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Where is pressure greatest in a turn and why? How much does a knowledge of physics help a ski instructor be a better instructor? - Page 2

post #31 of 54
Quote:
Originally Posted by borntoski683 View Post

well I thought the discussion was about pressure yes?

 

Yes you are correct in stating that motion comes from relieving pressure in the right ways.


The conversation was about the validity of instructors understanding physics. 

 

 

 

I assure you...you can take an inatimate object like a tobaggan, but it on a ski hill, and it will slide down the hill all by itself - "no right ways" or pressure releif required.  "Relief" implies the pressure was there to begin with.

 

You are missing major concepts in your understanding here.  Normal Force.  Look it up.  You also seemed to combine the forces acting from the skier to the snow with the forces acting on the snow to the skier and called that the "diagonal resultant"...incorrect.  You need to pick one frame of reference, and stick to it.  I presented the two options in my post earlier.

post #32 of 54

instead of trying to  discredit people and put them down, why don't you try to find some common ground and get back to the discussion about pressure in a ski turn?  I find nothing wrong with anything I have said.  Sorry to hear you can't understand it properly.

post #33 of 54
Quote:
Originally Posted by borntoski683 View Post

instead of trying to  discredit people and put them down, why don't you try to find some common ground and get back to the discussion about pressure in a ski turn?  I find nothing wrong with anything I have said.  Sorry to hear you can't understand it properly.


There is no common ground, when you are just wrong.  This is basic stuff.  I am not trying to discredit or put you or anyone down.  Just stating facts.  Normal force - look it up.  Its not an "opinion" or "point of view".  If you care to re-word your posts to algin with basic laws of newtonian physics - GREAT - I and others will read it. 

 

If you think I am wrong - please state where and why.

 

Where is pressure greatest?  I answered it.  Where disruption to our mass is greatest.  Physics proves that.  If you dont understand physics, you wont understand why this is true.  This is why its important at the high levels anyway for instructors and coaches to understand physics. 

post #34 of 54

ski dude I don't actually care if you are right are wrong.  I think you are actually just describing different things and creating conflict for nothing.

 

I know I am not wrong about what I have said.  It seems you either you did not understand my words or you just like conflict.  

post #35 of 54
Quote:
Originally Posted by borntoski683 View Post

ski dude I don't actually care if you are right are wrong.  I think you are actually just describing different things and creating conflict for nothing.

 

I know I am not wrong about what I have said.  It seems you either you did not understand my words or you just like conflict.  

 

Ok - so lets say I misunderstood what you wrote.

 

Care to explain it for us then?  Perhaps start here:

Quote:
Gravity is always making your mass feel heavy towards the ground and the ground pushes back up at you with as much pressure as your body weight.  In terms of "pressure", the pressure force vector is straight up.  If you are in a ski turn then your CoM feels like it wants to topple outside of the ski turn radius as centripetal forces push back laterally towards the inside of the turn, ON YOUR FEET.  The two pressure force vectors add up to a diagonal resultant force vector that is pushing upwards/inside ON YOUR FEET.

 

 

You also wrote this - I didnt raise it before - since despite your view - I actually do make an effort to give the benefit of the doubt and try to read things in a way that make sense...I was sorta able to do that here...but why dont you clarify for us as I am sure alot of people did't understand.  The underlined bits in particular.

 

Quote:
Back to the question about where is pressure the greatest in a turn, as JAMT was trying to explain you can see that particularly on steeper runs the force vectors will line up with each other at the end of a turn to be closer to the same direction, which means the resultant force vector will be stronger, ie, more pressure.  UNLESS you do certain things with your body to change things and control the pressure.  Gravity pulls in a direction that is more in line with the bottom of your skis at the bottom of the turn.
post #36 of 54

I don't actually believe you that you're being sincere in asking for clarification.  Rather you are quoting out of context and looking for trouble.

 

I was pretty clear in my long post that I don't particularly favor the inward force vs outward force representation any more than the other, and in fact that I myself tend to think of my CoM as feeling the outward (as your diagram), while my feet feel it the other way.

 

I made a reference to newton's third law only, about equal and opposite forces, That is newton's 3rd law.  I was not at all getting into all the other things contributing to motion...you have added that to the discussion and that is fine, but still there is nothing wrong with what I said up until that point.

