or Connect
EpicSki › The Barking Bear Forums › Ski Training and Pro Forums › Ski Instruction & Coaching › Centripetal Force How Does It Relate To Skiing?
New Posts  All Forums:Forum Nav:

# Centripetal Force How Does It Relate To Skiing? - Page 2

Quote:
Originally Posted by ChuckT

Yes, Bob is a fantastic writer. I agree he gives a good and useful description of what a skier feels. But someone has said about skiing that "perception is not reality". Bob's accelerating self as frame of reference would not produce a real force (centrifugal) to balance a real force (centripetal) in the earth frame of reference. Let's not forget that motion is relative to a frame of reference but force is not. It doesn't matter what frame of reference you choose, the net force on an object is always the same. There is no real centrifugal force to balance centripetal force. It is a convenient concept in some situations, but should always be used with the understanding that it is an artificial construct. Grab a text book on physics. Heck, let me google it for you. Here comes the University of Virginia, Physics Department

http://phun.physics.virginia.edu/topics/centrifugal.html

Bob's writing is good not only because it is vivid, clear, and organized, but also because it is correct.

I understand what you are saying perfectly well.  I just don't agree.  In some sense all forces are fictitious. They are constructs of a certain phenomenological level of description.  They are not there in the fundamental theory (written in terms of path integrals or Lagrangians or whatever craziness the string guys come up with). But when we restrict our discussion to a limited range of conditions (like everyday life), make some approximations, and rearrange the equations in convenient ways, we lump some of the terms into groups that are convenient to call "forces."

On a less metaphysical level, forces are different in different frames.  A lot of complicated, real-world computations are done in non-inertial frames.

As I said in the thread Bob linked, numerical weather prediction is done in an Earth-linked frame.  And one of the most important forces is Coriolus.  It's not fictional  -- it causes real hurricanes.

Inertial navigation is done by integrating the forces sensed by accelerometers and the rotations sensed by gyros, and it is usually done in the sensor's own frame.  Rigid body motions are much simpler in a body-fixed frame.

Fluid mechanics is usually done in co-moving frames, too.

Another example.  If you do satellite trajectories, the most convenient way is to describe the position in terms of orbital elements.  Most of them are constant, and one increases linearly, for a spherical Earth.  No forces here.  When you put in the real world, there are small perturbations that change the orbital elements gradually.  So here, the only forces are tidal forces (from a not-perfectly-round Earth) and drag forces from the upper atmosphere.

By the way, I recently read a note that pointed out that forces transform as Christoffel symbols.  I haven't had time to work it through, but it makes a lot of sense.

mdf,

I do see the error of my way. Not that I think I am wrong in insisting that you and Bob are incorrect in saying that centrifugal force is just as real as centripetal force. I was wrong in thinking that I could clarify something for someone. I should have read the entire thread Bob linked to.

Nothing I said had not been said clearly by two contributing physicists in that thread, amiles and PhysicsMan. They (and I) of course have complete agreement on the physics. After all, as amiles put it, "...it is certainly not a topic of debate within the professional physics community." What I find very instructive is the exchange between them on whether and how to discuss the matter with other skiers. A wise and practical assessment from PhysicsMan (emphasis mine):

"I feel that for folks who are not well trained in physics or mechanics, say, most ski students and instructors, in the context of a casual internet or on-the-hill discussion, putting *any* modifier such as "pseudo" or "fictitious" or "apparent" in front of the phrase "centrifugal force" does nothing more than re-fuel the confusion and misunderstanding of this subject which rears its head all too often. Such folks can not be expected to understand the nuances of the differences between inertial and non-inertial coordinate systems in the space of a few minutes or pages of discussion, and instead, they will attempt to use common-language definitions of the above terms (i.e., they will look in a dictionary), and wind up becoming even more confused.

My personal take on this issue is that if someone really wants to "get into it" (say, as sometimes happens here on Epicski.com, e.g., this thread), and a physicist or engineer has the time to teach a mini-course on the subject, fine, otherwise, its better to simply acknowledge the "forces" in their non-inertial frames (which are quite real to them) and not get into discussion of "fundamental or not", "real or not", "pseudo or not", etc
."

I sure would love to ski and have a beer with them.

The fact that we disagree in this issue of physics, a subject where "right" and "wrong" are ultimately not a matter of opinion, explains why there are passionately held opposing ideas about skiing. Belief is tough to change.

