 Poster: A snowHead
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guys...just go have fun and ski 
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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GrahamN
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Don't forget the other things that happen as well though. The period of registering the higher weight happens only while you are actively increasing the rate at which your body rises (i.e. the time when you are actively pushing).
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I accept that other things are happening. In skiing there are many other forces, but I was picking up on the comment that you cannot apply pressure yourself. I contend that we apply force and feel the resultant pressure when we extend our legs and raise our body mass upwards in a gravitational field. The scales will confirm this.
I am not sure that I need to increase the rate at which my body is rising. To maintain a constant upward movement against a force would require and additional force.
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Once you stop that push, your "weight" has returned to normal, but your body is still moving away from the scales,
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True, I would expect the lowered reading to stay lowered for a bit after I had stopped pushing. Ignoring the machanical nature of my legs, I would expect my body to continue upwards and to decelerate at 9.8 m/s/s ish, so the reading would return to normal as my body finally became stationary. Legs and all the other stuff will cause damping and drag and so forth.
Once I stop the push, if that is all I do, I would expect a lowered reading then a return to a normal reading.
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At some point you run out of leg length, and you can no longer even sustain the normal support of your body weight.
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I would be standing straight up. I could sustain it, but possibly you mean that I would not need to because I would no longer be in contact with the scales. I would basically have jumped. That's a pretty good illustration of the principles here.
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od of increased "weight" to make up the deficit. The bottom line is tha' don't get owt fer nowt - if you have a constant surface, and remain in a the long term moving a constant overall rate with respect to it, then the long term average of your apparent weight also has to remain a constant.
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Indded it will, though I might lose a bit of weight with all that exercise
For the purposes of skiing, we can increase the pressure on a ski or skis, edge the skis and so bend them. If we are moving, the skis will now turn and the tendency of our mass to continue in a straight line will keep the force acting downwards on the skis so keeping them bent. We may now BEND OUR KNEES.
To asses the force it is useful to refer to the pressure we fell in our feet.
I think that about wraps it up in accordance with Newtonian principles.
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You need to Login to know who's really who.
You need to Login to know who's really who.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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| thirty06 wrote: |
| I accept that other things are happening. In skiing there are many other forces, but I was picking up on the comment that you cannot apply pressure yourself. I contend that we apply force and feel the resultant pressure when we extend our legs and raise our body mass upwards in a gravitational field. |
I agree with you, but FastMan's point was that these are only transitory effects and that the main effect is achieved through the interaction of our body mass and our curved path over the snow. My point is that if we use these transitory effects (as in up/down-unweighting) then other transitory effects necessarily also follow - irrespective of any other forces we may be neglecting for the sake of clarity in the analysis.
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| I am not sure that I need to increase the rate at which my body is rising. To maintain a constant upward movement against a force would require and additional force. |
Sorry, but you are wrong there. Newton's 2nd law: force = mass * acceleration (not velocity), or force = rate of change of momentum (not mass*position). When standing in a constant position with respect to your skis, you are balancing your body weight by the force through your legs/feet/boots. When you push you accelerate your body away from the skis, so you change the upward velocity. If you then reduce your leg push to normal, you are again exactly balancing your body weight, but your body is still moving away from the skis, and will continue to do so until you reduce the force in your legs to below normal. I think you're sort of on the right lines, but just a little unclear in your own mind, as your next para then goes on to agree with me.
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Once you stop that push, your "weight" has returned to normal, but your body is still moving away from the scales,
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True, I would expect the lowered reading to stay lowered for a bit after I had stopped pushing. Ignoring the machanical nature of my legs, I would expect my body to continue upwards and to decelerate at 9.8 m/s/s ish, so the reading would return to normal as my body finally became stationary. Legs and all the other stuff will cause damping and drag and so forth.
Once I stop the push, if that is all I do, I would expect a lowered reading then a return to a normal reading.
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Yes, and the amount by which that reading is lowered, multiplied by the time for which it is lowered, exactly matches the initial higher reading. You may make the period of lowered force long and small (for the smoothest ride), or short and sharp (for a jump or suck-up type turn).
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At some point you run out of leg length, and you can no longer even sustain the normal support of your body weight.
