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# Let's get PHYSICS-cal - Page 3

TDK6, Allow me to pose this question:  If you are cruising in a gliding wedge (smallish size) at a 45 degree tangent to the fall line, I think we can agree the predominance of your weight would be on your down hill ski? yes?  Now if you release the edge grip from that ski, with the intent to turn your tips into the fall line, where would your weight shift to? Or another way of thinking about it….if you were walking and you take a step forward from one foot to the other, where does the weight shift to?  When taking a step, do you make any effort to tip your head out over the weighted foot to walk like Charlie Chaplin? Probably not.  Do you angulate over that foot? No, but there is a definite and immediate weight shift. This is what occurs in a passive weight shift in skiing at slow speeds.  There is no need for any big tipping movements, just the simple inclination of the lower leg and appropriate pressure distribution along the length of the ski is sufficient and everything moves in the direction of the turn.

Quote:
Originally Posted by bud heishman

Who here is associating a weight shift, passive or active, with "leaning on the inside ski", rotation and banking?  How could these things be associated with a weight transfer to the outside ski?  Wouldn't they indicate the opposite?  I continue to believe that you don't understand the mechanics of a passive weight shift?

Sorry a bit short on time.... tennis this time..... Anyway, as always I could be wrong but I'm still an expert skier  on the internet

Quote:
Originally Posted by tdk6

How about I just extend off my shorter uphill leg to release my down hill ski's edge?  where would my weight be then and what would my skis do?  Rather than trying to prove how this doesn't work, perhaps offer us the benefit of the doubt and try to understand how it actually does work quite well.

If you were standing with your skis parallel across the fall line and, you did NOTHING with your head or upper body, lifted your downhill ski up off the snow...what would happen?  I think you would discover that your weight shifted 100% immediately and that your body fell down the slope? yes?  Now take this thinking back into my first post about traveling across the hill in a wedge and apply there.  When the downhill edge is released it is NOT weighted rather unweighted which shifts the weight to the outside turning ski without any gross upper body movement out over the outside foot.  You can see this in action watching the various videos from PSIA instructors here on Epicski.  Come on man... see the light for me! and take your foot off the brakes!

Giving a Level II exam this week and can assure you that if any candidates initiate a wedge christie the way you advocate, their demo will be a FAIL.  So this isn't just a Budism it is a PSIA standard for all instructors.  I guess then perhaps there is a whole Nation over here that has it all wrong because TDK6 says it does not work???

Quote:
Originally Posted by LiquidFeet

If Bob were to enact a "total relaxation" wouldn't he fall down?
not if you are on the way up before the transition
Quote:
Originally Posted by Jamt

Quote:
Originally Posted by LiquidFeet

If Bob were to enact a "total relaxation" wouldn't he fall down?
not if you are on the way up before the transition

Right! Even a sack of potatoes continues on it's journey unless acted upon.

Hey Tog, maybe you have coined a new catch phrase "Tossin the potatoes", "Tossin the sac"??? well....maybe not.

Hey Bud. That could actually work. There is one small but.... but I get back to you on that later when Im on a computer.

Quote:
Originally Posted by bud heishman

How about I just extend off my shorter uphill leg to release my down hill ski's edge?  where would my weight be then and what would my skis do?  Rather than trying to prove how this doesn't work, perhaps offer us the benefit of the doubt and try to understand how it actually does work quite well.

If you were standing with your skis parallel across the fall line and, you did NOTHING with your head or upper body, lifted your downhill ski up off the snow...what would happen?  I think you would discover that your weight shifted 100% immediately and that your body fell down the slope? yes?  Now take this thinking back into my first post about traveling across the hill in a wedge and apply there.  When the downhill edge is released it is NOT weighted rather unweighted which shifts the weight to the outside turning ski without any gross upper body movement out over the outside foot.  You can see this in action watching the various videos from PSIA instructors here on Epicski.  Come on man... see the light for me! and take your foot off the brakes!

