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# Dan Dipiro's Mogul Book - Page 22

Been there; done that; on both skis and bike.  Just pointing out the difference to highlight the different mechanisms involved.

Quote:
Originally Posted by Ghost

Been there; done that; on both skis and bike.  Just pointing out the difference to highlight the different mechanisms involved.

No, actually the same mechanisms are involved in pumping on a bike and on skis, and in absorbing on a bike and on skis.

And, yes, you can both pump and absorb on skis, and pump and absorb on a bike.  There are not different mechanisms involved.

Quote:
Originally Posted by borntoski683

TE your CoM doesnt have to actually rise. If your momentum is directed up by snow reaction force, Gravity is working to slow you down immediately. If gravity slows you down just as fast as the upward acceleration from the snow, the net effect is gravity steals away momentum without actually moving you up and thus not acquiring potential energy so to speak. Yes we have conservation of energy but gravity is a constant external force. Resistance against the face of the bump provides the upwards acceleration out of our momentum, but Gravity is constantly working against that. If upwards acceleration exceeds force of gravity then we will move up and yes that will create potential energy. But we can resist just enough, imperfectly absorbing, such that the upwards acceleration does not exceed gravity, in which case no potential energy is created from moving up but gravity will definitely have reduced some of the momentum. Conversely if you absorb the face of the bump perfectly with aggressive flexing, then there will be no upwards acceleration and therefore momentum will be moving TOWARDS gravity and gravity will accelerate us to go even faster.

The only way that gravity can reduce your momentum is to increase your potential energy.  That's physics.  I'm willing to work out these concepts with you.  Answer questions.  Give equations.  Or point you to the right literature.  But these are facts.  There are two conceptual truths that can help you solve this problem.  First, if you drop into a halfpipe that has no friction you'll just keep going back and forth forever.  Next, forces acting normal (90 degrees) to the direction of travel do not slow you down.  This is why the earth can travel around the sun without slowing down even though the sun applies a force on the earth.  The force is always normal to the direction of travel, just like going up a half pipe.  Understanding these truths will answer these question by breaking down the velocity, gravity, and friction forces into vectors for each moment the skier moves down the slope and balancing the equations.  I'm happy to elaborate more if you'd like.  But just know that if you ski on a very gentle slope, you will not go fast, and that's the essence of it.  At fast speeds the expert skiers say pull those feet up before the bump.  Physics backs that up.

Quote:
Originally Posted by CTKook

Quote:
Originally Posted by Ghost

Been there; done that; on both skis and bike.  Just pointing out the difference to highlight the different mechanisms involved.

No, actually the same mechanisms are involved in pumping on a bike and on skis, and in absorbing on a bike and on skis.

And, yes, you can both pump and absorb on skis, and pump and absorb on a bike.  There are not different mechanisms involved.

What we have heeaah is a failure to communicate.

Absorbing on the front of the bump and extending on the back of the bump makes the bike go faster.

Absorbing on the front of the bump and extending on the back of the bump makes the skier go slower.

Please explain the difference in mechanisms that makes this difference.

Quote:
Originally Posted by Ghost

What we have heeaah is a failure to communicate.

Absorbing on the front of the bump and extending on the back of the bump makes the bike go faster.

Absorbing on the front of the bump and extending on the back of the bump makes the skier go slower.

Please explain the difference in mechanisms that makes this difference.

The red line is incorrect. You don't absorb the front of the bump to 'pump' on a bike, you extend into the transition (up the face) absorb the crest and extend into the transition (backside), that is 'pumping' on a bike.