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I would be standing straight up. I could sustain it, but possibly you mean that I would not need to because I would no longer be in contact with the scales. I would basically have jumped. That's a pretty good illustration of the principles here.
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Correct - you cannot sustain it, not because you aren't strong enough, but because your legs can't continue to extend indefinitely. The other thing it's important to realise here, is that we've now applied that extra push, followed it by a period of lower pressure/force, but we're now standing further from our skis. So we have to follow this by essentially the same process in reverse, to allow our stance to return to normal - so a period of lower force to allow us to return from that over-extended position to a more normal stance, and then a period of higher force/pressure to ensure we don't continue and end up in a low crouch. If the original intention was an up-unweighted/jumped turn we'd obviously make these as smooth (and so innocuous) as possible, but they'd still be there - you cannot escape them.
In mathematical terms - to return the body to be moving at the same velocity as the skis, you need the integral of force (that additional to body weight) over time to be zero, i.e. the area under the the graph of force against time, aka its zeroeth moment, to be zero; to return it additionally to moving at a constant position relative to the skis, you need the first moment of the graph of force against time to also be zero.
The point of my post was to make it clear that the period of increased pressure you talked about is necessarily followed by periods of lower than normal. In some places this is definitely advantageous, as the additional forces arising from what Fastman is talking about are sometimes disadvantageous, and if you can tie in the reduced pressure periods from these transitory effects to conincide with his external forces you get a win-win situation - and in instructor-speak this is essentially "managing the pressure through the turn" (and is what they're talking about, even if they don't always realise it). Get it the wrong way around though and you make things worse and end up with a very jerky means of skiing - with the possibilities of forcing the skis to break away at undesired (but, using the above analysis, mostly predictable) moments.
Hurtle Don't worry, this is nothing that an instructor should be saying on the hill, but is "about how skiing works", and is stuff he should be aware of, or at least be providing him some degree of background, if he starts getting into detailed technical points of pressure management.
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You'll need to Register first of course.
You'll need to Register first of course.
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GrahamN,
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Don't worry, this is nothing that an instructor should be saying on the hill, but is "about how skiing works", and is stuff he should be aware of, or at least be providing him some degree of background, if he starts getting into detailed technical points of pressure management.
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I know, and have no problem with it.
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I am not sure that I need to increase the rate at which my body is rising. To maintain a constant upward movement against a force would require and additional force.
Sorry, but you are wrong there. Newton's 2nd law: force = mass * acceleration (not velocity), or force = rate of change of momentum (not mass*position). When standing in a constant position with respect to your skis, you are balancing your body weight by the force through your legs/feet/boots. When you push you accelerate your body away from the skis, so you change the upward velocity. If you then reduce your leg push to normal, you are again exactly balancing your body weight, but your body is still moving away from the skis, and will continue to do so until you reduce the force in your legs to below normal. I think you're sort of on the right lines, but just a little unclear in your own mind, as your next para then goes on to agree with me.
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In free fall we might only have to consider the intial force, but I can assure you from experience that to maintain a body in a constant upward dorection with respect to the action of gravity requires a greater force than that required to maintain a body at a constant distance from the centre of the earth. The acceleration must needs be proportional to the resultant of all the forces imposed on the body. In my upwards extension, I will register a raised weight during all of the rising phase, not just during the intial acceleration from zero to whateveritis m/s
While you are applying a force to achieve upward movement, there will be a greater force and hence pressure measured by the scales. I can prove this by practical experiment.
Apart from that, I am in entire agreement. Given the scenario we are discussing here, the difference in timing and whatever may be negligible.
Gravity is of course relative when we are moving on skis, the resultant forces mean that the weight of the skier may act in a varying direction. Just don't tell the intermediates that or they start trying to lean over like wall of death riders.
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and if you can tie in the reduced pressure periods from these transitory effects to conincide with his external forces you get a win-win situation - and in instructor-speak this is essentially "managing the pressure through the turn" (and is what they're talking about, even if they don't always realise it). Get it the wrong way around though and you make things worse and end up with a very jerky means of skiing - with the possibilities of forcing the skis to break away at undesired (but, using the above analysis, mostly predictable) moments.
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Quite. Back to skiing, one of the hardest things for people to get seems to be the timing of extension and retraction.