Please don't take me wrong. I am just as fixated on my believes as the next human and usually not aware of my blind spots. (But in this matter, I have the backing of the professional physics community over more than 200 yrs. And that is as "correct" as you can get in science )

Regards,

Chuck

Oh, PhysicsMan, where have you gone? Where are you when we need you?

ChuckT--while it may not be "a topic of debate" in the physics community, it most certainly is a topic of disagreement, differences of understanding, and yes, even differences of opinion, if not in fact, certainly in regards to how best to explain or express the phenomenon of centrifugal force. With some amusement, I have witnessed physicists argue about it on more than one occasion. I have known PhD physicists who struggled with the concept of frames of reference just as much as many lay people do--they knew what "the books" said about these things, but their true understanding did not run deep.

And if you think Tom/PM (PhysicsMan) would agree with your contention that centrifugal force is any less real than centripetal force, then I am certain that you have misread him. There have been many discussions about this topic here at EpicSki, and PhysicsMan has contributed many great posts. I wish that the links in that other thread still worked, but a diligent search through the archives should prove fruitful.

Regardless, the root of my disagreement lies not in your preference for one, or some, frames of reference over others. It is in your suggestion that most people think and describe motion consistently in terms of the inertial frame of reference that defines the earth's surface as "stationary." As the many examples in my first post above clearly show, it is extremely common for people to think and observe and describe motion from other frames of reference, particularly the accelerated frame of reference that moves with them--the one in which I do not move, because I am always "here"--the one in which centrifugal force is very very real, and extremely important.

I don't object to your preference for the inertial frame of reference. I fully agree that other frames of reference are not strictly "needed," and that if you choose to explain motion only from that inertial reference frame, then there is no need to explain centrifugal force, because it truly does not exist (as a motive force) in that reference frame. But that does not mean that other frames of reference are not equally valid and real, and a personal preference for one certainly does not preclude others from preferring or relating to another one. While any frame of reference can be used to describe any motion, the description can be often simplified by selecting a particular reference frame purposefully--and I have provided examples of this in my first post as well.

So, where are you?

Best regards,
Bob

Chuck

Your problem seeing things the "right" way stems from your understanding of Newton's third law, " there is no centrifugal force to balance centripetal force".   Slow down for a minute and think about it. "Centrifugal force" does not arise "to balance centripetal force" .  What there is force acting on an object in one direction, and an equal in magnitude force acting on another object in the opposite direction.  Sometimes these two directions are centripetal and centrifugal by coincidence.  The balancing has nothing to do with centrifugal forces per se.  Centrifugal is merely an adjective describing a direction "fleeing the centre",  "Centrifugal Force" in the generally understood meaning of the term in the physics world is  the body force that must act on all objects in a particular  accelerating frame of reference ( a circular one) in order for Newton's laws of motion to work in that frame of reference.  Saying it does not exist is like saying gravity does not exist.  It is as real as any force in any frame of reference in that it is necessary for it to exist for F=ma to apply.

Of course, forces are merely imaginary constructs we use to better rationalize our lives.  "Nothing is real" ....John Lennon.

Quote:

Originally Posted by Ghost

What there is force acting on an object in one direction, and an equal in magnitude force acting on another object in the opposite direction.

[response deleted due to inaccuracy.]

Edited by jeff2010 - 12/17/11 at 4:51am

First, there is no disagreement about the equations or what the outcome of a particular calculation would be.  The disagreement comes when we try to talk about what it "means."  While that is important for suggesting directions to explore for new innovations in theory, it has no impact on the practice of the established parts of physics.  Hence the (perhaps apocryphal, but it sounds like him) Richard Feynman quote, "Shut up and calculate!"

But this discussion we have been having is relevant to, of all strange things, the foundations of quantum mechanics (QM).  When people argue about the Copenhagen interpretation of QM or the Everett many-worlds interpretation, or an interaction-based instrumentalist interpretation, there is a sense that the strange and mysterious nature of QM is the source of the disagreement.  But this discussion shows it is possible to have a similar, fervent, "religious" disagreement about something as cut and dried as Newtonian physics!

JASP's objection that there actually is a preferred frame of reference, the one defined by our sense of vision, is an interesting point that I don't believe has ever been raised in these discussions before.  And it is certainly true that vision is important in balance. (That is part of the reason skiing in a whiteout is so difficult.  Or compare the difficulty of balancing on one foot with your eyes open and closed.)

