71% of skiers go 'out of their depth' - insurance report

A snowHead

SimonN, David Goldsmith said that the carve will slow you if you continue on an uphill trajectory.... no skier in the world will disprove this without powered skis.... snowHead

This topic has relevance beyond skiing. If we were talking about annoying and endangering others, there are other sports where we could apply this. My other sport is golf, I see plenty of people who dont know the rules, who dont appreciate others and who go around 'duffing' or firing the ball at right angles. In many respects, those doing that are more of a problem than a skier out of their depth....

A snowHead

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I have just read what David Goldsmith has said and I disagree even more. He states

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Do you scare yourself on slopes you should have avoided?
Yes.

Do 'insurance scare articles' terrify you?
No.

Are you immune to danger and totally fearless?
No, but I will try most anything. Haven't yet had a true near-death experience while boarding.

Are you insured against your own stupidity?
No. I think this must be more common in the UK/Europe than in Canada?

Also:

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SimonN, Hi Simon, now, forgive me, my physics degree is some time ago, but I don't see how you can continue accelerating once you are no longer moving downhill. The motive force in skiing is gravity - although I do dearly want to mount a pair of model turbojets on the back of my old DH boards - nothing else is going on.

Maximum acceleration is in the fall line. And, there's just no way you will continue accelerating uphill. All forces on you are downhill, therefore all acceleration is downhill.

Unless you have cracked Heim physics, in which case, why aren't you in hyperspace right now?

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Sorry, ponder, but that's not quite right but I do conceed that my claim of accelerating up hill might only be theoretically possible! I might have to get the text books out to get the formula but I will try to put it in the right terms.

Simply put, if you increase the force acting on the ski when carving, without the ski skidding, you accelerate. Now, if that acceleration is greater than the gravitational forces of going up hill slow you down, you will continue to accelerate. However, here lies the challenge. Can the carve be tight enough so that the centrefugal g-forces are still there when you begin to go up hill? I don't actually know but if you can carve a small enough curve, then, yes, you would accelerate up hill.

I

After all it is free

SimonN, to be quite pedantic, if you're not skidding at all and you increase the turning force, yes, you will accelerate. You just won't go any faster. (Acceleration being a vector, change of direction means you're accelerating in that direction...)

I need to have my dinner now but I will leave you with incontrevetable proof in the shape of "The Up Hill Ski Club" which clearly wouldn't exist if you couldn't accelerate up hill. QED Very Happy

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SimonN, is correct, when an body is undergoing circular motion as in a ski turning, if you reduce the radius of the motion then the body will accelerate. To demonstrate this twirl a cotton reel on a thread, and then suddenly pull the thread to decrease the radius, the cotton reel will twirl faster.

It's to do with resolving the forces acting on the ski, when you turn tighter, the reaction of the snow against the ski increases, and so when resolved against the centripetal force causes an increase and change of direction of the acceleration vector.

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SimonN, A man with a well defined and developed sense of humour clearly. Nice one!

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SimonN, while I've been thinking about this DM's got there - the force you feel through your legs is there for changing your direction (i.e. the velocity of your centre-of-mass), but as it's essentially perpendicular to the skis it won't be changing their speed. The maximum acceleration in the direction of the ski will be when pointing down the fall line, but the maximum speed will be achieved when traversing directly across it (i.e. when gravity no longer has a component along the length of the ski)

Kramer, sorry but I disagree there. Yes, if you pull on the thread the bobbin seems to move faster, and it spins around at a higher angular frequency, but the bobbin speed stays unchanged. Think about conservation of angular momentum: mv = mwr (can't do omega, so omega=w). d/dr (mwr) = 0, so as r goes down w goes up, but the tangential speed of the bobbin v=wr is unchanged. If we're splitting hairs there will be a small increase in v while the radius of the curve is changing as there is a very small radial component proprtional to the rate of change of curvature.

[edited afterthought]
The one thing that could affect this all though (but not to the extent of appreciableacceleration uphill) is the fact that we are not (once we get beyond beginners' snowploughs, ideally anyway Wink

) rigid bodies, and can move ourselves with respect to the skis. So it's possible to build up a bit of stored potential energy by sinking into a turn, then explode upwards at the apex of the turn, pushing perpendicular to the skis at that time and gaining momentum across the slope, i.e. giving our bodies a bit of a slingshot, which translates to additional in-line speed only once the skis have followed around and completed the turn. At least I think this is the physics behind what a coach was trying (without much success) to get me to do when giving me some GS coaching back in October.

[quote="David Goldsmith"]

You know it makes sense.

Otherwise you'll just go on seeing the one name:

My name is Kramer and I am a ski geek.