Most common fault is no retraction at all. the turn is initated and the pressure\force is applied to enter the turn. The skier remains extended as the skis turn through the fall line and then remains extended for a greater or lesser period.
Some drop down suddenly as the skis as nearly across the slope, resulting in the 'unweighting' you have analysed , followed by a hard edge stop and disruption of their rhythm.
Some just don't vary their height after the inital turn entry and end up in the diagonal slidey'turn' that has them fighting to push their outer ski with a straight leg. They tend to adopt a very wide stance and\or a wedged stance.
Some do the whole thing backwards and use the negative phase weight effect that you describe. They start as prescribed, but the relaxation starts to come later until they are crouched as they are aligned with the fall line and standing high as they are in the transition phase.
I think I did this for while back in the seventies, I'm not even sure how I manged to ski at all with that going on. As far as I can recall, the effect is a series of rotating jumps with the skis coming into full contact at the transition point (maximum angle to fall line) and much less pressure at the apex of the turns. In extremis, it was possibly to leave a series of diagonal 'tracks' with no curved part. Being a teenager enabled me to carry out this bizzare athletic manoeuvre and not notice that it was nothing like what I was supposed to be doing. On steeper or more difficult slopes it results in a wide, strenuous, slideslip followed by a drop\rotate\push to slow down and so on. The skis get scratched to blazes and the edges wear off quickly, the skier gets battered by any rough surfaces and is largely unable to ski soft snow. People go on like this for years. I think it's called 'the plateau'.
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| thirty06 wrote: |
I can assure you from experience that to maintain a body in a constant upward dorection with respect to the action of gravity requires a greater force than that required to maintain a body at a constant distance from the centre of the earth. The acceleration must needs be proportional to the resultant of all the forces imposed on the body. In my upwards extension, I will register a raised weight during all of the rising phase, not just during the intial acceleration from zero to whateveritis m/s
While you are applying a force to achieve upward movement, there will be a greater force and hence pressure measured by the scales. I can prove this by practical experiment.
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If you are so convinced of this, then I look forward to the publication of your paper refuting Newton's First Law, and your ensuing Nobel Prize for Physics. In the mean time, do not be led astray that in order to maintain that upward movement you do still have to be doing work (both in the non-technical sense, and the technical sense of expending energy), as you are moving that force through a distance, and giving your body increasing potential energy. That force required to maintain a constant velocity is though still only that required to match your normal body weight .
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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It seems to me that a lot of the time the instructor may as well say "force" rather than "pressure", as they are both alluding to the same thing. It makes me feel a bit uncomfortable talking about pressure with a constant area when you could just say force. I do have a physics/mechanics background incidentally. I know it's an issue of semantics but it bugs me.
I like the example of lifting up one foot and instantly doubling the pressure on the other, this would be a situation when it's definitely relevent to talk about pressure.
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Right, so after a week or so trying out different sets of scales, it would turn out that GrhamN is right. However, looking it up in a book doesn't actually count as knowledge and furthermore "Smarty gave a party, but nobody came".
Anyway, I agree with Musehead that we should question the words used to describe the parts of skiing. If the instructor is repeating patter that he has been told to use and does not fully believe the words used, then I believe that the effect could be negative.
If you have to say "Feel for the pressure" and doubt what you are saying, it could come across.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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One of the things that strikes me is that there is a lot of misuse of physics terms in ski instruction and some of it is done knowingly because it "kind of works" in the sense that people learn to do the right things (even if the explanation was wrong).
I was having a lesson last year and my instructor and I were traversing a slope off-piste in nasty heavy crust. We needed to carry speed to get over a rise but there were some tricky patches - subtle speed control is a bit of an issue in that kind of snow. He suggested just sinking lower to "pressure" the skis and that this would allow me to shed speed without putting in turns or catching a ski in the crust. It worked brilliantly (just made you gently turn up the hill without changing direction much). But I couldnt get my head round the physics of the pressuring. He smiled when I asked him
"Ah you caught me. You're not pressuring the ski to control speed. You are actually just angulating more as you flex and carving up the hill. Sinking down actually reduces pressure on the ski for a moment. The thing is that the idea of pressing to control speed just works for people"
And I think he was probably right and wrong. IYSWIM
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