And on a metaphysical / cosmological level, it is sort of like Mach's principle (the not-quite-successful attempt to derive inertia from the influence of the rest of the stuff in the universe, making "inertial frames" those which are inertial with respect to the average). Even though it didnt literally work, it was one of the threads contributing to the development of General Relativity.  Or on a more modern note, there is clearly "something" special about the rest frame of the cosmic background radiation.

I admit to not having read the entire thread, but can't it just be simplified to this?:

-Centrifugal force is the force of the mass/weight of the skier, wanting to go in one direction once moving, aka inertia.

-Centripetal force is the resistance supplied by the skis/edges, causing the mass of the skier to follow an arc around a central point, instead of going in the direction of inertia.

-That resistance to natural inertia is why we feel g forces when carving turns, and must lean into the turn: the inward projection of our weight must match the outward centrifugal force, which is in turn contained by the centripetal force provided by the ski.

The best analogy is a roller coaster. The cars stick to the track in a loop because the track supplies centripetal force to counter the train's centrifugal force, ie, the track keeps the cars from sailing into the air. The deflected inertia creates the g forces passengers feel.

I think that is what we have been trying to get at here, in various ways.

Or are we already way past that?

Edited by LiveJazz - 12/16/11 at 12:27pm

It's only preferred as long as our eyes are open. Can't shut it off otherwise. Actually we can with a blindfold but most risk managers poo poo that idea nowdays. Which is funny since they allow blind skiers to use a coaches eyes while skiing. I remember a balabce and stance clinic where we unbuckled our boots and skied with a sight coach. I also remember an avalanche clinic where we worked on clearer radio communications by performing an indoor search blindfolded and by using only radio commands. The people we had to rescue had a map to guide us to them and we had to communicate to them what we discovered as we felt our way out of our starting room, down a long hallway with several turns and finally into their room. It pointed out just how much we rely on our sense of sight. Sometimes to the exclusion of what our other senses are telling us. So in that way I would say Bob's using an internal frame is similar since it shifts our focus to what we are feeling instead of what we are seeing. The fact that both of the clinics I mentioned happened at least fifteen years ago and they are still as vivid as the day I did them say there is a lot of value in learning to listen to what our other senses are telling us.

That doesn't mean I want to ski trees blindfolded though, which is the bottom line here. We might focus on a particular sensory input but rarely to the exclusion of that bigger world around us.

Quote:
Originally Posted by jeff2010

This is where you are wrong. The "equal and opposite force" situation *only* applies if the object is at rest or is moving in a straight line! An object moving in a circle is acted on by a force which is *not* counterbalanced by an equal and opposite force, which is precisely why the object turns instead of moving in a straight line. That force is called the centripetal force.

-Jeff, former physicist

I'm glad you included"former" in the sig.  What is forcing the object around in that circle (eg. string pulling a tether ball, Earth pulling the Moon, snow pushing skier's ski base)?  The object is exerting an equal in magnitude and opposite in direction force on whatever is making it travel in that circle (eg. ball is pulling back on the string, moon is pulling the Earth, ski is pushing the snow).

I'm here Bob - I was going to stop, but since you asked, I thought it would be impolite not to respond. I will once more address what I see as your incorrect interpretations (the highlighted parts of the quote below)

1. while it may not be "a topic of debate" in the physics community, it most certainly is a topic of disagreement, differences of understanding, and yes, even differences of opinion

It is semantic. There is neither debate nor disagreement regarding the nature of centrigual force within the professional physics community. There may be debate or disagreement on how to educate students on it, but that's a pedagogical issue, not physics. Please give me a physics text reference or a professional physics website that says centrifugal force is a real force.

2. if you think Tom/PM (PhysicsMan) would agree with your contention that centrifugal force is any less real than centripetal force, then I am certain that you have misread him.

I am certain I did not misread him. Please re-read his exchange with amiles in that threat. (I am fairly certain that he is a nicer and wiser member of the physics community than I am. He pointed out the futility of arguing about this topic with "folks who are not well trained in physics or mechanics, say, most ski students and instructors, in the context of a casual internet ".) PhysicsMan can pop in and resolve our disagreement easily enough.