I've made the first move gents, I think that it's time that you followed. Very Happy

A snowHead

Kramer, but we are all just mere pale imitations languishing in the shadows of the great ski geek that is physicsman Laughing

A snowHead

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buns, agree

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Just to add my penny worth
I find what I’m happy to ski on depends more on the light than if the run is blue red or black.
I’ve skied blacks which on the first attempt in good light were fine then returned to them latter with the light going flat and managed to scare myself silly. I’ve certainly found myself stretched when I can't see the bumps. Though I’ve always got down in almost one piece

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Wohoo!!

On the bus back and forth to Banff I've been thinking further.... ignoring friction, it is pretty much impossible to go faster than you can by travelling directly down the hill. If you do the integration, any additional acceleration down the slope a carve will bring, will be negated as you carve back the other way. To some respects it would be like integrating under a sine wave.

So considering the experiment of the 'hare' going straight down the slope and the carvers taking the same time - I believe this has nothing to do with the acceleration due to the carved turns. My suspicion is that it could be down to (as GrahamN suggestes) work done by the body but I reckon the biggest factor will be friction. While on your base, you have a large surface area in contact with (potentially) soft snow. On edge, you have much less ski area in contact with the ground. In addition, the large pressure exerted whilst on edge will compact the snow it is contacting, so it is likely that the carved edge contact surface is going to be alot firmer (icier maybe) and have less resistive effect.

Adam

After all it is free

Here's an easy way to think about it:
Lets take an ideal situation, ignoring friction, which in fact would only slow you down more no matter what. You are at point A. If you are travelling uphill, you will be going to a point higher up from point A, which we will call point B. Since point B is higher up from point A, you will have more potential energy at point B. Since this is an ideal system, all of your energy is made up of kinetic and potential energy, and no energy is lost or gained (in a real system energy would be lost). So since you have more potential energy at point B, you must have less kinetic energy, and thus less velocity. Even without friction, this proves that it's impossible to gain speed while travelling uphill (without an outside source of energy, e.g. rocketpack, herring-boning uphill, etc.), and in a real system with friction it's even more impossible.

Or even simpler yet.... if you could actually continue gaining speed as you carved uphill..... we wouldnt need lifts and we would occupy ourselves by skiing UP and not down the skihill snowHead

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Ok, I'm no physicist (sp?) and no great shakes on skis either Very Happy

, but I refer the 'speed' debate back to my waterskiing analogy. Imagine two powerboats moving due north at 30 knots, both pulling skiers. One skier simply holds a straight line the other cuts across the line of travel 'carving' big esses behind his boat. How fast are they travelling?

Well of course in a northerly direction the are both doing an AVERAGE of 30 knots, but the one who is carving across is accelerating as he begins each turn, going well over 30 as he crosses the wake, and slowing almost to a stop as he finishes each turn (Has anyone ever done this? you know that it's true). In fact if you carve out hard enough, and you are good enough you can actually get enough 'slingshot' acceleration to pull alongside and (briefly) overtake the boat. Clearly the carving skier covers a lot more distance in the same time (measured by his trail in the water, not by the trail of the boat) and so MUST be doing a higher average speed.

Now substitute snow skiers, and gravity, on a perfectly smooth piste, assume perfect carved turns and the model holds true. One last point, Isn't it true (I'm sure this has been said with regard to downhill racers many times) that they lose speed when they lose contact with the piste? Isn't this because they lose the force of the carve resisting the turning force? (bearing in mind that downhill courses are not in a straight line). I think SimonN and Kramer are right in what they are saying. Smile

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eng_ch, Get an instructor to lead you down and not tell you that you're on a black until you get to the bottom (make sure you are unobservant enough not to spot the give-away pist markers) Very Happy

This is EXACTLY how Eric got Mrs Ax to do her first black last year and she was so surprised when we got to the bottom and he pointed out what we'd done, that we had to go back up and do it again! Very Happy

As you say, 90% of the difficulty is in our heads. (The rest has to do with getting the skis to point in the right direction at the same time!) Very Happy

eng_ch, if any of my friends are a bit wobbly on blacks then I just get them to follow my (aggressive) line. It stops them thinking about anything else, and also gives them the confidence that if you ski aggressively it makes most marked pistes a lot easier.

IMO loads of people get in trouble on steeper pistes because they spend far too much time going back and forth across the fall line, whilst getting more and more frightened looking down the piste.

You know it makes sense.

AxsMan, I have spent whole lessons almost exclusively on blacks without turning a hair because I trust the instructor not to take me anywhere I can't do. If we're on our own, I don't trust my or our judgement. Especially unknown ones - I won't do them. The problem with blacks is they can cover anything from a steepish red gradient to a vertical wall and I seem to be petrified of encountering a sheet ice vertical wall or couloir, unable to either get down it or go back. Someone on Epic the other week had a technique I want to try out, though - "don't look down the damn hill!". As Kramer said, psychologically I know I'm not going to encounter anything I can't do on a red (like is a different matter) because I can "do" blacks. But there isn't anything worse than a black to have the same mental effect. Or is there?

