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# Let's get PHYSICS-cal - Page 4

Quote:
Originally Posted by JESINSTR

Magnifico, Yes I do feel the centrifugal force (force between the bottom of  my ski and the snow) ...

No. Centrifugal force is applied at each part of your body. It is a force deriving from a force field, an acceleration (like gravity is). If you sum up all these little forces for each mass points of your body you obtains a force called..centrifugal force which origin is CoM. Your body feels gravity. Your body feels centrifugal as well.

Here is a drawing :

CG is CoM

P is wheight

C is centrifugal force

FA in red  is apparent force in the skier reference.

That apparent force bends your skis :-)

Quote:
Originally Posted by Ghost

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.

gravity is a field of force. Do you understand that?

Jumping foff cliff and considering you as the reference makes an other field of opposed to gravity. That means in the reference of the solid (you) field of force is zero.

Believe me, feet on ground you feel every molecule of your arm submitted to gravity.

I was assuming an earth-based reference system, as you did not specify, only that you could feel the force on your arm.

"Nothing is real except the way that I feel"

Quote:
Originally Posted by Magnifico

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

Does centrifugal force bend your skis while carving?

Magnifico no problem.... now I understand

The centrifugal force (that being the force out from the bottom of the ski into the snow) does bend the ski PROVIDING there is snow there for the edges to grab.

Quote:
Originally Posted by Ghost

I was assuming an earth-based reference system, as you did not specify, only that you could feel the force on your arm.

"Nothing is real except the way that I feel"

You feel forces only in from your point of view that means from reference of yourself.

Feet on ground : earth-based reference system=body reference as you dont moove :-)

Quote:
Originally Posted by JESINSTR

Magnifico no problem.... now I understand

The centrifugal force (that being the force out from the bottom of the ski into the snow) does bend the ski PROVIDING there is snow there for the edges to grab.

OK fine. See my drawing  above, it is the red force FA (centrifugal+weight)  bending your skis...

Quote:
Originally Posted by Magnifico ...

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:-)...

JESINSTR, you see know what we are going to reach?

Quote:
Originally Posted by Magnifico

No. Centrifugal force is applied at each part of your body. It is a force deriving from a force field, an acceleration (like gravity is). If you sum up all these little forces for each mass points of your body you obtains a force called..centrifugal force which origin is CoM. Your body feels gravity. Your body feels centrifugal as well.

Here is a drawing :

CG is CoM

P is wheight

C is centrifugal force

FA in red  is apparent force in the skier reference.

That apparent force bends your skis :-)

I appreciate all the effort you have put into this.  As  zenny (Zentune) has stated in a previous post,  we have the Yin and the Yang.  I have to leave it at that. I wish you well.

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

Sigh.

There is a reason why Newton made the first law. Centrifugal force is not a force, it is inertia. Inertia isn't a force, that is why we define it as something else. Props to Ghost because he gets this. Inertia doesn't twist anything. It is force applied to objects and said objects inability to transfer force throughout it that bends objects.

Quote:
Originally Posted by tdk6

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.

Please explain the difference to me between standing on your floor and lifting one foot to balance on the other and lifting one foot, and doing nothing with your upper body to counter balance?  What will happen differently?  In both cases what are the similarities?  Does the weight move to the stance foot in both instances?  Now imagine you are cruising in a wedge with even pressure on both feet and you very slowly began to lift and flatten one foot without moving your head to counter balance.  What would happen?

Quote:
Originally Posted by Spooky

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

Sigh.

There is a reason why Newton made the first law. Centrifugal force is not a force, it is inertia. Inertia isn't a force, that is why we define it as something else. Props to Ghost because he gets this. Inertia doesn't twist anything. It is force applied to objects and said objects inability to transfer force throughout it that bends objects.

Double sigh....

Spooky, as you know, this has been discussed and re-discussed ad nauseum. Anyone interested can search the archives. You either understand the concept of frames of reference--and therefore, probably, understand the reality of the phenomenon known and defined as centrifugal force, or you do not grasp the concept of frames of reference, and therefore surely do not understand the issue.

