the pull of gravity.

now which participant in this had the greater role? the energy stored in the skis ? or gravity. dont bother with equations on this, as i'm dumb;)

incidentally, i feel that it is this "not finishing our turns" idea which causes many runaway skiers to feel that they are deriving a lot of acceleration from their insufficiently arced skis (some, not all...great skiers experience rebound and jet as well).

respectfully

zenny
btw, i was in no way suggesting you, majortato, or anyone else here was incapable of good turn shape

zenny

Zenny,

I think we can all agree that gravity and turn shape has much bigger role in dictating the speed of a skier than anything else.

If you read my latest post carefully, the conclusion can be summarized with the statement "ski rebound used properly can allow us to be faster, but it's not what makes us faster."

There is a subtle, yet key difference.  It's the idea that the energy in the ski can aid in the timing of our movements to be faster.  The energy might not be much, but it is there, it does help, and it is significant enough to matter in a sport where hundredth of seconds count.

Quote:
Originally Posted by Jamt

This acceleration out of the turn is the preservation of angular moment I mentioned before.
So this is essentially to say that we slingshot out of the turn?
Quote:
Originally Posted by majortato

I was hoping to make things simple by focusing on the x-axis.  If we can increase speed in either component, then we can increase speed overall.  With discussions around ski rebound, I wanted to focus on the x-axis because that's where most of it happens.  There's no need to bring in the Y-axis into the discussion.  I know its not 100% transfer...doesn't need to be.

Not true. An object moving at a constant speed in a circle will have its x- and y-axis speed oscillating 90 degrees out of phase.

IMHO, the simplest way to look at any possible change in speed between the turn entry (top of the turn) and exit (bottom) as affected by ski rebounding is to look at energy. Assuming there is no muscular pumping, energy conservation dictates that the maximum increase in speed at the bottom of the turn is g*h/(average speed) where h is the drop in elevation, completely independent of the skis. Ski can cause you to go slower, but not faster (I think SkiDude already said something like this). As you load the ski, you are converting your own kinetic energy into the potential energy of the bent ski. The more the ski bends, the slower, relative to a non-bending ski, you must become. When the ski "rebounds", the best you can do is converting all that stored potential energy into kinetic energy which will speed you up, but the total change in speed is till limited by the expression above. Now, how much you slow down and the speed up due to the bending of the ski is dictated by how much energy is stored in a bent ski. I think it has been well demonstrated here that it's not very much.

That's the best anyone can do (ignoring pumping). Intermediates like me dissipate our kinetic energy in friction, scrapping the snow, drifting, etc... as we turn and are a lot slower.

Under the right feet, high performance skis allow turns to be made with minimum loss of energy. They cannot create or add speed.

As an aside, the centripetal force is provided by the snow/ski contact for the most part and some from the bending of the ski. When you completely relax or retract the legs, the centripetal force is zero. Your "rebound force" is called the restoring force in physics lingo. That force is not acting on the ski like a centripetal force to keep the ski turning like you said. In this case, both skier and skis fly off in a tangent when the turn is released suddenly.

Edited by ChuckT - 2/20/13 at 5:50pm

Hmm. I haven't had the patience to read the entire thread for which I apologize but I can imagine a person standing on an icy surface having his feet suddenly fly out from under him. Isn't that acceleration? What if he is standing on skis on a slick surface? Perhaps he is already moving when his skis suddenly jet forward. That isn't acceleration of the entire body though, just a part of it with respect to the rest. What about a muscular contribution to speed? We are certainly familiar with the acceleration imparted to a x country skier by his diagonal stride. Why is it not probable that some kind of forcible muscular extension might be imparted within a turn to create a similar acceleration, however slight? What if much of the energy that is absorbed by friction in a turn is suddenly released as the skis flatten and the direction of travel straightens? Wouldn't that lead to a momentary burst of acceleration?

I tend to side though with those who maintain that this is negligeable. I think that often we get the impression that a person accelerates out of a turn simply because his apparent speed is so much more noticeable when he/she suddenly moves across your direction of view. The speed of the skier in the turn as he was moving towards you more or less enfilade in the turn seems to be somewhat less even when  the speed is more or less constant.