Giving a Level II exam this week and can assure you that if any candidates initiate a wedge christie the way you advocate, their demo will be a FAIL.  So this isn't just a Budism it is a PSIA standard for all instructors.  I guess then perhaps there is a whole Nation over here that has it all wrong because TDK6 says it does not work???
Quote:
Originally Posted by bud heishman

Hey Tog, maybe you have coined a new catch phrase "Tossin the potatoes"...

"Get those potatoes down the hill!"?

What people will do in wide open spaces:

http://youtu.be/5n8XGm8ruxw

All,  I originally created this thread to discuss the physics and understanding of Centripetal vs Gravitational Force in skiing. Somewhere along the line I responded to a post that had a question related to Active vs Passive Weight Transfer.  I totally apologize for that and the "off thread" discussion that followed regarding this subject.

Although the discussions on weight transfer were very interesting can we come back to the original intent of this thread?  If you want to continue with weight transfer maybe a new thread on that subject can be started.

All those coming to this thread for the first time, please see my original post.

Thanks!

Jesinstr--do you not find weight transfer to be related to, and relevant to, a discussion of those forces of a turn? In a "passive weight transfer," those forces are the cause, are they not?

Best regards,
Bob

From a traverse, gravity can pull the front of your skis, steering them into a turn with completely released edges, or the engaged edge of your skis can push them into a turn.  Two different mechanisms, one works better from a near stand still, one requires a bit more forward motion.

Exactly. The more forward momentum the sooner and/or higher the edge can be tipped.  This has everything to do with the forces present in a turn.

Quote:
Originally Posted by JESINSTR ...

The second we begin moving and put a (shaped) ski on edge we are creating an arc (circular path), and so begins dynamic changes in our body's relationship with gravitational force to deal with the onset of centripetal force...

euh, on my skis I wont feel a centripetal force but the opposite one: the centrifugal force.

Both forces wheight and centrifugal are applied on the CoM.

Centrifugal force is calculated with speed, mass and the radius of arc made by your CoM (not the arc of skis, but a good approximation is the radius of skis trajectory of course since the path of CoM and skis are linked when the skier is not in transition phase).

JESINSTR, you should reconsider your problematic and instead see in deep what is the contribution of the centrifugal force with variation of the angle of edges !!

In deep means what is exactly the process here. You will reach then the actual problematic of how a skier perform the shortest radius as possible:-)

btw, thx people for welcoming me here.

Quote:
Originally Posted by Magnifico

euh, on my skis I wont feel a centripetal force but the opposite one: the centrifugal force.

Both forces wheight and centrifugal are applied on the CoM.

Centrifugal force is calculated with speed, mass and the radius of arc made by your CoM (not the arc of skis, but a good approximation is the radius of skis trajectory of course since the path of CoM and skis are linked when the skier is not in transition phase).

JESINSTR, you should reconsider your problematic and instead see in deep what is the contribution of the centrifugal force with variation of the angle of edges !!

In deep means what is exactly the process here. You will reach then the actual problematic of how a skier perform the shortest radius as possible:-)

btw, thx people for welcoming me here.

Hmmmmm..... centripetal vs centrifugal. Scientists are arguing over it so lets not bother. We all know what it is and what it does.

Quote:
Originally Posted by bud heishman

How about I just extend off my shorter uphill leg to release my down hill ski's edge?  where would my weight be then and what would my skis do?  Rather than trying to prove how this doesn't work, perhaps offer us the benefit of the doubt and try to understand how it actually does work quite well.

If you were standing with your skis parallel across the fall line and, you did NOTHING with your head or upper body, lifted your downhill ski up off the snow...what would happen?  I think you would discover that your weight shifted 100% immediately and that your body fell down the slope? yes?  Now take this thinking back into my first post about traveling across the hill in a wedge and apply there.  When the downhill edge is released it is NOT weighted rather unweighted which shifts the weight to the outside turning ski without any gross upper body movement out over the outside foot.  You can see this in action watching the various videos from PSIA instructors here on Epicski.  Come on man... see the light for me! and take your foot off the brakes!