Quote:
Originally Posted by The Engineer

2.  Many discussions and definitions treat moguls as natural, unmovable features.  The truth is that they are made by skiers (mostly) and change over time.  For example, the troughs form by skiers consistently skiing over the same path.  The skis dig this lowest point by compressing the snow or pushing it to the side.  The troughs represent the net effect of the average path of the skiers, the road most traveled.  The style of the population of skiers determines the shape of the trough (how direct, how curved, how abrupt...).  When someone discusses picking a path other than the trough, it means picking a path different than the majority of the skiers that made those moguls.  If the majority of the skiers are making slow round turns, the trough will have a large angle, and a skier may take a more direct path that's not as angled.  But, if the whole population takes the same direct path, the troughs will reform under these skiers, and this direct path will now be in the troughs.  I've seen it happen, I've made it happen, it must happen because troughs are dug by the skis.  When skiers turn very little, the troughs are pointed almost straight down the hill, and there isn't a more direct path.  So, sometimes the fastest, most direct line will be in the troughs, sometimes it won't depending on the type of skiers on the mountain.  And, defining paths based on the shape of the moguls is only useful for a given population of skiers.  On a competition course, if all of the skiers are the same skill level skiing the same path, eventually they must all be skiing in the troughs if the slope is given enough time to reform (can happen in just a few loops if there's enough traffic and the surface is soft enough).  Proposals that everyone ski a line other than the trough is not sustainable, because if everyone skis a different path, the troughs will reform in that new path.  Also, if everyone is skiing a random path there will be no moguls.  Consistent moguls are formed by most people taking the same path.  So, path choice outside the trough is only defined by picking a different path than most other skiers.  If you have a really great ride not in the troughs you might want to keep it secret, because it probably relies on everyone else skiing differently.

While you're pumping away at the other answers, what do you mean by "Proposals that everyone ski a line other than the trough is not sustainable,..." ?

People aren't entirely random. Hundreds of skiers, none good zipperliners, will still form moguls. Even if none of them follow the same path. One can weave in and out of paths.

We are not goats. "Live Free or Die" was translated from the original in Ænglisc, "I am not a goat".

Summer time makin' mogul troughs.

A lot of what you guys are saying is true, but the convoluted way you are getting there is painful to read.
The missing piece, which I am sure some of you know, is ( change in energy) = work = (force times distance).
It is actually a vector thing so the force and distance have to be in the same direction.

Are there cliff notes for this??

Can someone bottom line the physics so like Donald Trump can build another Sky Scraper? There's not much time.

Quote:
Originally Posted by Whiteroom

Quote:
Originally Posted by Ghost

What we have heeaah is a failure to communicate.

Absorbing on the front of the bump and extending on the back of the bump makes the bike go faster.

Absorbing on the front of the bump and extending on the back of the bump makes the skier go slower.

Please explain the difference in mechanisms that makes this difference.

The red line is incorrect. You don't absorb the front of the bump to 'pump' on a bike, you extend into the transition (up the face) absorb the crest and extend into the transition (backside), that is 'pumping' on a bike.

My excuse is Socrates had keener students!

Sure you extend to start with more elevation, but if you absorbed the whole thing and pushed on the back side you would go faster, not slower.  What absorbing and extending in the same place in skiing moguls result in slower not faster.

Quote:
Originally Posted by Tog

Are there cliff notes for this??
Can someone bottom line the physics so like Donald Trump can build another Sky Scraper? There's not much time.

Nonsense! There's all summer and only 600 some posts so far...
Quote:
Originally Posted by Ghost

What we have heeaah is a failure to communicate.

Absorbing on the front of the bump and extending on the back of the bump makes the bike go faster.

Absorbing on the front of the bump and extending on the back of the bump makes the skier go slower.

Please explain the difference in mechanisms that makes this difference.

Yes, you don't know what pumping is.

Absorbing the front of the bump and extending into the trough makes the skier go slower.  The bump skier isn't pumping the backside.  The bump skier is not pumping when extending.

If I were showing a bunch of high-end intermediates how to make speed on a gravity traverse, however, I might tell them to be light on the face and heavy on the backside, because I would want them to pump.

Just as for a beginner on a bike on a pump track.

What Whiteroom said,

Quote:
Originally Posted by Whiteroom

...You don't absorb the front of the bump to 'pump' on a bike, you extend into the transition (up the face) absorb the crest and extend into the transition (backside), that is 'pumping' on a bike.

is also correct, but represents a higher level of skill and/or roller geometry that make this possible.

Learn to pump and learn to absorb and it will make sense.

Now we are getting somewhere.   Please elaborate on the difference between the extending and the pumping.

I don't think you're listening TE.  This is my last post to you on this subject as you just seem to want to be disagreeable and I'm actually trying to support your point of view about flexing the legs not sucking up speed as claimed by some.

I agree with you that flexing your legs doesn't absorb speed.  You're arguing against the wrong guy.  I've been saying that here for a long time.  Energy is always conserved, I agree!