But one thing is certain: centrifugal force and inertia are not the same thing. Yes, one way to look at centrifugal force (like any inertial force) is that it is caused by inertia--and acceleration--but the two are certainly not the same.

You are right, though, that Ghost described it accurately: "There is no argument among scientists between centripetal and centrifugal; as Magnifico later stated, it simply depends on the choice of reference system."

Regardless of how you define it, or what frame of reference you choose to view it from, the phenomenon is real, quantifiable, measurable, experienced by all of us since birth, and really not worth debating beyond simple definition. It "exists," and it is not unreasonable to refer to the concept of centrifugal force in a discussion of the mechanics of skiing. I really do not know why some people get so up in arms about this one.

If you want to argue that you don't have to lean into a turn for balance, then perhaps there is a valid discussion to be had. But as long as you agree (and I assume you do) that balancing in a turn involves leaning into the turn (inclination), then it is pointless to get too caught up in definitions or arguments about why.

Best regards,

Bob

Frames of reference.

Simulating walking in the gravity field of the moon on earth.

Quote:
Originally Posted by Tog

This guy was suspended from cables to simulate the reduced gravity of the moon. They would then walk and run in this position. The feet don't know that the body is sideways.

Francis B. Smith wrote in his paper "Simulators For Manned Space Research,"

"I would like to conclude this talk with a discussion of a device for simulating lunar gravity which is very effective and yet which is so simple that its cost is in the order of a few thousand dollars at most, rather than hundreds of thousands. With a little ingenuity, one could almost build this type simulator in his backyard for children to play on. The principle is ...if a test subject is suspended in a sling so that his body axis makes an angle of 9 1/2 degrees with the horizontal and if he then "stands" on a platform perpendicular to his body axis, the component of the earth's gravity forcing him toward the platform is one times the sine of 9 1/2 degrees or approximately 1/6 of the earth's normal gravity field.

That is, a 180 pound astronaut "standing" on the platform would exert a force of only 30 pounds - the same as if he were standing upright on the lunar surface."

Published in James R. Hansen, Spaceflight Revolution: NASA Langley Research Center From Sputnik to Apollo, NASA SP-4308; Francis B. Smith, "Simulators For Manned Space Research," Paper for 1966 IEEE International Convention, New York, NY, March 21-25, 1966.

https://archive.org/details/NIX-EL-2000-00433

Quote:
Originally Posted by Bob Barnes

Double sigh....

Double sigh squared.

Spooky, as you know, this has been discussed and re-discussed ad nauseum. Anyone interested can search the archives. You either understand the concept of frames of reference--and therefore, probably, understand the reality of the phenomenon known and defined as centrifugal force, or you do not grasp the concept of frames of reference, and therefore surely do not understand the issue.

You know Bob, I don't harass you when you nit pick about your PSIA terms ad nauseum. If you don't want to listen to me, don't respond. You're a big boy. I am sure you can handle that task.

But one thing is certain: centrifugal force and inertia are not the same thing. Yes, one way to look at centrifugal force (like any inertial force) is that it is caused by inertia--and acceleration--but the two are certainly not the same.

It isn't a way of looking at it. It is it. Some use of the term centrifugal force here is the one of non-physical reality. The fleeing from center is not a force, it is the phenomena of inertia. Calling it the centrifugal force term in an arbitrary non-inertial frame doesn't make it real, or its own separate entity in reality that has/needs a cause.

You are right, though, that Ghost described it accurately: "There is no argument among scientists between centripetal and centrifugal; as Magnifico later stated, it simply depends on the choice of reference system."

Yes, there is no argument because it is clearly defined by Newton's laws. I am sure that you can agree that the realm occupied by dynamics of skiing adheres to these axioms? Skiing at half of the speed light would be sweet though.

