 Poster: A snowHead
|
| veeeight wrote: |
| GrahamN, In my book, P = F/A therefore P is a result(ant) product of a Force over area, so change one of those two things and yes, you change the pressure. As the units are N/m2 I think that points towards the product (or result) of a Force being applied. Hence resultant. |
You could equally state F=PxA and say that force is a result of pressure - think of the force on a piston caused by the increase in pressure as fuel is burned in a cylinder - and quote the result in Pa.m2
| Quote: |
| Force is not what gets your body to change speed/direction. |
Erm ... you've just flatly contradicted Newton's 2nd law of motion!
|
|
|
|
|
|
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
|
Well, if you guys want to continue using the term Force if that's your preference, who am I to stop you, but you'll be going against every ski system and book that utilise Pressure in the sense that I've used it.
The more astute books/ski systems correctly refer to "Pressure Control" - Sensory Skill as opposed to just "Pressure" - with the caveat that it's a sensory skill, as opposed to an applied one.
| Quote: |
| You could equally state F=PxA and say that force is a result of pressure...... and quote the result in Pa.m2 |
Sure you can, mathematically, but in the application of skiing not that many Pascals are used.
|
|
|
|
|
|
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?
|
| veeeight wrote: |
GrahamN, In my book, P = F/A therefore P is a result(ant) product of a Force over area...Hence resultant.
|
No disagreement there but "Pressure is a resultant force" and "Pressure is a result of applying force" are two completely diffrent statements. I'f your going into bat on the difference between pressure and force, don't break its handle off first (apologies for plagiarism, Sir Geoffrey).
| Quote: |
You can extend, or apply a Force in powder, but unless a platfrom is built then no change in speed/direction occurs. So in bottomless Utah powder, you can go on applying all the force you want, but until you get pressure built from a platfrom nothing will happen. So no, Force is not what gets your body to change speed/direction.
|
Sorry...back to O-level physics there, or Mr Newton. F=ma. No mention at all of pressure. Agreed that extension will achieve nothing if you have no platform, but that's because there's zero force resulting from that extension, as there's nothing to push against (OK there's no pressure either). This is verging on angels and pinheads here though - but it's a fundamental misunderstanding that it's pressure not force that moves a body.
| Quote: |
| Quote: |
It certainly is possible to "apply pressure [force]" early in a turn, by extending the legs, so projecting the CoM away from the skis
|
If accompanied by nothing else then all that happens is that the skis are likely to break away. In reality and hopefully, you are using you ankles, knees, hips in that order so in reality you are getting some increase in edge angle as you extend. As I've said before, I can live with applying force, but as pressure is a resultant, it is technically incorrect to "apply pressure". If you want to apply force early in the turn this should be in conjuction with increasing edge angle so that the skis aren't tempted to break away. I can also live with the phrase building the pressure early in the turn.
|
While those should all be going on for good skiing, I'm not convinced that they are necessarily related. Compare two people going round the same turn a) the park-and-rider with a 30 degree edge angle and b) the active carver extending into the turn with the same edge angle. b) will come out of the turn faster but would have used the same edge angles. Of course at some point there will be a limit at which the ski will break away. Thinking about this a bit more, doesn't the maximum grip come at about a 45 degree edge angle - the more it's a V and less of an L the more grip you get (the higher the contact pressure  and the greater the local deformation in the snow and the deeper the ski will cut) - beyond which the geometry starts increasing the amount of edge in the snow, reducing the contact pressure and the sideways component also increases, starts exceeding the snow's critical limit.
| Quote: |
| Quote: |
| I don't see that increasing the edge angle in itself change the pressure on the snow. |
That's a pretty fundemental CSIA L1 question, what is the relationship between edging and pressure, yes it does.
|
So can you explain how please? As I explained earlier I'm assuming there that the ski is on edge to start with. So say you have a 30 degree angle and change it to 40 degrees. How does that, of itself, change the "pressure"? The only thing I can think of here is the change of edge profile with angle above, but that's contact pressure rather than force/pressure we're talking about (i.e. what you'd feel in your legs).
|
|
|
|
|
|
You need to Login to know who's really who.