However bumps can be used to slow us down.  You missed the point I was trying to make, I wasn't trying to say energy dissappears into thin air.  But I will leave it to you to figure out where it goes.  There is always slope below us and gravity waiting to pull us, pretty much always on an open slope.  Energy is always conserved.  Speed is not.  You don't have to lose energy to lose speed.

I'm simply saying you can reduce your speed by letting the snow accelerate you up while gravity accelerates you down.  You essentially stall out any change of vertical height if you use sense of touch and leg flexing to absorb some of the upward acceleration so that the upward acceleration matches or is less then the downward acceleration from gravity.  Just like you slow down as you approach the top of a half pipe, the same is true from a bump.  The difference is that in the half pipe you are clearly moving upwards, but as you approach the top you can flex your legs so that upwards acceleration is reduced, even while gravity continues to pull the other way.  Why is upwards acceleration reduced?  Because by flexing to reduce pressure you are reducing the action/reaction pressure on the snow that is supposed to be creating the upward acceleration, so acceleration from gravity starts to win and slow you down.  Is there a lot of potential energy at that moment?  Sure, but the rider doesn't have to be launched into the air, they can literally stall out and stop right there, stand on the edge and come to a complete stop.  Mission accomplished.  Speed was reduced and they aren't suddenly falling back down and they didn't launch into the air.

A bump is not a perfect upwards acceleration like a half pipe, it only accelerations a little more more up then before, but same concept applies, gravity is accelerating you the opposite way and if you use just the right amount of resistance against the bump so that upwards acceleration matches acceleration from gravity, then you don't move up...your CoM moves down the hill on a smooth path, and yes speed can be reduced because gravity is the only external force and it worked against the direction of momentum.

It requires some amount of resistance, in order to create the upwards acceleration that counter acts the acceleration from gravity.

Obviously if you resist fully with stiff legs, your upwards acceleration will exceed gravity and you will jump in the air.  If you actually pump it, it will be even worse.  If you flex to reduce pressure, the action/reaction force from the snow will be be reduced, while gravity is not reduced so you can accomplish zero vertical change, while slowing down at the same time.  The reason you slow down is because the upwards acceleration does not come from an external source, it comes from energy that is converted into upwards acceleration, but gravity is an external force that counter acts that acceleration and essentially squelches some speed.  I leave it to you to figure out the energy conservation physics.

If you flex so much that there is no upwards acceleration due to reducing pressure and reactionary forces, then your momentum will be continuing to move in the same general direction as gravity and gravity will continue to accelerate you without any opposing acceleration...ie...you'll speed up.

This kind of overly deep physics discussions rarely go well on this site.  its not easy stuff and even smart guys get it wrong and everyone else doesn't know WTF is being discussed, but the bottom line is that if you flex so much that you get no upwards acceleration from the face of the bump, then you will not be slowed down by the bump at all other than insignificant amounts of friction.  if you resist too much you will get some slowing down, but you will also go airborne and get more directly available potential energy created from moving up into the air.  The upwards acceleration still counter acts gravity during that time, and your speed down the mountain WILL be reduced.  And you will not suddenly start moving fast down the mountain again right after that.  You will not like getting kicked around, but even slamming into the face and going airborne does not initially speed you up, it also slows you down, it just does it in a very bumpy way.    But if you find just the right amount of pressure aborbtion such that the upwards acceleration matches gravity, then you will get speed reduction, you will stay on the snow and the show will go on.

Real moguls. May 3. Killington.

The center below the cat track was groomed the night before. Despite the lack of bonafide zipperliners, the moguls did indeed form from Brownian motion skiers slogging their way down oblivious to General Relativity.

It was an absorbing experience. The problem was there was no turning the moguls off. Every damn area had them. I can confirm it takes work to ski them for 5 hours.

With sugar corn mash snow, there's also blasting through the top of the mogul for minor speed control, shin, sometimes face shots, and general hilarity. Slaloms work well for blasting but are not large fording ladders for deep troughs so there's a limit. (I know that's not "real" mogul skiing and since it's not skiing the troughs it's not "sustainable", but throw us a bone.)

If you follow the traverse where the two guys are and keep going to the right, you end up on Skyhawk. A short steep pitch with some very large moguls. One guy took the ziplock path straight down on the tops. Essentially using the tops of moguls as pillows, he just jumped and launched from one to the other. The sugar corn mash helps in absorbing the impacts.