 

With all due respect ski dude, I don't think you sincerely want to know more about my thoughts, you are just looking for conflict.  If anyone else does not understand what I wrote earlier I will be happy to discuss further.

post #37 of 54
Quote:
Originally Posted by borntoski683 View Post

I don't actually believe you that you're being sincere in asking for clarification.  Rather you are quoting out of context and looking for trouble.

 

I was pretty clear in my long post that I don't particularly favor the inward force vs outward force representation any more than the other, and in fact that I myself tend to think of my CoM as feeling the outward (as your diagram), while my feet feel it the other way.

 

I made a reference to newton's third law only, about equal and opposite forces, That is newton's 3rd law.  I was not at all getting into all the other things contributing to motion...you have added that to the discussion and that is fine, but still there is nothing wrong with what I said up until that point.

 

With all due respect ski dude, I don't think you sincerely want to know more about my thoughts, you are just looking for conflict.  If anyone else does not understand what I wrote earlier I will be happy to discuss further.


How is it out of context?  In both instances I quoted the entire paragraph!

 

My comments have nothing to do with inward vs outward - I only pointed out, you need to pick one.  You combined them to create a resultant.  That is one error.  I highlighted the other things that were wrong with what you wrote in my post #26 and 29.

 

 

I am not looking for conflict - just clarity. 

 

 

 

Here is a question for you:

 

A skier who say weights 200lbs is skiing down a slope of 25 degrees - just going straight - no turns.

 

According to you - is the ground pushing up at:

 

A) some value greater then 200lbs?

B) 200lbs exactley?

C) some value less then 200lbs?

post #38 of 54

obviously C. And that is not in conflict with anything I said

post #39 of 54
Quote:
Originally Posted by borntoski683 View Post

obviously C. And that is not in conflict with anything I said

 

C is correct.

Quote:
Gravity is always making your mass feel heavy towards the ground and the ground pushes back up at you with as much pressure as your body weight.  In terms of "pressure", the pressure force vector is straight up.  If you are in a ski turn then your CoM feels like it wants to topple outside of the ski turn radius as centripetal forces push back laterally towards the inside of the turn, ON YOUR FEET.  The two pressure force vectors add up to a diagonal resultant force vector that is pushing upwards/inside ON YOUR FEET.

 

Here is the whole paragraph so I dont get accused of taking quotes out of context.  Note the underlined and bold.

 

Also note the italicsied bit.  This combined with the sentence above lead me to believe you did not understand Normal Force or Resultant Forces.  So by "Straight Up" you really meant inline with the resultant. 

 

 

This may seem like semantics - but it isnt.  Look at post #10. 

 

 

Can you now explain the blue bit?  I dont get it.  Maybe draw the diagram for us using the one from my post 26 as the base.

post #40 of 54

Ski Dude this is my last response to you on this.

 

You can take a whole paragraph and its still out of context.  Any one of us could be referring to other posts by ourselves or others..  Myself I tend to make long posts and you have to take all of it in. 

 

If you seek to understand sincerely, its clear enough.  I don't have the energy to go back and prove it to you.

 

I'm happy to discuss this with anyone else...

post #41 of 54

And now for something completely different.

 

Spin a yoyo around quickly in a near-horizontal loop, horizontal enough that gravity is small compared to the string tension..  The string tension applies the force to accelerate the yoyo in a circle.

 

Now spin the yoyo in a vertical loop (I think I used to call this trick "around the world").  The yoyo's acceleration is the result of ALL the forces acting on the yoyo, string tension and gravity (forgetting air friction for the moment because it is small).  Notice the string pulls harder when the yoyo is at the bottom of it's arc.  Gravity and the string tension together make the yoyo go in a circle.  The string doesn't have to do as much work  (no work done at all due to orthagonal directions) pull as hard at the top of its arc, because gravity is pulling its fair share.  The string has to pull doubly hard at the bottom because, now gravity is pulling in the wrong direction to make the yoyo turn.

 

It's the same with the skier.  Your acceleration is the result of all the forces acting on you, gravity and your skis (ok and a bit of air friction, but let's ignore that as it's small at most recreation skiing speeds; once you understand the concept you can add as many forces as you can handle).  Your skis have to exert less force on you to get you to go around an arc at the top of your turn, because gravity is doing it's fair share of pulling you into an arc.  Your skis have to exert more force at the bottom of the arc because gravity is working against them, trying to pull you out of the arc.