Giving a Level II exam this week and can assure you that if any candidates initiate a wedge christie the way you advocate, their demo will be a FAIL.  So this isn't just a Budism it is a PSIA standard for all instructors.  I guess then perhaps there is a whole Nation over here that has it all wrong because TDK6 says it does not work???

IMO you would have to shift your weight over to your new outside ski when you release your new inside ski. Your outside foot supports you and when you pick it up you don't remain hanging in the air. And there are no sufficient turn forces jet. That's when you shift your balance. You angulate. Not much if your wedge is very small. I think if you read through your own posting you will see that you kind of say it yourself.

Please link some videos so I can once again see there is no active weight shift.

tdk6, it is not a "fight" between centripetal and centrifugal here. Both exit depending of choice of reference (ground or skier)  :-)

While skiing I feel the centrifugal force, the one which pull me outside the curve. Thats all. So lets call a cat a cat not a tac...

The main point in my last post is not that but :"the contribution of the centrifugal force with variation of the angle of edges".

Quote:
Originally Posted by Magnifico

Quote:
Originally Posted by JESINSTR ...

The second we begin moving and put a (shaped) ski on edge we are creating an arc (circular path), and so begins dynamic changes in our body's relationship with gravitational force to deal with the onset of centripetal force...

euh, on my skis I wont feel a centripetal force but the opposite one: the centrifugal force.

Both forces wheight and centrifugal are applied on the CoM.

Centrifugal force is calculated with speed, mass and the radius of arc made by your CoM (not the arc of skis, but a good approximation is the radius of skis trajectory of course since the path of CoM and skis are linked when the skier is not in transition phase).

JESINSTR, you should reconsider your problematic and instead see in deep what is the contribution of the centrifugal force with variation of the angle of edges !!

In deep means what is exactly the process here. You will reach then the actual problematic of how a skier perform the shortest radius as possible:-)

btw, thx people for welcoming me here.

I "feel" the centripetal force applied at my skis pushing me around the corner and toward the centre of my turn.  I don't tend to feel body forces, like gravity or centrifugal force although I do feel the contact forces that arise because of them.

There is no argument among scientists between centripetal and centrifugal; as Magnifico later stated, it is simply depends on the choice of reference system.

Centrifugal forces are needed in accelerated reference systems in order for the math to work out such that F=ma.  For example consider a car going around a corner with a golf ball starting out in the centre on the floor of the trunk.  Consider it from a frame of reference fixed to the road. As the car enters a corner and starts to turn the golf ball will continue in an unaccelerated motion until it is acted upon by a force, then when it hits the wall it will be accelerated by the wall pushing it around the corner (centripetal force).

Now consider it from a frame of reference fixed to the trunk.  The ball will start to move toward the wall.  F=ma must be true, so there has to  be a force (centrifugal force = center fleeing) accelerating this ball towards the wall.  When the ball gets to the wall, the contact force of the wall on the ball will cause it to stop moving (in the frame of reference system fixed to the trunk.

This is really no different that the body force felt when an elevator accelerates.  Frame of reference fixed to building ===> floor pushes you up and f= ma, a = f/m and you accelerate up.  Frame of reference fixed to elevator ===> extra body force pulls you down as though there were more gravity, floor resists and you feel the floor pushing against your weight + extra "weight", all forces acting you sum up to zero and you go nowhere.

Quote:
Originally Posted by Ghost

I "feel" the centripetal force applied at my skis pushing me around the corner and toward the centre of my turn.  I don't tend to feel body forces, like gravity or centrifugal force although I do feel the contact forces that arise because of them.

There is no argument among scientists between centripetal and centrifugal; as Magnifico later stated, it is simply depends on the choice of reference system.

Centrifugal forces are needed in accelerated reference systems in order for the math to work out such that F=ma.  For example consider a car going around a corner with a golf ball starting out in the centre on the floor of the trunk.  Consider it from a frame of reference fixed to the road. As the car enters a corner and starts to turn the golf ball will continue in an unaccelerated motion until it is acted upon by a force, then when it hits the wall it will be accelerated by the wall pushing it around the corner (centripetal force).

