The Bobski Blog

The Man and the Bracken revisited. If you have not already read it, you might enjoy going to www.bobski.com/technical papers  and read "The Man, The Bracken, and the Sport Psychology".

So.  It had not been cleared. A year later and when, in early May, the man went to the bracken areas, there it was, healthier than ever. True, in some places where it had previously been there was now either none or much less. But in others, it was flourishing in an abundance greater than previous years. There were even some areas where pulling it up was no longer even an option - it would have to be cut, at least for a while.

So how was he to "handle" this? What sort of things about it would he be likely to say to himself, and his self, about it? Should he call himself all sorts of fool for having even attempted the job; or for having believed he might have cleared it in just one or two seasons? Perhaps it wasn't possible: how would he know?

Clearly the possibility arose for depressing himself about it. It wasn't possible for the bracken to depress him; bracken is just bracken and totally indifferent to him or any one else. But if he were to do this, why would he do it? What would be achieved?

Would it be possible NOT to depress himself about it? And if he did, what would be the PROCESS of doing it? What would need to be in place for him to be able to? The man thought about this, and about what might be different to last year. Perhaps the circumstances had changed? If so, how might they have done so?

Slowly some differences became apparent. Firstly, last year in order to get to places where the bracken was he had had to clear brambles, rosebay willow herb, small areas of blackthorn and so on. This had allowed him access to the bracken but in the process had let in more light so this year's bracken growth was enhanced. So, clearly the graph of his bracken clearance would not be a straight line - it would have accelerative phases and troughs.

Secondly, the weather this year was much better than last year; temperatures are higher, rainfall just right for growth. So he must be careful not compare like with un-like. At this point Sport Psychology came in again. His end goal, his dream, was to clear these two large areas of bracken, and it is important to have a dream and a long term objective. But if that had been ALL he had - if this had been the only kind of goal he knew about - then it could easily have been disappointing and he might have depressed himself.

Fortunately the man knew a little more about goal setting and he knew that he could also choose to create and adopt other kinds of goals. He could if he chose set himself PERFORMANCE goals; say, more yardage of cutting, in fewer minutes of work. But the man had tried these sort of goals in other areas of his life and while they had helped then, the idea didn't seem to fit in well with this job.

He was afraid that he would find that by accepting goals of this sort he would become too intense about it, and miss all the good things going on around him. When he had first set out to do this job, he had made that mistake. He had been so focused, so intense that he found hiself missing the bird song, missing the sound of the river, and missing the opportunity to stop and look at the distant hills.

So, he began considering PROCESS goals as an option. And this is the type he chose. With a process-goal mind-set he could set a goal of "making sure he did at least one area a day". He could even change that if he later wanted to, by making it "at least 12 days out of every 14" or some such. That would still quite likely be a challenge - what about the mornings he didn't feel like getting up, or the days the weather was lousy. Yes, sounded good.

He could break it up and set himself the goal of pulling up half of it, and cutting the other half; then swapping the halves over.  He could include in his daily goal "stopping at least four times, to rest and look at the view, listen to the birds and hear the river singing along in the valley" why not? What a beautiful sort of goal. Wouldn't be a bad sort of goal to set yourself when you were skiing in the mountains.

He noticed that one effect of thinking things through like this was that he found he was not falling for goals that made him impatient, or inadequate. It didn't even matter that perhaps the dream goal of total clearance might not even be "realistic" - who knew, perhaps expecting total clearance in one lifetime was just pie in the sky.

It didn't matter, what mattered was sticking to the task, AND ENJOYING THE PROCESS.  Maybe the world was a slightly better place if this bit of it had some bracken, who was he to say? What right did he have to dictate what would happen, perhaps it was better if he just stuck to what HE was intending to do, and leave the rest to the fates.

Maybe, if he didn't ever become quite the skier he had once dreamed of, that was a better solution because it meant he would always have the possibility of improvement, and after all it was in working toward that improvement wherein lay the real pleasure.