Tog on Sky Hawk. As usual, it doesn't look that steep from the top. (Also I am off to the side, not directly above him.) I believe this is the steepest pitch at Killington. Notice how deep that trough is.

I did finally read my copy of the book a week ago. Ok but not an OMG experience. Tog reminded me to absorb actively and it does make a difference. Crude blasting with only passive absorption can be fun too, though, and it helps groom the slope.
Quote:

TE wrote:

Ski in ways where friction gives you a better advantage over gravity to control speed.

You are onto something TE, but I don't believe you realize what you wrote.

Instead of "A&E", whatever absorption means anyways, the key to speed control is finishing the turn, getting the skis across the fall line while brushing the tails.

Quote:

TE wrote:

At fast speeds the expert skiers say pull those feet up before the bump.  Physics backs that up.

Ah, no again. The pivot /skid is not the fastest technique. Even the pivot makers don't flex until they contact the bump.

The fastest skiers, WC Bumps, are using the deflected carved turn.  They are not dependent on the pivot/skid for speed control.  The fastest skiers extend into the turn finish and don't flex/retract until their toes clear the top of the bump.  They do not flex BEFORE the bump or even when they  first contact it, they extend into and through the mogul sidewall as they deflect off of it and flex/retract as their feet crest the top.

Telling people to flex before they slam the bottom of the bump to gain some speed control is just plain bad advice.  While it may may work in low angle bumps, it's generally a bad idea.

The whole A&E scheme is a joke, the focus should be on how, when and where to finish the turn, because that's where the most speed control can be gained if needed with minimal impact.  Absorption / compression happens naturally and should not be focused on, rated or judged.

Nail

Edited by Nailbender - 5/4/15 at 10:53pm

My 2 cents.

This does not have to be complicated. On average the force acting on the skis will be so that you stay on the surface on average. If the force was larger you would fly.

The only thing you can do is to use your body to distribute this force differently in time and space.

If you distribute the force so that it is primarily acting when the ground is flat, like in the left picture below, the speed will not increase so much.

If you distribute the force so that it is primarily acting when the ground is steep, like in the right picture below, the speed will increase very rapidly.

If the conditions are like in the bottom picture, you may even come to a stop.

If you don't see it, Imagine what happens if you jump between the spots shown in the pictures.

Angular momentum only confuses things. Conservation of angular momentum is only applicable when rotating around a fixed point. No such thing in moguls.

Here's Moseley, he's is acheiving speed by moving his CoM on curved surfaces. The timing is close to how he would have "pop" at the kickers. In principle, speed can be reduce by doing the opposite.

Quote:
Originally Posted by borntoski683

I don't think you're listening TE.  This is my last post to you on this subject as you just seem to want to be disagreeable and I'm actually trying to support your point of view about flexing the legs not sucking up speed as claimed by some.

I agree with you that flexing your legs doesn't absorb speed.  You're arguing against the wrong guy.  I've been saying that here for a long time.  Energy is always conserved, I agree!

However bumps can be used to slow us down.  You missed the point I was trying to make, I wasn't trying to say energy dissappears into thin air.  But I will leave it to you to figure out where it goes.  There is always slope below us and gravity waiting to pull us, pretty much always on an open slope.  Energy is always conserved.  Speed is not.  You don't have to lose energy to lose speed.

I'm simply saying you can reduce your speed by letting the snow accelerate you up while gravity accelerates you down.  You essentially stall out any change of vertical height if you use sense of touch and leg flexing to absorb some of the upward acceleration so that the upward acceleration matches or is less then the downward acceleration from gravity.  Just like you slow down as you approach the top of a half pipe, the same is true from a bump.  The difference is that in the half pipe you are clearly moving upwards, but as you approach the top you can flex your legs so that upwards acceleration is reduced, even while gravity continues to pull the other way.  Why is upwards acceleration reduced?  Because by flexing to reduce pressure you are reducing the action/reaction pressure on the snow that is supposed to be creating the upward acceleration, so acceleration from gravity starts to win and slow you down.  Is there a lot of potential energy at that moment?  Sure, but the rider doesn't have to be launched into the air, they can literally stall out and stop right there, stand on the edge and come to a complete stop.  Mission accomplished.  Speed was reduced and they aren't suddenly falling back down and they didn't launch into the air.