Now consider it from a frame of reference fixed to the trunk.  The ball will start to move toward the wall.  F=ma must be true, so there has to  be a force (centrifugal force = center fleeing) accelerating this ball towards the wall.  When the ball gets to the wall, the contact force of the wall on the ball will cause it to stop moving (in the frame of reference system fixed to the trunk.

This is really no different that the body force felt when an elevator accelerates.  Frame of reference fixed to building ===> floor pushes you up and f= ma, a = f/m and you accelerate up.  Frame of reference fixed to elevator ===> extra body force pulls you down as though there were more gravity, floor resists and you feel the floor pushing against your weight + extra "weight", all forces acting you sum up to zero and you go nowhere.

Good post Ghost...especially your first 2 points.  Allow me to elaborate/embellish my thoughts regarding your first point and see if you agree.  Your "thought picture"  about  "pushing you around the corner"   is right on.  One of the biggest issues I deal with when working with skiers on their carving turns is that they don't maintain the necessary body positions to manage centripetal force and complete the turn.  Somewhere after the apex and heading into the completion phase of the turn I see a breakdown of body position.. ie they stop pushing around the corner. Visually I see the shoulders opening up down the hill instead of continuing in direction of travel. Some of us call it "hitting the wall".  Most often source of  the poor body position (my opinion) is the under development of the inside half. A strong inside half says to the outside half.."Hey follow me here" vs the outside saying to the inside half: "Hey get out of my way".

The other thing you stated which I believe is very revealing is that we don't feel the centrifugal force but we do feel the contact force (pressure) that arise from them.  And this brings up another interesting macro thought....In skiing (as well a many things in life), we react to what we feel not necessarily cause of what we are feeling.  Well done!

When a carnival ride turns sharply and you feel thrown toward the outside of the bucket you are riding in, do you pay attention to yourself pushing up against the outside abruptly, or do you think about how the outside wall of the bucket is suddenly pushing on you so rudely?

How does the way you conceive of this situation and similar ones in ski turns affect how you ski?

Quote:
Originally Posted by Ghost

I "feel" the centripetal force applied at my skis pushing me around the corner and toward the centre of my turn.  I don't tend to feel body forces, like gravity or centrifugal force although I do feel the contact forces that arise because of them...

???

We need some experiment here

If I extend my arm horizontally I can feel gravity force pulling down my arm. I can feel it baby: for every mass element of my arm. In fact I feel here a force couple trying to put my arm down (not up hey!?).

The same with centrifugal force...I can feel it. In fact while skiing I feel the sum of forces: weight + centrifugal both  belonging to a force field.

I can feel also an opposite force from ground (I dont call it centripetal): it is ground reaction force. That is why in my own reference sum of all forces equal zero.

Quote:
Originally Posted by LiquidFeet

When a carnival ride turns sharply and you feel thrown toward the outside of the bucket you are riding in, do you pay attention to yourself pushing up against the outside abruptly, or do you think about how the outside wall of the bucket is suddenly pushing on you so rudely?

How does the way you conceive of this situation and similar ones in ski turns affect how you ski?

I said to JESINSTR : ".. see in deep what is the contribution of the centrifugal force with variation of the angle of edges !!

In deep means what is exactly the process here. You will reach then the actual problematic of how a skier performs the shortest radius as possible:-)"

Quote:
Originally Posted by Bob Barnes

Jesinstr--do you not find weight transfer to be related to, and relevant to, a discussion of those forces of a turn? In a "passive weight transfer," those forces are the cause, are they not?

Best regards,
Bob

Bob,  Yes I agree.  In a carved turn, do you feel that the force that causes the passive weight transfer is governed by the alignment of our mass along the centripetal radius?

John.

Quote:
Originally Posted by Magnifico

???

We need some experiment here

If I extend my arm horizontally I can feel gravity force pulling down my arm. I can feel it baby: for every mass element of my arm. In fact I feel here a force couple trying to put my arm down (not up hey!?).

The same with centrifugal force...I can feel it. In fact while skiing I feel the sum of forces: weight + centrifugal both  belonging to a force field.