Regardless of how you define it, or what frame of reference you choose to view it from, the phenomenon is real, quantifiable, measurable, experienced by all of us since birth, and really not worth debating beyond simple definition. It "exists," and it is not unreasonable to refer to the concept of centrifugal force in a discussion of the mechanics of skiing. I really do not know why some people get so up in arms about this one.

I have never said the effect wasn't real, or not felt, ever on this forum. Like you nit pick others about your PSIA terms, I am nit picking something that is important to me. The fleeing from center is an effect of inertia; it most definitely is real, but it is not a force.

Sometimes I see a back and forth between confusion between the reactive force and the inertial force. It is in my opinion much better to just talk about the centripetal force you apply to change your direction which is invariant to changes of coordinates and countering the effects of inertia. If you don't care to listen, continue rocking in the free world my friend.

The animations/diagrams still make sense in a loose world defined by "feelings" without knowing the underlying truth. I have learned how to ski past an intermediate level just by your videos/animations. I am not dismissing the quality or challenging the correctness  of what you are saying.

If you want to argue that you don't have to lean into a turn for balance, then perhaps there is a valid discussion to be had. But as long as you agree (and I assume you do) that balancing in a turn involves leaning into the turn (inclination), then it is pointless to get too caught up in definitions or arguments about why.

I'd rather not; I will leave that to you to dominate. I know as much about skiing as you know about physics.

Best regards,

Bob

Quote:
Originally Posted by Tog

Frames of reference.

Simulating walking in the gravity field of the moon on earth.

That is sweet. Further proof that the moon landing was faked.

I'm bragging here, but I have done lunar gravity on the vomit comet.

Quote:
Originally Posted by Spooky

Sigh.

There is a reason why Newton made the first law. Centrifugal force is not a force, it is inertia. Inertia isn't a force, that is why we define it as something else. Props to Ghost because he gets this. Inertia doesn't twist anything. It is force applied to objects and said objects inability to transfer force throughout it that bends objects.

tss...you are right when you say centrifugal is inertia. Inertia is about acceleration, lets say gamma.

Second part of your proposal you are wrong: inertia (thus acceleration) can affect objects.I can fell centrifugal inertia: it affects me, my molecules :-).

It is "like" a force: F=M.gamma. That is why we say(not only me) centrifugal force.

gravity is acceleration too: g. I can feel it baby and WE call it weight :-)

Quote:
Originally Posted by Spooky
Sometimes I see a back and forth between confusion between the reactive force and the inertial force. It is in my opinion much better to just talk about the centripetal force you apply to change your direction which is invariant to changes of coordinates and countering the effects of inertia. If you don't care to listen, continue rocking in the free world my friend.

there is no confusion here AFAIK :-)

In fact Spooky you are missing it here. I made efforts in that thread to make people understand that:

That apparent force FA bends your skis. You can calculate it, that is the core of skiing as far as carving is concerned. I will go further on this later.

C=M.v.v/r

Sorry but if you stay at your level of centripetal etc you miss the main point.

CG is CoM

P is wheight

C is centrifugal force

FA in red  is apparent force in the skier reference.

That apparent force bends your skis :-)

Originally Posted by Magnifico

No. Centrifugal force is applied at each part of your body. It is a force deriving from a force field, an acceleration (like gravity is). If you sum up all these little forces for each mass points of your body you obtains a force called..centrifugal force which origin is CoM. Your body feels gravity. Your body feels centrifugal as well.

Here is a drawing :

CG is CoM

P is wheight

C is centrifugal force

FA in red  is apparent force in the skier reference.

That apparent force bends your skis :-)

originally Posted by JESINSTR

I appreciate all the effort you have put into this.  As  zenny (Zentune) has stated in a previous post,  we have the Yin and the Yang.  I have to leave it at that. I wish you well.

You are welcome.

Let s go to the realm  of ski now:

what append when the FA force bends your skis while carving?

Q1: what appends to the radius of your turn when you have more edge angle (angle between skis and snow)?

Q2: what appends to the FA force when radius disminish?