I hope this helps and I am sorry if this is ground that has already been covered.

Quote:
Originally Posted by oisin

Hmm. I haven't had the patience to read the entire thread for which I apologize but I can imagine a person standing on an icy surface having his feet suddenly fly out from under him. Isn't that acceleration? What if he is standing on skis on a slick surface? Perhaps he is already moving when his skis suddenly jet forward. That isn't acceleration of the entire body though, just a part of it with respect to the rest.

That's correct, not accelerating the person as a whole.

What about a muscular contribution to speed? We are certainly familiar with the acceleration imparted to a x country skier by his diagonal stride. Why is it not probable that some kind of forcible muscular extension might be imparted within a turn to create a similar acceleration, however slight?

Certainly any muscular force in the right direction can add to overall mechanical energy, i.e. increase speed like what racers do at the start of a race.

What if much of the energy that is absorbed by friction in a turn is suddenly released as the skis flatten and the direction of travel straightens? Wouldn't that lead to a momentary burst of acceleration?

Unfortunately, what is absorbed by friction is completely lost, cannot be "released" or converted to mechanical energy  (Some folks like to mention the "2nd law of thermodynamics" in discussions like this)

I tend to side though with those who maintain that this is negligeable. I think that often we get the impression that a person accelerates out of a turn simply because his apparent speed is so much more noticeable when he/she suddenly moves across your direction of view. The speed of the skier in the turn as he was moving towards you more or less enfilade in the turn seems to be somewhat less even when  the speed is more or less constant.

I hope this helps and I am sorry if this is ground that has already been covered.

Quote:
Originally Posted by majortato

Zenny,

I think we can all agree that gravity and turn shape has much bigger role in dictating the speed of a skier than anything else.  With all due respect possible, majortato, I have to tell you a secret. I posed my question to you because I strongly suspected that you wouldn't guess correctly. I suppose I fibbed a bit when I said it wasn't a trick question (but it WAS an easy one).

The reason I felt you wouldn't get it was that you were so focused on the idea that energy from the ski is able to help us be faster that you would over look the obvious...which you have just stated above. Good Job

If you read my latest post carefully, the conclusion can be summarized with the statement "ski rebound used properly can allow us to be faster, but it's not what makes us faster."

There is a subtle, yet key difference.  It's the idea that the energy in the ski can aid in the timing of our movements to be faster.  The energy might not be much, but it is there, it does help, and it is significant enough to matter in a sport where hundredth of seconds count.  So many more variables go into it than this miniscule effect. We've covered a lot of the proper movements/tactics, but how about snow type? Angle(s) of slope? Wind resistance? Waxing? Conditions of skis? etc...why get so hung up on this tiny detail, when there's so much more to it?

P.s., Am I the only non-Engineer/Physicist here?

zenny

Quote:
Originally Posted by ChuckT

"Unfortunately, what is absorbed by friction is completely lost, cannot be "released" or converted to mechanical energy  (Some folks like to mention the "2nd law of thermodynamics" in discussions like this"

You're right. I think I was referring to whatever forces are acting on the skier to produce his speed, some of which is robbed by increased friction in the turn. When the friction eases the same force and momentum ought to produce an increase in speed. The acceleration in this context would be produced by the (constant) force of gravity though and not by the turn. Nonetheless the skier would seem to accelerate out of the turn.

Quote:
Originally Posted by zentune

P.s., Am I the only non-Engineer/Physicist here?

zenny

"here is a subtle, yet key difference.  It's the idea that the energy in the ski can aid in the timing of our movements to be faster.  The energy might not be much, but it is there, it does help, and it is significant enough to matter in a sport where hundredth of seconds count. "

Wouldn't that stored energy (in the ski) have to first be placed in the ski by a force (presumably downward) that would produce an accompanying increase in friction that would cause a reduction in speed? I'm not saying the release of this small amount of stored energy might not be useful at  some point in time, I'm just saying that it might be a net loss.

Quote:
Originally Posted by oisin

"here is a subtle, yet key difference.  It's the idea that the energy in the ski can aid in the timing of our movements to be faster.  The energy might not be much, but it is there, it does help, and it is significant enough to matter in a sport where hundredth of seconds count. "

Wouldn't that stored energy (in the ski) have to first be placed in the ski by a force (presumably downward) that would produce an accompanying increase in friction that would cause a reduction in speed? I'm not saying the release of this small amount of stored energy might not be useful at  some point in time, I'm just saying that it might be a net loss.