I can feel also an opposite force from ground (I dont call it centripetal): it is ground reaction force. That is why in my own reference sum of all forces equal zero.

Here is the Definition for Centrifugal

Centrifugal force (from Latin centrum, meaning "center", and fugere, meaning "to flee"[1][2]) is the apparent force that draws a rotating body away from the center of rotation. It is caused by the inertia of the body as the body's path is continually redirected. In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a"fictitious" force) observed in a non-inertial reference frame, and a reaction force corresponding to a centripetal force

Here is the Definition for Centripetal

Centripetal force (from Latin centrum "center" and petere "to seek"[1]) is a force that makes a body follow a curved path: its direction is always orthogonal to the velocity of the body, toward the fixed point of the instantaneous center of curvature of the path. Centripetal force is generally the cause of circular motion. In simple terms, centripetal force is defined as a force which keeps a body moving with a uniform speed along a circular path and is directed along the radius towards the centre.[2]

So my question is: To which definition are those \$1500 skis designed to apply?  Do we spend our time fighting the force that is pulling us out and away or are we using the force that is trying to  follow  a curved path?

Ahhh yes, the yin and the yang...two sides of the same coin.Perhpas ultimately, there is no distinction between the skis and the skier and the forces involved. Let's get META-PHYSICS-cal, and think outside of the box!

zenny
Quote:
Originally Posted by JESINSTR ...

So my question is: To which definition are those \$1500 skis designed to apply?  Do we spend our time fighting the force that is pulling us out and away or are we using the force that is trying to  follow  a curved path?

In the skier reference both exist. You have the ground force (from skis to CoM) opposed to weight+centrifugal.

In last post you said you don't feel centrifugal while skiing. Do you?

I do feel it baby.

Now the main point is that centrifugal force twists your skis no? And this is not meta-physi-cal indeed

Quote:
Originally Posted by Magnifico

In the skier reference both exist. You have the ground force (from skis to CoM) opposed to weight+centrifugal.

In last post you said you don't feel centrifugal while skiing. Do you?

I do feel it baby.

Now the main point is that centrifugal force twists your skis no? And this is not meta-physi-cal indeed

Magnifico, Yes I do feel the centrifugal force (force between the bottom of  my ski and the snow) and it is key feedback as to my state of dynamic balance.  Centrifugal force doesn't twist my skis when I am carving.

A.  When I'm carving, I feel the pleasurable power of the snow guiding my skis where I want to go.  Let's call that centripetal force.

B.  When I'm "steering" a skidded turn, I feel the inertial properties of my body wanting to go straight out there towards the trees while I am telling those skis to make me go in a curve in the other direction.  We (centrifugal force and me) are fighting.  I win, but it's still me versus centrifugal force; a battle or skirmish ensues.

On groomed trails, or in crud, I prefer A.

Bumps are a different matter.

JESINSTR, sorry for my poor english.may be twist is not the right word...

Does centrifugal force bend your skis while carving?

Quote:
Originally Posted by Magnifico

Quote:
Originally Posted by Ghost

I "feel" the centripetal force applied at my skis pushing me around the corner and toward the centre of my turn.  I don't tend to feel body forces, like gravity or centrifugal force although I do feel the contact forces that arise because of them...

???

We need some experiment here

If I extend my arm horizontally I can feel gravity force pulling down my arm. I can feel it baby: for every mass element of my arm. In fact I feel here a force couple trying to put my arm down (not up hey!?).

The same with centrifugal force...I can feel it. In fact while skiing I feel the sum of forces: weight + centrifugal both  belonging to a force field.

I can feel also an opposite force from ground (I dont call it centripetal): it is ground reaction force. That is why in my own reference sum of all forces equal zero.

OK, I'll play...

Do it after you jump off a cliff.  Your arm will feel weightless, as it and you are accelerated by a body force that you cannot feel.  Perhaps it is you resisting gravity by forcing your arm up against gravity that you feel, the resistance originating at your feet where they meet a solid platform.

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