3. the root of my disagreement lies not in your preference for one, or some, frames of reference over others. It is in your suggestion that most people think and describe motion consistently in terms of the inertial frame of reference

You wrote eloquently how people often use an accelerating frame of reference and I had claimed they rarely do. Here is the thing: The same people turn around and talk about F = ma which is OK if and only a is the acceleration relative to an inertial frame of reference. I can see now that people indeed think of their accelerating selves as reference, but it is clear that they don't know how to do that properly as a technical matter (which I assume is what we are talking about here, not just human feelings). Again, to quote the polite PhysicsMan: "Such folks can not be expected to understand the nuances of the differences between inertial and non-inertial coordinate systems"

4.  While any frame of reference can be used to describe any motion, the description can be often simplified by selecting a particular reference frame purposefully (my addition: only if correctly done)

I do not disagree, as I had said before. A trained physicist can use any frame of reference but understand that not all are equivalent and treat them differently. We do this in a lot of situations. Hiking last weekend in the late afternoon with my young kids, I estimated the time before the sun "went behind" the mountain as if the sun moved around the earth in a circle in a day. I had to caution my kids that it's just a model with apparent motion to derive a simple solution, not that the sun actually moved like that lest they share our fun with their friends or teachers and cause an uproar in school that their supposedly highly educated dad is a flat earth guy. That apparent motion appeared extremely real to them.

With respect to mdf observation that "this discussion shows it is possible to have a similar, fervent, "religious" disagreement about something as cut and dried as Newtonian physics!", I would say No, not among physicist for at least 200 years. Only among folks who do not understand the nuances of the differences between inertial and non-inertial coordinate systems would such disagreement happen.

I believe none of this matters in the learning, teaching and practice of skiing (of which you are a respected professional). People don't typically go to the slope for physics lesson with ski instructors. However, when we invoke physics to give the discussion the appearance of the rigor, usually implied and associated with this venerable discipline, we should do it right. It is not a matter of opinion.

Regards,

Chuck

Quote:
Originally Posted by Bob Barnes

Oh, PhysicsMan, where have you gone? Where are you when we need you?
ChuckT--while it may not be "a topic of debate" in the physics community, it most certainly is a topic of disagreement, differences of understanding, and yes, even differences of opinion, if not in fact, certainly in regards to how best to explain or express the phenomenon of centrifugal force. With some amusement, I have witnessed physicists argue about it on more than one occasion. I have known PhD physicists who struggled with the concept of frames of reference just as much as many lay people do--they knew what "the books" said about these things, but their true understanding did not run deep.
And if you think Tom/PM (PhysicsMan) would agree with your contention that centrifugal force is any less real than centripetal force, then I am certain that you have misread him. There have been many discussions about this topic here at EpicSki, and PhysicsMan has contributed many great posts. I wish that the links in that other thread still worked, but a diligent search through the archives should prove fruitful.
Regardless, the root of my disagreement lies not in your preference for one, or some, frames of reference over others. It is in your suggestion that most people think and describe motion consistently in terms of the inertial frame of reference that defines the earth's surface as "stationary." As the many examples in my first post above clearly show, it is extremely common for people to think and observe and describe motion from other frames of reference, particularly the accelerated frame of reference that moves with them--the one in which I do not move, because I am always "here"--the one in which centrifugal force is very very real, and extremely important.
I don't object to your preference for the inertial frame of reference. I fully agree that other frames of reference are not strictly "needed," and that if you choose to explain motion only from that inertial reference frame, then there is no need to explain centrifugal force, because it truly does not exist (as a motive force) in that reference frame. But that does not mean that other frames of reference are not equally valid and real, and a personal preference for one certainly does not preclude others from preferring or relating to another one. While any frame of reference can be used to describe any motion, the description can be often simplified by selecting a particular reference frame purposefully--and I have provided examples of this in my first post as well.
So, where are you?

Best regards,
Bob

Quote:
Originally Posted by ChuckT

With respect to mdf observation that "this discussion shows it is possible to have a similar, fervent, "religious" disagreement about something as cut and dried as Newtonian physics!", I would say No, not among physicist for at least 200 years. Only among folks who do not understand the nuances of the differences between inertial and non-inertial coordinate systems would such disagreement happen.

Well, I understand the nuances very well.  And this disagreement is getting pretty "religious".  So either your statement is wrong, or ...