A bump is not a perfect upwards acceleration like a half pipe, it only accelerations a little more more up then before, but same concept applies, gravity is accelerating you the opposite way and if you use just the right amount of resistance against the bump so that upwards acceleration matches acceleration from gravity, then you don't move up...your CoM moves down the hill on a smooth path, and yes speed can be reduced because gravity is the only external force and it worked against the direction of momentum.

It requires some amount of resistance, in order to create the upwards acceleration that counter acts the acceleration from gravity.

Obviously if you resist fully with stiff legs, your upwards acceleration will exceed gravity and you will jump in the air.  If you actually pump it, it will be even worse.  If you flex to reduce pressure, the action/reaction force from the snow will be be reduced, while gravity is not reduced so you can accomplish zero vertical change, while slowing down at the same time.  The reason you slow down is because the upwards acceleration does not come from an external source, it comes from energy that is converted into upwards acceleration, but gravity is an external force that counter acts that acceleration and essentially squelches some speed.  I leave it to you to figure out the energy conservation physics.

If you flex so much that there is no upwards acceleration due to reducing pressure and reactionary forces, then your momentum will be continuing to move in the same general direction as gravity and gravity will continue to accelerate you without any opposing acceleration...ie...you'll speed up.

This kind of overly deep physics discussions rarely go well on this site.  its not easy stuff and even smart guys get it wrong and everyone else doesn't know WTF is being discussed, but the bottom line is that if you flex so much that you get no upwards acceleration from the face of the bump, then you will not be slowed down by the bump at all other than insignificant amounts of friction.  if you resist too much you will get some slowing down, but you will also go airborne and get more directly available potential energy created from moving up into the air.  The upwards acceleration still counter acts gravity during that time, and your speed down the mountain WILL be reduced.  And you will not suddenly start moving fast down the mountain again right after that.  You will not like getting kicked around, but even slamming into the face and going airborne does not initially speed you up, it also slows you down, it just does it in a very bumpy way.    But if you find just the right amount of pressure aborbtion such that the upwards acceleration matches gravity, then you will get speed reduction, you will stay on the snow and the show will go on.

Borntoski-I'm sorry the conversation isn't going well for us.  It's true that you seem to have the most similar perspective on moguls compared to me.  You tend to be the person I agree with most often when discussing how to ski moguls.  I believe your end conclusions are very similar to mine.  I believe you know how to ski moguls and know how to describe how you ski moguls.  The problem comes when you get into the discussion of why things work.  You end up using physics to describe it whether you intend to or not and you say things that are categorically wrong such as "gravity steals away momentum without actually moving you up".  As long as people are discussing physics while saying categorically wrong things and not willing to address it then the discussion is futile.  It's not just you that does it, so this forum may not be the place for these discussions.  However, physics is my expertise, and for those that are interested I feel it's what I have to offer, though I'm quite willing not to discuss physics.  If you are engaging me on a discussion involving physics, I can't let categorically wrong statements pass.  I agree it's best for us to drop it, but if you would like to continue the discussion we would have to work on one statement at a time and we would have to keep recycling your statements until nothing is a clear violation of verifiable facts in physics.  Some of these things are like 2+2=4 for me.  I am highly trained and highly skilled in these areas.  When talking about physics it would be best to treat me as a physics professor and work to understand what I'm saying rather than arguing with me.

Quote:
Originally Posted by Nailbender

You are onto something TE, but I don't believe you realize what you wrote.

Instead of "A&E", whatever absorption means anyways, the key to speed control is finishing the turn, getting the skis across the fall line while brushing the tails.

Ah, no again. The pivot /skid is not the fastest technique. Even the pivot makers don't flex until they contact the bump.

The fastest skiers, WC Bumps, are using the deflected carved turn.  They are not dependent on the pivot/skid for speed control.  The fastest skiers extend into the turn finish and don't flex/retract until their toes clear the top of the bump.  They do not flex BEFORE the bump or even when they  first contact it, they extend into and through the mogul sidewall as they deflect off of it and flex/retract as their feet crest the top.

Telling people to flex before they slam the bottom of the bump to gain some speed control is just plain bad advice.  While it may may work in low angle bumps, it's generally a bad idea.

The whole A&E scheme is a joke, the focus should be on how, when and where to finish the turn, because that's where the most speed control can be gained if needed with minimal impact.  Absorption / compression happens naturally and should not be focused on, rated or judged.