Otherwise you'll just go on seeing the one name:

AxsMan, the leading you onto the wrong colour is how my mate got me onto a black run at my first day at verbier at new year.

A snowHead

AxsMan, the problem with your waterskiing analogy is that the accelerating force (the skiboat) is not a constant: as the skier tracks to the side there is a higher force in the tow rope which would tend to slow the boat down, so the boat has to up the gas to keep at a constant speed, so it's doing more work. Gravity cannot do that - you only get the same downhill force whichever direction you are skiing, gravity does the same amount of work on you for the same vertical drop, so the same amount of kinetic energy.

Now to thinking about SimonN's demonstration of a carver getting down the hill at the same rate as the straight-liner. This would indicate that the carver does have more kinetic energy than the straight-liner. I personally doubt that this is possible without a significant amount of work from the carving skier, but I can see how this may just about be achievable with the sort of compression/extension work I described above.

But this would say that the carver should be able to get down the hill faster than the straightliner if they stopped carving at the apex of one curve and straightlined it from there. So why don't we see downhill racers weaving their way down the first part of straight sections of the course and then gliding out the bottom part of that section? And finally is not the loss of speed when the skier is in a jump loss of a ground effect, and so increas of wind resistance, rather than loss of "carving force".

Energy-based arguments are a bit difficult to make watertight though, particularly when the skier is moving at speed, as there is a significant energy loss to due wind resistance (just stand upright and stick your arms out and see how quickly you slow down).

A snowHead

The reason that a skier in the air is slower than one on the snow is to do with resolving the forces acting on them.

For the skier in the air, the only force (apart from wind resistance) acting on him is gravity, which is accelerating him straight down, with no forward component.

For the skier on the snow, gravity is acting on him straight down, but you also have the reaction of the hill to his weight, which is perpindicular to the gradient (as well as wind resistance). When you resolve the vectors of these two forces you end up with a proportional forward accelerative vector which is what makes him gain speed down the hill.

eng_ch, there are always itinerary runs, which most resorts have now made their worst blacks into.

Well, the person's real but it's just a made up name, see?

Kramer, of course....doh rolling eyes

! On that same argument though, if the jump results in a landing uphill from the takeoff point wink

, the skier is better off in the air (as on the uphill slope the component of gravity acting along the slope is backwards).

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Firstly, everybody is getting a little worked up and confused. I have never said you will continue to accelerate by turning up hill. What I have stated is that, if due to a force acting through the skis you are accelerating, there is a period of time when the graviational forces of going up hill are not great enough to counteract that acceleration and therefore for a period, however, brief, you will accelerate up hill.

The waterskiing analogy is, IMO, correct. The speed of the speedboat is constant to within .5 mph. It needs to be for competitive reasons. The boat goes down middle of the course at a set speed or in snow ski terms the fall line and the set speed = gravity. By setting the ski across the course of the boat, you accelerate and has been pointed out, you can get to the point where you catch the boat and the line goes slack. You are definately going significantly faster than the boat.

As for what you feel through when turning, it is the result of a g-force which you are resisting and this is what leads to increased speed.

Now, if you guys how say that you only decelerate once you leave the fall line, explain those who ride pipe. Why do they go from side to side to pick up speed? It is because they set an edge and use the effects of increased g-forces to build speed.

Lets try to take another look at this. Assume at all times you are carving WITHOUT sideslip.OK, you are going down the fall line at max warp! Now, if you turn there are a series of forces that come into play. There is a decellerating force due to the change of slope angle and if you do it very gently, this will slow you. However, when going at max in the fall line, your body was ony applying its own weight in a downward direction and you aren't accelerating any more. As soon as you set your edge across the direction of graviational pull, your body gets subjected to an increased force, which we know as g-force. This has the effect of increasing the weight that goes through the ski. As you are carving and not sliding, this force has to go somewhere and it gets translated into forward motion.

Now, the acceleration due to the increased load can be less than the deceleration due to turning into a less steep trejectory and you slow, but it is also possible to have the acceleration greater than the deceleration and therefore the turn makes you accelerate.

It is no different from the forces that mean that "skating" accelerates you. One is powered by human mussle, the otherby gravity.

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eng_ch, itinerary runs are the runs that aren't pisted or patrolled, but that are avalanche cleared. They tend to be the really steep narrow runs in a resort.

Far more scary than a black run!

After all it is free

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SimonN, gravity is a constant. It can't increase the g-force anywhere (gravitational waves notwithstanding).

"once you are at max speed in the fall line and are no longer accelerating, a correctly carved turn increases your speed".

I am afraid, no, not if what you mean is you can go faster.

If you are at max speed in the fall line and can't change your wax mid schuss, flatten your skis, attach a rocket pack, that is as fast as you are going to go. You will be at "terminal velocity". Turning can ONLY slow you down, although you might find that you accelerate in a sideways direction...

I am afraid those are just the sad realities of Newtonian physics.

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71% of skiers go 'out of their depth' - insurance report

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