You need to Login to know who's really who.
|
veeeight, I think that what you've described as "pressure", a physicist or engineer would call "reaction" - which is a force.
Thus if the skier applies a force by leg extension, in light snow the ski will just accelerate through it, in more consolidated snow a reaction will be generated which will provide the centripetal force for a turn.
|
|
|
|
|
|
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
|
|
GrahamN, great minds, or perhaps those having followed an identical academic path, think alike.
|
|
|
|
|
|
You'll need to Register first of course.
You'll need to Register first of course.
|
In my non-professional skier's mind I've always kind of thought of applying pressure as how to make a ski accelerate turn by turn, involving work and getting tired, and pressure control as how to make skiing as effortless and mechanically economical as possible.
Disclaimer: I know it's semantics, that they're not mutually exclusive etc etc
|
|
|
|
|
|
|
|
| veeeight wrote: |
| Well, if you guys want to continue using the term Force if that's your preference, who am I to stop you, but you'll be going against every ski system and book that utilise Pressure in the sense that I've used it. |
But it's you who was the one who has first quibbled about "applying pressure" (in several previous threads), and wants to distinguish it from force. It's then incumbent on you to use those terms correctly - if you live by the sword you die by it too. If the ski manuals use the terms incorrectly - then maybe you should be taking them to task as well . As is obvious from above, I'm not worried about "applying pressure" or "applying force" in general conversation - it just becomes important when we get to the technical nitty-gritty.
Last edited by Then you can post your own questions or snow reports... on Fri 14-09-07 14:14; edited 1 time in total
|
|
|
|
|
|
|
|
|
GrahamN, laundryman, don't have nearly the instant command of mechanics you two clearly do ( 'A' level physics and maths long since faded into distant past) but even I could see that immediately
|
|
|
|
|
|
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.
|
|
slikedges, me too - maybe we've had the same teachers!
|
|
|
|
|
|
|
|
Saying that you can "apply" pressure is like that old chesnut, where people claim to be able to "stress" something.
You can "strain" something, and stress may be exhibited as a result of that strain.
It's the same case with pressure - it's the reaction (thank you laundryman, or as a result (thank you GrahamN)) of a force being applied. Any yes, I can see now how you thought I was using the term "resultant force" to mean "net force" as opposed to a result being produced by a force 
If I press my palm against your chest, I'm technically incorrect if I say that I'm applying pressure against you. You will feel an increase in pressure when I push harder, but no way am I applying more pressure to your chest. More force, yes, pushing harder, yes, applying more pressure no, you feeling more pressure, yes.
So - in the same way that you cannot stress something, you cannot apply pressure (as it's a result of a force being applied).
| Quote: |
| Thinking about this a bit more, doesn't the maximum grip come at about a 45 degree edge angle - the more it's a V and less of an L the more grip you get |
The determining factor of a gripping edged ski is the "critical edge angle" - see Ron LeMaster!
Last edited by Ski the Net with snowHeads  on Fri 14-09-07 14:31; edited 3 times in total
|
|
|
|
|
|
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
|
|
veeeight, doesn't it depend on the size of her chest?
|
|
|
|
|
|
And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
|
|
Wear The Fox Hat, Always!
|
|
|
|
|
|
brian
brian
Guest
|
| GrahamN wrote: |
Force, not pressure, is what gets your body to change speed/direction.
....
But pressure, not force, is what gets the ski to bite into the snow
|
Pressure is simply a measure of the force applied per unit area of a surface (acting perpendicular to the surface because force is a vector).
You can't really say one rather than the other is doing blah, since one is just a measurement of how the other is applied.
|
|
|
|
|
|
brian
brian
Guest
|
| veeeight wrote: |
If I press my palm against your chest, I'm technically incorrect if I say that I'm applying pressure against you. You will feel an increase in pressure when I push harder, but no way am I applying more pressure to your chest. More force, yes, pushing harder, yes, applying more pressure no, you feeling more pressure, yes.
|
Oh dear oh dear, you really don't get this do you ?