Bob Valentine Trueman

My fellow coach Dave Tapley reported to me that one or two skiing blogs have recently been filled with (largely rubbish) posts from well-meaning but clearly confused instructors. The kind who can ski well no doubt, but who's comprehension of skiing is shall we say rather shallowly based.

The discussion hinges on whether or not "parallel" skiing is actually possible. Dave quoted his own observation that when you are "carving" perfectly you can inspect your skis' tracks and they look to be perfectly parallel. However, they are not drawing the same radius arcs (part-circles).

So, After some thought I wrote to a pupil and friend of mine,  physicist Tony York. Here is the e-mail like what I wrote.

Let's say we have a skier effecting an arc, a perfectly "carved" arc - an arc during which both skis slide perfectly (no skid) -and let's say that his skis are parallel to one another all the way round that arc.

For this to happen, the inner ski must perforce travel a shorter distance than the outer ski.  For this to happen without skidding, the inner ski must either, tilt more, or bend more, or a combination of both. Were this not to be the case, they would necessarily be describing segments of arcs of non-concentric circles.

To bend more it would need to be receiving greater centripetal force, which we know would be very unstable for the skier, so optimally no more than 50% of the force should be being resisted by the inner ski. Unless - I wonder - being nearer to the circle's centre it inevitably receives more force ? ?

Even in this scenario, the inner ski must be tilted slightly more than the outer ski, or it would skid. This is because were it to be tilted to the same degree it would be describing a circle of the same diameter as the outer, but in a different location - they would not be concentric; and if you draw this out on a piece of paper it becomes obvious that the two circles must cross (twice) which thereby denies the "parallel" requirement of this experiment.

Now, there is plenty enough bio-mechanical movement in the hips and ankles to permit this variation, but here a little confusion arises in my mind ( which is rather unusual  -  because usually there is a lot of confusion in my mind; I must do this again!).

There will be one aggregate centre of mass for the skier, supported against the centripetal force by two platforms.  Here then is where my confusion arises.

Where, precisely is the centripetal force's own centre of origin? Or is this a daft question?  Is there, for example, just one centre of centripetal force, or since there are two platforms, are there also two centres of this force? After considering this I feel there must be two, because each ski (platform) is resisting a force, and I feel that this necessitates having two forces, coming from two slightly different directions. This being the case, then there are two reasons for the inner ski to tilt more - 1) in order to present a platform at 90 degrees to the force, and 2) in order to enable the ski to slide perfectly around a circle of smaller radius.

But if this is so, then if you followed the directional lines of these forces (or this force) from whence do they emanate? Is it for example on the snow's surface? Or precisely at the interface between the platform and supporting surface? Or - does it emanate from somewhere else, underground? And if so, how far away/down?

I think it must be at the interface only, which is where the force and the resistance meet. Am I right? After all ( I conjecture) unless there is resistance, there will be no centripetal force - in effect they are one and the same???? Without the one, you cannot have the other.

Bob

PS - It's just occurred to me that the bend in the ski is created at least in part by a force from ahead of it, acting on the shovel through a couple between the shovel and the ski's centre. The shorter the radius of the circle being followed for any given tangential speed, wouldn't the force be inevitably greater? So might we not get more bend anyway even though the skier's mass was being equally distributed between the two skis?

Tony, after considerable cogitation answered as follows, and I'm very grateful to him.

OK, (he said) here are my thoughts so far:
 
Since the skis are going round curves of different radii, and are therefore travelling at different speeds, it is mathematically easier to say they are both moving with the same angular velocity (ie they would both take the same time to complete a full circle).  The expression for the force is then mw²r (m is mass, w is angular velocity, r is radius).  Because r is greater for the outside ski, there will be more force, which is what the skier needs, in order to be stable.
 
So far so good - but then how do the skis provide this force?  If the outer one is producing more of the centripetal force, and they are both at the same angle, it will bend more, making it impossible for both skis to be "carving", as the inner one is following a tighter curve.  If the inner one is tilted more, perhaps it could be describing a tighter arc, but be bent less, consistent with it producing less force.  I should stop now while I'm ahead, but I have a horrible feeling that if you look at a still photo of a racer in a turn, the outside ski is tilted more! (Yes, but you'll usually see that the inner ski is all but "floating" and is not actually carving, even though that's what they would like. Bob)
 
The bending of the ski is a result of the snow pushing against it, but that won't be simple either.  Even in the simplest imaginable scenario of the same force from the snow against each cm of the ski, the front of the ski will have more bending moment, as it is longer than the tail.  Whether this leads to more actual bending depends on the stiffness of the ski, which varies along the ski in a very complex manner, I would imagine.
 