 

We can flex at the end of the turn (bottom of the arc) and release the pressure, but when we do that our CM comes out of its arc.  The more pressure we exert at the top of the turn, and the less pressure we exert at the bottom of the turn, the less we fight gravity, and the faster we reach the chair lift loading area; remember that fact when you want to get in a 2nd "last run".

post #42 of 54

YO YO!!!

OH!! this is why Pressure is the greatest at the END of the turn.. or bottom of the arc...or just before the edges are released. OR OR.....

 

But too much knowledge can be dangerous. A good understanding of physics helps the ski instructor to answer the questions, WHY, WHERE, WHEN and HOW? The answer is because............................

post #43 of 54
Quote:
Originally Posted by Tek Head View Post

YO YO!!!

OH!! this is why Pressure is the greatest at the END of the turn.. or bottom of the arc...or just before the edges are released. OR OR.....

 

But too much knowledge can be dangerous. A good understanding of physics helps the ski instructor to answer the questions, WHY, WHERE, WHEN and HOW? The answer is because............................


icon14.gif Bingo,

 

Despite what many believe, "proper" technique isnt just made up at the whimsy of a few "ol boys" sitting around in a dark secret room.

post #44 of 54
Quote:
Originally Posted by Tek Head View Post

YO YO!!!

OH!! this is why Pressure is the greatest at the END of the turn.. or bottom of the arc...or just before the edges are released. OR OR.....

 

But too much knowledge can be dangerous. A good understanding of physics helps the ski instructor to answer the questions, WHY, WHERE, WHEN and HOW? The answer is because............................

I don't know a single CSIA L4 who has even a basic understanding of even Newtonian mechanics*.  However they do know by experiment that pressure builds up as you turn past the fall line and what some of the options are.  

 

A friend of mine actually taught a real life boffin and he asked "exactly how much edge angle do I need on this slope?".

 

Ski Instructor = Penny.  Ski Instructor != Sheldon.

 

*Newton was wrong anyway ;).

post #45 of 54
Quote:
Originally Posted by Skidude72 View Post

 

Where is pressure greatest?  I answered it.  Where disruption to our mass is greatest.  

I don't know how you define disruption, but in my definition would be something like the turn radius at a certain speed of the CoM. You also have to take gravity into account, Like Ghosts's Yoyo example, the "pressure" force is larger in the bottom when you spin it because of gravity. But then again, maybe you are including gravity in your definition of disruption.

 

Also regarding the Normal force. Like I mentioned before it depends on how you define it. Is it perpendicular to the slope or to the platform that your ski is building. 

In general I think either is fine and forces can be called whatever as long as you define them clearly enough. It does help to use common conventions though.

Quote:
Originally Posted by DLF7 View Post

I don't know a single CSIA L4 who has even a basic understanding of even Newtonian mechanics*.  

Well Skidude is L4 and he seems to have a very good understanding.

post #46 of 54

Well Skidude is L4 and he seems to have a very good understanding.

I have not yet had the pleasure of meeting Skidude so I don't know his educational background.  I can only speak about friends of mine and quite frankly they were lucky basic literacy wasn't a requirement when they passed their L4s roflmao.gif.

post #47 of 54
Quote:
Originally Posted by fatoldman View Post

 

First is the pervasive myth that toward the end of one of the arcs we call turns gravity and the inertial forces of the turn combine to make the pressure greatest in that phase of the turn.

 

 

 

 

Beside all the physics, I'm not sure if this statement has been addressed. I have an interest in the answer because the I believe the "myth" to be reality, and stated so in the EB thread. Of course if we let the turn/arc complete to uphill, it will be devoid of energy, but that's not what we are talking about, I hope.

 

I feel (not physics) that the forces build up in the turn to the point where I use them to make the crossover, very much like Bob's trampoline man. I can't imagine making that move if all the energy in the turn has dissipated. More to the point, I feel that the lack of energy we see in many turns, is due to the turn being rushed and not allowing those forces to build. As a style preference, I prefer long sweeping completed turns full of timing, rhythm and energy.

 

So, what is the myth and what is the reality here............depends right?

post #48 of 54

I didn't read that other thread and so I'm not sure of the full context fatoldman was trying to get to. 

 

I will only say that its not a complete myth, the steeper the hill, the more you can feel it, and its quite obvious.   Several of us have tried to explain the physics behind it, and its been explained on this forum many many times. 