P = F/A. If F goes up, so does P. It's really not that complicated (for mortals anyway).
|
|
|
|
|
|
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
|
veeeight,
| Quote: |
Saying that you can "apply" pressure is like that old chesnut, where people claim to be able to "stress" something
|
There is nothing wrong with shorthand, which is frankly the only windmill you seem pointlessly to be tilting at: the phrase 'to apply pressure' is commonly used and understood, even if not strictly correct (on this point, I bow to the superior knowledge of physics demonstrated by you and others here). To use 'stress' as a verb is semantically incorrect.
|
|
|
|
|
|
Poster: A snowHead
|
|
Frames of reference, brian, frames of reference. Besides which, am I applying P, or F?
|
|
|
|
|
|
brian
brian
Guest
|
| veeeight wrote: |
| Frames of reference |
Must remember that the next time I'm trying to bluff my way out of something I obviously haven't a scooby about
| Quote: |
Besides which, am I applying P, or F? |
You are applying F. P expresses how that F is applied to the surface in question.
|
|
|
|
|
|
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?
|
Hurtle,
Don't get phased by this sort of thread ...all you have to understand is what works for you and how you interpret a body movement. Everyone is different and goes into this with varying degrees of depth. All you need to do is ask a direct question of what you do or do not understand. I don't think the posters here will mind if you ask them to explain it to you so you get it...and if they do, then remind them to get over themselves....  . Fortunately, this Board isn't infamous for that sort of stuff.
Most just enjoy sharing their knowledge and experience.....which is the best thing about the place, IMV
|
|
|
|
|
|
You need to Login to know who's really who.
You need to Login to know who's really who.
|
C'mon guys, let's keep this civil.
| brian wrote: |
| You can't really say one rather than the other is doing blah, since one is just a measurement of how the other is applied. |
Actually you can, since they're different properties. Try using a pole with a piste basket to assist with a jump turn on soft snow and it'll sink in, but use a powder basket and it won't. Same amount of force but totally different outcome, as it's the pressure the snow can withstand that's important. Conversely, if you stick your head above a ridge on a windy day on Cairngorm you'll probably be OK, but stand up on that ridge and you'll probably get blown off. Same pressure from the wind, but a greatly increased force.
Still looking forward to the explanation backing up
| Quote: |
| Quote: |
I don't see that increasing the edge angle in itself change the pressure on the snow.
|
That's a pretty fundemental CSIA L1 question, what is the relationship between edging and pressure, yes it does.
|
with specific reference to the problem I outlined above .
Last edited by You need to Login to know who's really who. on Fri 14-09-07 16:43; edited 1 time in total
|
|
|
|
|
|
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
|
|
|
|
brian
brian
Guest
|
| GrahamN wrote: |
Actually you can, since they're different properties. Try using a pole with a piste basket to assist with a jump turn on soft snow and it'll sink in, but use a powder basket and it won't. Same amount of force but totally different outcome, as it's the pressure the snow can withstand that's important. Conversely, if you stick your head above a ridge on a windy day on Cairngorm you'll probably be OK, but stand up on that ridge and you'll probably get blown off. Same pressure from the wind, but a greatly increased force.
|
Yes, obviously, in the first example you're showing how pressure varies with area and in the second how it varies with force. I do understand the physics
Without force, there is no pressure, so going back to what I originally quoted:
"But pressure, not force, is what gets the ski to bite into the snow"
The ski will bite the same amount for the same pressure but this pressure can be achieved either by reducing the area or increasing the force. There must be a force applied or there would be no pressure.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
brian
brian
Guest
|
|
|
|
|
|
|
GrahamN, sorry, not biting into yet another ping pong session, the schoolboy taunting isn't going to change that. Yyou can read it for yourself in Ron LeMaster's book. I'm not giving away all the answers! Or some other informed instructor can have a go. Or you can ask at your next race training session.