Frame of reference fixed to skier:

Centrifugal force pushes on skis, snow resists, skis bend into a turn and skier is pushed into a turn (if in balance with accelleration V^2/R)

Frame of reference fixed to Earth:

Snow pushes on skis, skier's inertial resists and skis bend into a turn with acceleration V^2/R if in balance.

You may prefer the frame of reference fixed to the Earth thinking because the Earth is a non-accelerated reference, but the Earth is accelerating (in a curve around the sun), the sun is accelerating around the centre of the Galaxy,  and it is equally true to say that the universe is rotating about the self.  There is no absolute fixed non moving frame of reference;  Copernicus's Earth revolves around the Sun was no more true than the previously used system with the Sun revolving around the Earth; it just made the math easier.  I suspect that both Paul and Copernicus knew this fact, but Paul preferred the symbolism of keeping the Earth centric system despite it's complexity.

There is no "stationary aether" either, but then again there is only theory, or is there?

ok so let's choose frame of reference fixed to skier...more convenient hihi.

Q1: what appends to the radius of your turn when you have more edge angle (angle between skis and snow)?

If somebody here knows why just tell us...

For shaped ski on a hard surface it is pretty easy.  A simplified model is the sidecut approximates a circle of radius Rs, and if the ski is flat on the snow the side of the ski lines up approximately with a circle of that radius, but if you tip the ski to an angle of say 60 degrees, then when you press the middle down so it touches the hard surface along with the tip and tail the interface of the ski and snow will have a radius of Rs times the cosine of 60 degress.   Want to carve a 6.5 m turn with your 13 m sidecut radius ski?  Tip 'em up on edge to 60 degrees.  Easy peasy.

On soft snow or IN soft snow it is a bit different.  Pressure * area integrated over the surface of the ski bend differently shaped and different stiffness patttern skis in a more complex fashion.   Still more shapely skis give you a tighter radius and tipping more gives more of a turn, but softer skis even more so.

Exactly. Thx for answer Ghost. Now:

Q2: what appends to the FA force (weight+centrifugal) when radius diminish?

A2: Centrifugal force increases. So FA does.

If somebody here knows why Centrifugal force increases just tell us...

F= MV^2/R.

don't know where you're going with this, but I suspect you will be looking at the required tipping angle to hold that turn with forces in balance; you may want to refer to bob

Quote:

Originally Posted by Bob Barnes

PS--here's an animation that I made several years ago in an attempt to illustrate Critical Edge Angle:

Thanks for the reminder, Ghost!

...and another animation from the same time, that shows how platform angle/critical edge angle changes through turns to allow edges to grip and shape the turn in the shaping phase, then release to start the new turn. I tried to show also how the slope angle affects edge angle on the snow as the turn progresses, and how the center of mass moves about with changes in body position, the balance point changes smoothly from foot to foot, and a few other things, but I was never convinced that this animation worked very well. You have to watch it a bit, at the very least!

Best regards,
Bob

Thx Ghost.

I just wanted to show that as edge angle increase radius diminish and then centrifugal force increases and then FA increases.

It is very important here to see here that FA increases. Because for the skis to bend fully you need more and more force as angle of the edge increases.

With less force skis cannot bend maximally if angle of edge is important.

That is the magical of parabolic skis, if speed is sufficient the process for shaping phase of carving is the following :

FA increases(by itself) -> you can increase edge angle -> radius diminish -> FA increases (by itself) ->you can increase edge angle -> radius diminish...etc.

Nota: FA= weight+centrifugal force

Edited by Magnifico - 3/22/14 at 3:35pm

The interesting thing to me is that as you tip the skis on hard pack you require more force (FA) in order to hold the turn, so the net force applied on the ski, the red arrow in Bob's diagram above, becomes more horizontal, so in order to "carve" you need to tip your skis more on edge.  There is a limited range of speeds for a ski of any given sidecut radius which allow you to have a tipping angle that will hold the turn you have dialed up.