 

On the other hand, we also have a lot of pressure management options at our disposal.  With those options we can even things out a bit so that we might not feel quite as much pressure build up at the bottom, and we might feel more at the top.  Or we can opt to embrace pressure buildup too for various purposes.  There are a lot of options available and tactics for when to use them..

 

If you do a very simple park and ride turn, then yes you are going to feel the "mythical" buildup of pressure from top to bottom as you ski from a state where your CoM is in between your skis and the center of the earth, to the position at the end of the turn where your skis are between your CoM and the center of the earth.

post #49 of 54
Quote:
Originally Posted by borntoski683 View Post

I didn't read that other thread and so I'm not sure of the full context fatoldman was trying to get to. 

 

I will only say that its not a complete myth, the steeper the hill, the more you can feel it, and its quite obvious.   Several of us have tried to explain the physics behind it, and its been explained on this forum many many times. 

 

On the other hand, we also have a lot of pressure management options at our disposal.  With those options we can even things out a bit so that we might not feel quite as much pressure build up at the bottom, and we might feel more at the top.  Or we can opt to embrace pressure buildup too for various purposes.  There are a lot of options available and tactics for when to use them..

 

If you do a very simple park and ride turn, then yes you are going to feel the "mythical" buildup of pressure from top to bottom as you ski from a state where your CoM is in between your skis and the center of the earth, to the position at the end of the turn where your skis are between your CoM and the center of the earth.

 

Like this? (from Bob Barnes' Crudology at  http://vimeo.com/24100822)

 

 

post #50 of 54
Quote:
Originally Posted by Jamt View Post

I don't know how you define disruption, but in my definition would be something like the turn radius at a certain speed of the CoM. You also have to take gravity into account, Like Ghosts's Yoyo example, the "pressure" force is larger in the bottom when you spin it because of gravity. But then again, maybe you are including gravity in your definition of disruption.

 

 

Yes, by disruption I mean its momentum altered (either in direction or speed) and am including gravity and everything else - however caused.

 

 

Quote:
Originally Posted by Jamt View Post
Also regarding the Normal force. Like I mentioned before it depends on how you define it. Is it perpendicular to the slope or to the platform that your ski is building. 

In general I think either is fine and forces can be called whatever as long as you define them clearly enough. It does help to use common conventions though.

 

 

To me Normal Force would have to perpendicular to the platform the ski makes.  If you used the "general slope" I dont understand how you could get the vector diagram to balance. 

 

 

Quote:
Originally Posted by Jamt View Post

Well Skidude is L4 and he seems to have a very good understanding.

 

True, that guy is awesome!biggrin.gif

post #51 of 54

popcorn.gif

 

What about when skiing bumps?

 

What about skiing reverse camber skis?

 

What about our heartcarving friend?
 

There are as many different correct answers to "Where is pressure greatest in a turn" as there are different types of turns and different types of equipment.

th_dunno-1[1].gif
 

post #52 of 54
Quote:
Originally Posted by crgildart View Post

popcorn.gif

 

What about when skiing bumps?

 

What about skiing reverse camber skis?

 

What about our heartcarving friend?
 

There are as many different correct answers to "Where is pressure greatest in a turn" as there are different types of turns and different types of equipment.

th_dunno-1[1].gif
 

 

Well that is why my answer - it holds true in bumps, or any conditon for that matter, as well as any skis, or any skiing ability.

post #53 of 54
Quote:
Originally Posted by LiquidFeet View Post

 

Like this? (from Bob Barnes' Crudology at  http://vimeo.com/24100822)

 

 

 

hehehe.  

post #54 of 54

Ski instruction's interesting because we all actually care to a (non-zero) degree about understanding physics/forces, biomechanics, and skill development. 

 

I've been entering into the kiteboarding world and it's been fascinating to take lessons. In kiteboarding, lessons are mostly thought of as for beginners and low intermediates. There is no skill development model, and physics aren't directly discussed. Lessons typically consist of "point your board downwind", "lean back, push your hips forward, turn the hips towards the direction of travel, and look where you want to go". Once a kiteboarder can safely ride upwind, that's the last time 99% of kiteboarders take lessons. 

 

When I asked about building a skill development model for kiteboarding, responses were "why would we need that? Just ride!" It's a completely different culture from the ski instruction world. 

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