Nail

Nail- In all the discussion I've been having on this last round, I've only been talking about how just A&E controls your speed without turning, skidding, smearing, scarving etc.  And I agree that mogul skiers using only A&E to control their speed will be the fastest.  I understand that you like to finish your turns.  Sometimes I do too.

Quote:
Absorption / compression happens naturally and should not be focused on, rated or judged.

So many skiers can not ski the zipper line smoothly without impact because they do not know how to do A&E properly.

Quote:
Originally Posted by Jamt

My 2 cents.

This does not have to be complicated. On average the force acting on the skis will be so that you stay on the surface on average. If the force was larger you would fly.

The only thing you can do is to use your body to distribute this force differently in time and space.

If you distribute the force so that it is primarily acting when the ground is flat, like in the left picture below, the speed will not increase so much.

If you distribute the force so that it is primarily acting when the ground is steep, like in the right picture below, the speed will increase very rapidly.

If the conditions are like in the bottom picture, you may even come to a stop.

If you don't see it, Imagine what happens if you jump between the spots shown in the pictures.

Angular momentum only confuses things. Conservation of angular momentum is only applicable when rotating around a fixed point. No such thing in moguls.

Very true, except that if the bottom tactic gets you too much air, you are not given opportunity to use the friction of skis on snow in a non-arced short radius turn.  It depends.

Quote:
Originally Posted by Tog

While you're pumping away at the other answers, what do you mean by "Proposals that everyone ski a line other than the trough is not sustainable,..." ?

People aren't entirely random. Hundreds of skiers, none good zipperliners, will still form moguls. Even if none of them follow the same path. One can weave in and out of paths.

We are not goats. "Live Free or Die" was translated from the original in Ænglisc, "I am not a goat".

That's what I've been saying.  You've just described a population of skiers.  Sometimes that will not be the population, such as during a mogul competition.  Sometimes that population can change over time.  For example, maybe your favorite bump shape that gives you a line you love to take requires a certain percentage of zipper line mogul skiers.  If fads change and zipper line mogul skiers disappear you may never see your favorite line again, just hypothetically.  Most of these discussion about different lines are for a given population of recreational skiers, but even with that population, the mogul shapes change over the years based on changes to equipment and style, so the qualities of different line choices also changes.  I'm invested in people turning on every bump, because that makes the moguls I enjoy the most.  The more people that follow the direct line, the more lines there are to follow.  Mary Jane is one of my favorite places to ski, merely because there are lots of mogul skiers skiing the way I do, and they allow many of those nice bumps to form on many trails.  I wish that could happen at other places.

Quote:
Originally Posted by The Engineer

Borntoski-I'm sorry the conversation isn't going well for us.  It's true that you seem to have the most similar perspective on moguls compared to me.  You tend to be the person I agree with most often when discussing how to ski moguls.  I believe your end conclusions are very similar to mine.  I believe you know how to ski moguls and know how to describe how you ski moguls.  The problem comes when you get into the discussion of why things work.  You end up using physics to describe it whether you intend to or not and you say things that are categorically wrong such as "gravity steals away momentum without actually moving you up".  As long as people are discussing physics while saying categorically wrong things and not willing to address it then the discussion is futile.  It's not just you that does it, so this forum may not be the place for these discussions.  However, physics is my expertise, and for those that are interested I feel it's what I have to offer, though I'm quite willing not to discuss physics.  If you are engaging me on a discussion involving physics, I can't let categorically wrong statements pass.  I agree it's best for us to drop it, but if you would like to continue the discussion we would have to work on one statement at a time and we would have to keep recycling your statements until nothing is a clear violation of verifiable facts in physics.  Some of these things are like 2+2=4 for me.  I am highly trained and highly skilled in these areas.  When talking about physics it would be best to treat me as a physics professor and work to understand what I'm saying rather than arguing with me.

You are missing two points in your previous arguments.

As soon as you push off or absorb, the direction of travel of the skis and CoM is not parallel, which means the speed (and velocity) has changed.

Energy does not have to convert to gravitational potential energy, it can convert to heat in your legs. Likewise you can convert fat/carbo in your legs to kinetic energy.

Quote:
Originally Posted by mdf

Tog on Sky Hawk. As usual, it doesn't look that steep from the top. (Also I am off to the side, not directly above him.) I believe this is the steepest pitch at Killington. Notice how deep that trough is.