|
|
|
|
|
|
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
|
| rob@rar wrote: |
Not sure I agree. Even when I started skiing as a kid I wanted to know in close details what I needed to do, why it was important and what effect it had on the skis. Other kids were happy to be told to put the tips of their skis together and swoop like a bird (or whatever nonsense the instructor mumbled on about). |
that is so like me... and ex-boyfriends son was the same - drove ex slightly bonkers that the kid wanted it discussed rather than just following the movements like his older brother (and father).... OTOH he and I skied well together as I was quite happy to have discussions about why you sit in the seats the picture says on the chairlift, point pole tips forward and down, etc etc etc
|
|
|
|
|
|
And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
|
| little tiger wrote: |
| I was quite happy to have discussions about why you sit in the seats the picture says on the chairlift, point pole tips forward and down, etc etc etc |
My curiosity didn't quite stretch that far In fact, curiosity isn't the right word - I needed to know the information as I couldn't seem to understand what I had to do without that info. I sure I was a right PITA to teach!
|
|
|
|
|
|
|
|
|
|
|
You know it makes sense.
|
JT,
| Quote: |
Don't get phased by this sort of thread
|
I'm not phased. These are the points I wanted to make, irrespective of the skiing standard of the contibutors on here, which in some cases is ostensibly somewhere higher than stellar:
1) - this one bears repetition - easiski is unfailingly clear, concise, un-precious and un-patronising, and uses language which you don't have to have a doctorate in physics to understand, whether she is engaging with another instructor or with a relatively inexperienced skier. I have worked in other areas where jargon abounds - it is always unnecessary and I abhor it.
2) Martin Bell's post resonated strongly with me, and that obviously is not unconnected with the first point.
3) - this is entirely personal - some of the knowledge being aired is so very technical, rather like the finer points of Haydn's composing technique, that I neither need nor want to know it, indeed it might reduce my enjoyment of a sport which I only pursue for a few weeks in the year. I do not dispute that it might be interesting or valuable for competitive skiers. That is not to say that I wish to remain completely ignorant of skiing technique, I was merely saying that 'race techniques' - not my phrase, by the way - do not apply or need to apply to my own skiing.
4) to have a ding-dong about whether it is strictly correct to use the phrase 'apply pressure' strikes me as being so up itself as to have completely disappeared into a region where light no longer shines.
That's all. 
As to
| Quote: |
All you need to do is ask a direct question of what you do or do not understand
|
I do indeed ask - when there is something I want to know - and, as you say, people on here are mostly generous with answers. In fact they are sometimes over-generous with answers, but that's another issue.
|
|
|
|
|
|
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
|
|
easiski, Hi there! You appeared while I was composing my little diatribe! Nice to see ya!
|
|
|
|
|
|
Poster: A snowHead
|
Hurtle, I got lost in this thread a good few posts ago, so you're not alone. However, I disagree with:
| Hurtle wrote: |
| I was merely saying that 'race techniques' - not my phrase, by the way - do not apply or need to apply to my own skiing. |
The 'race techniques' that have been discussed in this thread are as equally applicable to your skiing as they are to some of the very good skiers in this thread (that doesn't include me, I hasten to add). You might not choose, or indeed to be able, to use them to the standard required to ski round some gates in a quick time, but that doesn't mean that you won't use those skills as you ski around the pistes at your own rate. By and large racers don't do anything that is different from the rest of us, they just do it much, much more effectively 
|
|
|
|
|
|
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
|
|
rob@rar, Possibly. Permit me another musical analogy: I learned piano to a fairly high standard and, obviously, had to learn a lot of technical 'moves' to get to that standard. But I realised that to undertake the work required to play extremely virtuoso stuff was pointless for me - my hands are small, I was not pursuing music as a profession and I didn't much want to practise more than four hours a day anyway. I therefore did not push myself to learn, say, the Brahms Concerti; I love them, but I can live without being able to play them.
|
|
|
|
|
|
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?
|
|
Hurtle, shock, horror. You don't actually have to read these threads, it isn't compulsory!
|
|
|
|
|
|
You need to Login to know who's really who.