Your replying to majortato, not me. FYI  I'm sure you know this

zenny

Quote:
Originally Posted by zentune

Your replying to majortato, not me. FYI  I'm sure you know this

zenny

Sorry, I just hit the "quote" button without looking carefully at the quote's origins.

You're not the only  non-engineer/physicist here.

Quote:
Originally Posted by oisin

You're not the only  non-engineer/physicist here.

Thank goodness!

zenny

Quote:
Wouldn't that stored energy (in the ski) have to first be placed in the ski by a force (presumably downward) that would produce an accompanying increase in friction that would cause a reduction in speed? I'm not saying the release of this small amount of stored energy might not be useful at  some point in time, I'm just saying that it might be a net loss.

When skiing, we are actually putting a lot of energy into the system that end up getting dissipated through friction and all kinds of stuff aside from accelerating our skis/bodies.  Really, the whole system is a big net loss.  The trick is to get what you can out of it, where you can and convert as much potential energy as you can into kinetic energy.

Depending on the end result you wish to achieve, storing energy might hurt you one way, but result in something positive another way.  In the case of skiing, it's possible that the energy spent to bend the skis comes straight from burning calories, and the resulting energy can be used to help us be faster.  Or maybe the energy spent does slow the ski down in some way...but the resulting energy can be used to assist in certain motions and movements that we can benefit even more greatly from with the goal of getting through a race course as fast as possible.

Quote:
So many more variables go into it than this miniscule effect. We've covered a lot of the proper movements/tactics, but how about snow type? Angle(s) of slope? Wind resistance? Waxing? Conditions of skis? etc...why get so hung up on this tiny detail, when there's so much more to it?

Cause this is the discussion we're having and it's making the time go faster before the next time I get to go skiing.

There's a lot of things to consider in skiing.  All those things you mentioned affects things a little..but that doesn't make them insignificant or non existent.  A few tenth here and a few tenth there...pretty soon you're talking seconds difference.

Also, I still don't think the effect is that miniscule.  Small relative to the major forces acting on us during skiing like gravity.  But probably about as significant as having a good tune on your skis.

very well then, majortato. since you wish to continue, i propose you take a different approach here. select some photo montages of various world cuppers each in different types of turns and elaborate (using these photos as visual references). try to tell us when and where they are making these movements.

just a suggestion

zenny
Quote:
Originally Posted by majortato

I was hoping to make things simple by focusing on the x-axis.  If we can increase speed in either component, then we can increase speed overall.  With discussions around ski rebound, I wanted to focus on the x-axis because that's where most of it happens.  There's no need to bring in the Y-axis into the discussion.  I know its not 100% transfer...doesn't need to be.

semantics.  That's why I said "push" in quotes.  I'm talking about resisting forces.  It feels like a push with your legs.  And btw, you can make the scale temporarily read higher than your weight if you pushed off it.

A bike turns with combination of tipping and the front wheel being able to steer...it's very different from a ski.  In any case, you could get chucked anywhere in the turn depending on what you did wrong.  Not sure where you're going with it, but I think it just adds complexity to the discussion as we have to take into account complicated bike mechanics as well.

Zenny, I'm confused...I thought you're arguing that you don't believe you can accelerate out of a turn.  But you're asking why a person might put their COM behind their BOS to generate more speed?

I know...I'm not talking about accelerating only in the x-direction...I'm talking about looking only at the x component of the acceleration force.

ok lets try the turn phases and power inputs/outputs....still somewhat simplified because listing everything would be incredibly complicated.

Before we begin though, I just want to make sure everyone understands exactly what centripetal force is (I know at least Jamt does), but in case you missed out on HS physics class.  Centripetal force is a force that acts perpendicular to the direction of travel, always inwards towards the center of a circle.  Think earth going around the sun...the sun's gravity is the centripetal force that's always towards the sun.  The earth orbits but never falls in because the earth has momentum, even though the force is always directly in the direction towards the sun.  It is a force that turns us without slowing us down or speeding us up in a turn.