I did finally read my copy of the book a week ago. Ok but not an OMG experience. Tog reminded me to absorb actively and it does make a difference. Crude blasting with only passive absorption can be fun too, though, and it helps groom the slope.

Looks like some fun late season bumps. I agree with you mdf, crude blasting in raw passive absorption has limited usefulness, we should almost always be actively retracting our legs during that phase. Personally I don't think there is any point discussing so called passive absorption in bump skiing. That's a theoretical concept only and not part of good bump skiing.

Good advise from tog. If you get a chance to go out again, play around with varrying degrees of retraction intensity. In this spectrum of pressure management you can get different results and it all effects speed. Retract too quickly and you get too much pressure loss and no speed control. Retract too slowly, which is probably more commonly seen, and you will get tossed around by the bump too much by excessive pressure on the face. We do need to actively retract our legs in order to reduce that pressure on the face, but there is more subtlety to talk about there. You don't want to reduce ALL of the pressure to zero, unless you're trying to avoid slowing down of course, then by all means retract faster, eliminate the blockage and go fast.

I also want to emphasize that this is not the only speed control tool. Extending on the backside of the bump is not about pumping it, which in theory would speed the skier up. Extension there is just enough to again maintain some pressure, just enough that the edges can be engaged and creating speed control with steering angle and turn shaping. Someone posted a Canadian freestyle video earlier and note they were talking about retracting more quickly then extending. The extension move shouldn't be a pumping move. It's just enough extension to maintain the required pressure to edge, not so much as to push yourself faster.

The crux of all of this is that pressure management is an extremely important skill in bump skiing. It is not just about flexing your legs out of the way of the bumps and reaching into the troughs. It's about very carefully managing the amount of pressure on the face of the bump, the back of the bump and pretty much all phases of the turn. This is all about touch.

The interesting thing is that all of this applies to groomer skiing too, true finesse comes from this sense of touch and pressure management, but the difference is that on a groomer we get to dictate where the virtual bump will be, in real bumps the consequences of missed opportunities to manage pressure are more severe.

Crude blasting is a very specialized technique that only works in extremely soft, late season bumps that blow apart when you hit them.  It is not normal skiing, but it is fun for awhile.

Quote:
Originally Posted by Jamt

You are missing two points in your previous arguments.

As soon as you push off or absorb, the direction of travel of the skis and CoM is not parallel, which means the speed (and velocity) has changed.

Energy does not have to convert to gravitational potential energy, it can convert to heat in your legs. Likewise you can convert fat/carbo in your legs to kinetic energy.

There will be a vertical component to the velocity vector and a horizontal component.  The legs respond to the vertical component, but have no effect on the horizontal component.  Only friction can effect that, this is the concept of normal forces (90 degrees) not effecting each other (orthogonal functions being incoherent).  The legs absorb the energy when walking down stairs and control the vertical velocity, but it's friction that allows control of the horizontal velocity.  In the example where the stairs are slightly sloped and slippery, the legs will have no ability to decrease the minimum terminal velocity.  If there is no friction at all on the stairs in this case even with the slightest of slopes, you will accelerate to time warping speeds without any ability for any part of your body to absorb the energy (in a vacuum).  Please don't send me the doctor bill.  Pushing on an upward ramp will redirect your velocity, but without friction you will not lose any energy.  Even worse, you will gain potential energy which then causes you to have to drop at the end of the ramp thus accelerating.

There is one case where the legs can absorb the energy.  This case is CTkooks example of pumping to change the angular momentum.  In this case using parametric pumping, the energy will be absorbed by the legs, but as I've argued before, there are many cases where you can not generate angular momentum, because this requires particularly shaped curved surfaces and spending time with your COM moving on those curved surfaces.  There are other competing forces and concerns that will overwhelm this technique in many situations.  A&E still works just fine by extending from flat section to flat to illustrate the stair step analogy, and in this case friction is the only form of dissipation for the horizontal component of the vector necessary to control velocity.

Quote:
Originally Posted by mdf

Crude blasting is a very specialized technique that only works in extremely soft, late season bumps that blow apart when you hit them.  It is not normal skiing, but it is fun for awhile.