You need to Login to know who's really who.
|
Hurlte (and easiski), as I said earlier, I have no problem with the phrase "apply pressure", even though it is technically incorrect, because we all know what it means - and in cases where I don't know how to achieve it I ask the instructor to explain more fully.
I did though start the pressure discussion because of this:
| veeeight wrote: |
This concept of "apply pressure" is one of those things I've been meaning to address in a skiing myth thread, but haven't got around to it. It does kinda do my nut in slightly when I hear instructors/coaches tell people to "apply pressure early in the turn".  |
Knowing from previous discussions that it's a bee in veeeight's bonnet I thought it useful to try and get his physics straight, as I didn't believe from that post (and this discussion has reinforced that belief) he's really got on top of it and properly thought it through. 90% of the time it's irrelevant - but there are occasions where understanding the basic mechanics does shed light. (And, BTW, none of that physics should come as a shock to anyone who's passed O-level/GCSE physics or maths - although it helps to still be able to remember it ).
While at an artistic level it's about emotion, exhilaration, grace etc, at a technical level skiing is purely applied mechanics.
|
|
|
|
|
|
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
|
|
GrahamN, What exactly is wonky about my physics? We've already established that it's technically incorrect to "apply pressure", that I miscommunicated a "resultant force" instead of a force as a result of? Your reluctance to accept the mechanics and relationship of a ski bending with edging and pressure does not make my physics wonky. Or are you just being provocative for the sake of being provocative?
Last edited by Anyway, snowHeads is much more fun if you do. on Fri 14-09-07 21:15; edited 1 time in total
|
|
|
|
|
|
You'll need to Register first of course.
You'll need to Register first of course.
|
| veeeight wrote: |
| Hurtle, shock, horror. You don't actually have to read these threads, it isn't compulsory! |
Indeed, but I think I 'do' "general skiing" as referred to in the thread title, so I might have learned something useful from reading it.
|
|
|
|
|
|
|
|
| veeeight wrote: |
| Force is not what gets your body to change speed/direction. |
Rubbish!
| veeeight wrote: |
| So the other viable alternative is to increase the edge angle, which in turn will increase the pressure, which in turn will bend the ski more, resulting in a tighter arc |
As I explained above, I don't believe that physics supports the bit I've emboldened as a direct consequence - I gave above what I believe is the physical sequence of events (i.e. the tightening turn radius causes the increased pressure, rather than the other way around) - and as you won't discuss it, I have to stick with that (dis)belief.
I'm honestly not trying the be offensive, just accurate.
|
|
|
|
|
|
|
|
|
If you want some homework to do, then there are at least 3 influencing factors as to why pressure will increase when the ski is tipped on edge.
|
|
|
|
|
|
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.
|
| Hurtle wrote: |
| Indeed, but I think I 'do' "general skiing" as referred to in the thread title, so I might have learned something useful from reading it. |
So what have we discussed, apart from the latest 'Epic' diversion?
1) One footed skiing for improvement of lateral and fore-aft balance (including "javelin" turns).
2) Dynamic extension/flexion to take the "park-and-rider" to the "dynamic carver"
3) Fine edge control, following a pivot turn (the "Feathering" discussion)
4) Reaching with the inside arm to prevent incorrect rotation
5) Getting more of the turn done before reaching the fall-line (which can result in reduced pressure later in the turn)
6) a whole host of basic skill improvements (I listed those that I had experienced myself in the third post on this thread).
7) ...erm, there's probably something else in there too!
So I hope there was something useful in that lot .
|
|
|
|
|
|
|
|
|
GrahamN, Yes, I concede that I did learn one or two useful things, particularly when they were in easiski-speak.
|
|
|
|
|
|
|
|
FAQ