Quote:
Originally Posted by Magnifico

Q1: what appends to the radius of your turn when you have more edge angle (angle between skis and snow)?

If somebody here knows why just tell us...

To answer your question, Magnifico, a few items from our archives here at EpicSki (there have been many discussions!):

What is best edge angle for max grip? (Link is to my post #17, but the whole thread is worth a read)

Sidecut vs. Turn Radius (Link to an early post by Tom/PhysicsMan, defining the formula for calculating carved turn radius from sidecut radius and edge angle)

That should do you, for a while.

---

So, Spooky, you really do not understand the concept of "frames of reference," then, do you? You seem to insist that motion is somehow absolute, and that therefore, there is only one way to look at it. But just about the only absolute about motion is that it is not absolute--it is relative to other things, relative to the frame of reference ("coordinate system," if you prefer) that you choose to observe it and describe it from. If you want to continue the discussion, at least with me, you're going to have to demonstrate that you do, in fact, have a grasp of that concept. Otherwise, it is nearly pointless to discuss a concept that is so heavily dependent upon that understanding.

Yes, I'm sure you'll insist, and you will be correct, that the force pulling on your hand is "merely" the result of the inertia and acceleration of the weight on the other end of the string. That is true--in fact, it is essentially the definition of "inertial forces," of which centrifugal force is an example. But it does not change the fact that the force pulling out, away from the center, on your hand is as real as any other force.

For you, and anyone else interested in more discussion of Centrifugal and Centripetal Forces, I will refer you to one of the better EpicSki discussions of the topics, this one initiated by Tom/PM or "PhysicsMan," and itself referring to several pertinent and prior discussions:

Some comments on centrifugal and centripetal forces

---

"There is no "stationary aether" either, but then again there is only theory, or is there?" Good question, Ghost! Do you know the difference between theory and reality? In theory, there is no difference between theory and reality, but in reality, well, sometimes there is.

And you thought we never got around to discussing physics, jesinstr! Welcome, again, to EpicSki!

Best regards,

Bob

Yes Ghost there is a sort of equivalence :

tip your skis more on edge <=> shorter radius <=> more pressure.

While we described the process we just did a proof of it.

Edited by Magnifico - 3/22/14 at 4:08pm

Waouh Thx Bob Barnes for information.

The most significant post linked with our thesis here is this one :

Quote:
Originally Posted by oldgoat

All of this math assumes that the skier is a passive participant. Turn Radius = cosine of edge angle to snow surface X sidecut radius--from Bob Barnes post #8. But edge angle is also determined by the turn radius: the smaller the radius the higher the centrifugal force at a given speed, and the higher the speed the higher the centrifugal force at a given radius--which means that faster the skier is going and the smaller the radius the lower the edge angle of the ski to the snow surface  possible without the skier flopping over, and thus the smaller the radius produced by the sidecut. In other words natural turn radius is determined not just by the geometry of the ski but by the speed of the skier and steering movements initiated by the skier. The point being that while some skis certainly tend to have smaller turn radii than others, the actual radius of the turn is as much due to the skill of the skier as the shape of the ski.  Skilled skiers can carve tighter curves than unskilled skiers. But we all knew that.

BB,

I said to JESINSTR that:

"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:-)"

We just saw (demonstrate) that: tip your skis more on edge <=> shorter radius.

We know here that at the end of move, angulation is the tool for tipping your skis maximum on edge.

So the last problematic (bolded above) becomes how the skier prepare angulation at best and how angulation helps fo tipping your skis more on edge?

Quote:

"There is no "stationary aether" either, but then again there is only theory, or is there?" Good question, Ghost! Do you know the difference between theory and reality? In theory, there is no difference between theory and reality, but in reality, well, sometimes there is.

And you thought we never got around to discussing physics, jesinstr! Welcome, again, to EpicSki!

Best regards,

Bob

Bob,  These have been very interesting and mind expanding posts.  Thanks again for your and everyone's contributions and for the link to PhysicsMan's  post.  It is a must have document for an instructor's tool kit.

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