Heheh yea I hear that, slush bumps are fun. But I want to challenge your thinking here. Is it truly and fully passive, even when you feel like you are blasting through? I am willing to bet you are still actively retracting your legs, you're just doing it with a sense of touch that allows more pressure. The slush factor softens the blow of that pressure. That actually helps control your speed even more due to the larger pressure. The soft snow softens the blow in much the same way that flexing your legs more aggressively would. Quite likely though if you try to be passive your head will be bobbing up and down a lot which is probably why tog told you to flex more
Quote:
Originally Posted by The Engineer

There is one case where the legs can absorb the energy.  This case is ...

Anytime you use your muscles, except static bracing, you get force times distance = work, and therefore they are putting energy into the system or taking energy out of the system.

Also, there are a lot of ways to dissipate energy into the snow surface that are not conventionally included in the "friction" category.  Scraping on ice shaves off fragments, which takes a fair amount of energy.  Compressing snow takes energy, and a lot of the effect is inelastic (meaning the energy stays there).  Just moving snow takes energy.  I suppose it is all friction broadly construed, but I wouldn't call it that.

Quote:
Originally Posted by The Engineer

There will be a vertical component to the velocity vector and a horizontal component.  The legs respond to the vertical component, but have no effect on the horizontal component.  Only friction can effect that, this is the concept of normal forces (90 degrees) not effecting each other (orthogonal functions being incoherent).  The legs absorb the energy when walking down stairs and control the vertical velocity, but it's friction that allows control of the horizontal velocity.  In the example where the stairs are slightly sloped and slippery, the legs will have no ability to decrease the minimum terminal velocity.  If there is no friction at all on the stairs in this case even with the slightest of slopes, you will accelerate to time warping speeds without any ability for any part of your body to absorb the energy (in a vacuum).  Please don't send me the doctor bill.  Pushing on an upward ramp will redirect your velocity, but without friction you will not lose any energy.  Even worse, you will gain potential energy which then causes you to have to drop at the end of the ramp thus accelerating.

There is one case where the legs can absorb the energy.  This case is CTkooks example of pumping to change the angular momentum.  In this case using parametric pumping, the energy will be absorbed by the legs, but as I've argued before, there are many cases where you can not generate angular momentum, because this requires particularly shaped curved surfaces and spending time with your COM moving on those curved surfaces.  There are other competing forces and concerns that will overwhelm this technique in many situations.  A&E still works just fine by extending from flat section to flat to illustrate the stair step analogy, and in this case friction is the only form of dissipation for the horizontal component of the vector necessary to control velocity.

It is not about horizontal or vertical. if i run into a friction-less tree on the flat my "horizontal" energy will be converted into cracked bones.

Also, if there is no friction there is no terminal velocity unless it is zero.

Further, as I said in a previous post. You have to be very careful if you are going to use the conservation of angular momentum argument. It is only valid around a fixed point of rotation.

Another example, take the half-pipe example where you implied you can only speed up.

Every time you come to a stop, jump to the bottom of the half pipe and absorb the perpendicular component.

Will the speed decrease or increase after each jump?

The first jump is simple and it is easy to show that at the bottom you will have a kinetic energy which is about 1/4 of what you would have if you only slided down the surface.

This mean you will only make it 1/4 up the other side. Now jump again to the bottom.

You will get an energy time series which converges to zero.

You could also jump to a place where the velocity at the point of impact is perpendicular to the surface. You would get a zero velocity after absorbing and then start sliding back much slower, or you could immediately jump to the corresponding place on the other side. You would never have a speed component along the surface and it would converge quite rapidly.

Quote:
Originally Posted by mdf

Anytime you use your muscles, except static bracing, you get force times distance = work, and therefore they are putting energy into the system or taking energy out of the system.

If you are flying through space with lead weights attached to your legs and radiation shielding all around you and you pull and push on the lead weights, your legs will use energy, but you won't slow down.  So, just the fact that energy is being used in your legs is not a relevant point without careful analysis.

Quote:
Also, there are a lot of ways to dissipate energy into the snow surface that are not conventionally included in the "friction" category.  Scraping on ice shaves off fragments, which takes a fair amount of energy.  Compressing snow takes energy, and a lot of the effect is inelastic (meaning the energy stays there).  Just moving snow takes energy.  I suppose it is all friction broadly construed, but I wouldn't call it that.

Yes, many times I described it as displacement of snow.

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