As implicit in last para, as far as the ski is concerned the force comes from the snow immediately in contact with it, but that snow is in turn supported by the snow beneath it, which is in turn supported by the ground beneath it.  This is of course why the skier sinks deeper into powder before there is enough force generated to support him/her.
 
I don't think the idea of a "centre of centripetal force" is useful.  The vector sum of all the forces from both skis must pass through the centre of mass of the skier and be directed towards the centre of the circle in which he/she is travelling.  One also needs to be careful in talking about reaction forces.  This vector sum is effectively a single force acting on the skier.  There is no sense in which the skier is in equilibrium; he/she is being continually accelerated towards the centre of the circle.
 
God knows how ski designers do the business, & God knows how any of us can actually get the skis to do what we want (sometimes).  I should probably stick to making furniture or high energy nuclear physics; that would be simpler. 

I am very grateful to Tony for his observations, and if anyone wants to join in, then please do so; it won't make anybody's skiing any better, but it keeps the old grey matter from atrophying any more quickly than is necessary! And it sure as Hell beats "doing turns!"

Bob Valentine Trueman

The greatest satisfaction I get, and have got from coaching, has been so repeatedly to watch my pupils change their beliefs about their own potential. This happens with both men and with women, but more so I think with the womenfolk.

Far too frequently, women "of a certain age" come to their first Bobski coaching week scarcely able to believe that they will be able to bring about any serious changes in their skiing. They typically arrive in a last-ditch, more-in-hope-than-expectation mode. Frequently I have been the absolute last hope - "If this doesn't work, I'm giving up, the family can go on their own and I'll take up macrame!"

So far, fingers crossed, I haven't had a failure. Big changes in technique have usually not happened quickly - and nor should they, skiing isn't easy! - but changes in self-belief are the norm. "Hang on a minute, this is something I am going to be able to do, if I work at it. I'm not a failure, or a dead loss."

What a marvellous thing to happen. What more could anyone do for another person, than to do something that helps them change their own self belief?

Here are extracts from a report of a recent scientific paper which described a research project into what the differences were between women who were overweight and stayed that way, and women who were able to change. I'd be interested in any feedback.

If you are what you eat, what you eat has a lot to do with how you think about yourself, says a QUT PhD researcher whose study is part of an international research project on the healthy ageing of women.

She surveyed more than 560 South-East Queensland women aged between 51 and 66 on their exercise and diet habits and found that although women in their 50s were keen to make healthier diet and exercise changes, they had few effective strategies to draw upon.

"This is an age when women's weight tends to peak, and almost two-thirds of the survey group were overweight "Ms Anderson said.

"Self efficacy is our belief that we can produce the result we want to produce, so a person with high dietary self-efficacy believes they can eat healthily no matter what - even when bored, upset, tired, on holiday or at a party.

"A person's level of self-efficacy determines how hard they try and how long they stick at things in the face of difficulties. People with high self-efficacy are motivated and optimistic - when the going gets tough, they keep going.

"People with low self-efficacy avoid difficult tasks and when things get tough they are more likely to give up. We can improve our self-efficacy by developing skills, having role models and getting encouragement from others."

"Education is also a factor - women with a tertiary education were more likely to have high self-efficacy for exercise."

Ms Anderson said her findings were timely given the population was ageing and women lived longer than men.

That last paragraph reminded me of Jackie Mason's gag about "Why do men die before their wives? - Because they want to."

The key element in all this extract, for me, is that We can improve our self-efficacy by developing skills. A great deal of what we do on my courses is aimed specifically at this; without it all the physical stuff just runs off like water from a duck's back.

Bob Valentine Trueman