• phase 1 - transition to right before fall line:  movements: legs extend to cause divergence of COM and skis.  The COM is also pushed in a direction down the fall line as well (upside down position).  power inputs: leg muscles, gravity.  power output: skis dig into snow and centripetal force is created to turn the skis.  Skis start bending as a result.  Gravity contributes to some speed increase and helping accelerate the COM down the hill.
• phase 2 - fall line: movements: drop COM inside of turn to stay balanced against forces generated as a result of the turn.  power input: leg muscles, gravity.  power output: skis against snow creates centripetal forces to turn the ski/skier.  Skis reach max bend as a result.  Gravity has most effect on forward momentum of skis and skier.
• phase 3 - after fall line to transition:  movements: legs relax to allow convergence of COM and skis. The retraction move isn't really a muscle move...it's the result of allowing the convergence of COM and ski to happen as the COM passes over the BOS.  Power inputs: gravity, ski rebound.  Power output:  Skis still on edge creates centripetal force that keeps the ski turning.  Meanwhile, the leg muscles have relaxed so the centripetal force acting on the COM goes away.  This allows the COM to exit the old turn and enter the new turn.  However! there's the case of the ski rebound.  It is in the direction of the centripetal force (perpendicular to direction of travel).  This force can be used to help the skis finish the turn while the COM starts moving into the new turn.

What's funny is after this exercise, it's even more clear that the direction of the rebound force being perpendicular to the ski is actually exactly what we need.  We need it to be a centripetal force so that it can continue turning the skis even after we have reduced the centripetal force created by the leg muscles.  It doesn't actually matter where in the turn this release happens..even if it's across the hill.  Additionally, it actually doesn't matter if this force is released quickly or slowly because it generally adds on to the centripetal force to turn the ski after the leg muscles have stopped pushing the skis against the snow.

So why is it faster? cause it allows us to project the COM into the new turn slightly earlier than we would be able to otherwise.  I don't need to wait the extra split second for the skis to finish its turn before projecting my COM..I can start projecting the COM and let the rebound of the ski help create the centripetal force to finish the ski's turn.

So I'm moving away from how much it speeds up the ski...rather it helps turn the ski and allows convergence to be quicker.  The end result is faster!

That is what I meant when I said that the stored energy can assist you in the retraction move. But you have hardly showed that a stiffer spring is faster because of this. Yes the retraction move becomes slightly faster, but due to conservation of energy it means that you have lost it somewhere else. It means that you "lost" some energy in the top of the turn instead.

Further, the push from the spring also pushes the CoM upwards, which means a higher position, so maybe even slower? I'm not saying it, but it needs to be considered.

Also, just because you are faster from one turn to the other does not mean you are faster as a skier. Heard of not rushing you turns?

If we make an analogy into motor sports, e.g. bike racing. What is fastest on a smooth track, a lot of energy going into the springs, or virtually no energy going into the springs?

(I think Razie is into this?)

Or how about introducing springs into BMX racing?

Quote:
That is what I meant when I said that the stored energy can assist you in the retraction move. But you have hardly showed that a stiffer spring is faster because of this. Yes the retraction move becomes slightly faster, but due to conservation of energy it means that you have lost it somewhere else

So you agree that this rebound (if used properly) helps with a certain movement..and that movement can allow us to be faster.  When it comes down to it, that's all I was really arguing.  The idea that one can use ski rebound to assist them in getting down the hill a little faster than they would if they didn't use it.

Losing energy elsewhere is not really a concern...that ski will bend no matter what you do.  It's inevitable when you turn, and therefore moot.  There's nothing you can do to prevent the ski from bending.  It's like talking about extra friction forces when you turn.....if you want to turn, you're going to increase frictional forces.  Like I said before, the whole system is a net loss.  Only a percentage of total input energy (gravity/muscle movements) can be converted into positive outcome (turning, increased speed, balance, etc.).  Conservation of energy is useful when applying physics theory using mathematics...in the real world, applying this idea doesn't do much because so much energy is dissipated through heat/friction (still conserved..but not in any meaningful way).

In the end, it's all about what you can do to reduce the amount of energy lost.

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