(edit- snark level toned down)

Edited by mdf - 12/16/11 at 8:41pm
Quote:
Of course, forces are merely imaginary constructs we use to better rationalize our lives.

No--it's much more important than that, Ghost. Forces are what we use for skiing!

(Great post, by the way.)

Best regards,
Bob
Quote:
Originally Posted by jeff2010

This is where you are wrong. The "equal and opposite force" situation *only* applies if the object is at rest or is moving in a straight line! An object moving in a circle is acted on by a force which is *not* counterbalanced by an equal and opposite force, which is precisely why the object turns instead of moving in a straight line. That force is called the centripetal force.

-Jeff, former physicist

Jeff--I may be wrong, but I think that that is exactly what Ghost was trying to say. While in a sense, centrifugal and centripetal forces are equal and opposite, they "exist" in different frames of reference, and/or they apply to different bodies, depending on how you look at it. With the classic ball-on-a-string example, centripetal force acts on the ball, making it move (accelerate) in a circle, while centrifugal force acts on your hand, caused by the turning ball. Both forces are actually the same thing, of course--the string. Both have the same magnitude, but they do pull in opposite directions. Both are just as real and just as measurable. But they should not be included in the same equation as forces that "cancel," for the reasons you've described.

There was a great discussion quite some time ago, in which PhysicsMan explained and analyzed the forces involved when two equal-mass balls connected by a string spin freely through space, each pulling on the other. The "system" would naturally rotate about a point in the middle of the string, as each ball pulls against the other, each creating the centripetal force that causes the other to move in a circle. Both forces are, of course, completely real....

Best regards,
Bob
Quote:
Originally Posted by justanotherskipro

It's only preferred as long as our eyes are open. Can't shut it off otherwise. Actually we can with a blindfold but most risk managers poo poo that idea nowdays. Which is funny since they allow blind skiers to use a coaches eyes while skiing. I remember a balabce and stance clinic where we unbuckled our boots and skied with a sight coach. I also remember an avalanche clinic where we worked on clearer radio communications by performing an indoor search blindfolded and by using only radio commands. The people we had to rescue had a map to guide us to them and we had to communicate to them what we discovered as we felt our way out of our starting room, down a long hallway with several turns and finally into their room. It pointed out just how much we rely on our sense of sight. Sometimes to the exclusion of what our other senses are telling us. So in that way I would say Bob's using an internal frame is similar since it shifts our focus to what we are feeling instead of what we are seeing. The fact that both of the clinics I mentioned happened at least fifteen years ago and they are still as vivid as the day I did them say there is a lot of value in learning to listen to what our other senses are telling us.

That doesn't mean I want to ski trees blindfolded though, which is the bottom line here. We might focus on a particular sensory input but rarely to the exclusion of that bigger world around us.

JASP--I doubt that anyone here would argue with you about the significance of using all of our senses, or the critical role that vision plays for most of us in skiing. But that really is not the point here. In fact, while our eyes detect motion, they do not therefore define a "more real" frame of reference. Opening your eyes and looking out the car or plane window does not "prove" that the earth is still and we are moving about on it--it merely shows that there is motion. Have you ever had the dizzying experience of looking out a train (for example) window and seeing another training passing you, but interpreting it as your train moving backwards? Our eyes are not deceiving us in that situation--they are really just "opening our eyes" to the possibility--and reality--of different frames of reference.

All motion is relative--meaning we can describe the motion of one thing only in relation to another thing, or to something we (arbitrarily, actually) define as "stationary," or to a "coordinate system" that can be centered and anchored on anything we choose--the earth, the sun, the deck of a ship, or our own body or some part thereof, or our center of mass.... It is not any more "real" to choose one frame of reference (coordinate system) over any other, although it can simplify the description and analysis of a motion if we choose one over another. Your eyes can tell you that distance between you and some object is closing, but they cannot tell you whether you're moving toward the object, or the object is moving toward you, or both--because all of these things are true!

But none of this discussion has anything to do with whether we should ski with our eyes open or closed! (Or write, for that matter.)

Best regards,
Bob
ChuckT--I really do appreciate your continued discussion here, and your attempt to explain your perspective. But I am also quite surprised by your adamant belief in two things: first, that all physicists agree on .... well, anything, and second that any one frame of reference is somehow "more real" than any other. Yes, of course, if the analysis in any frame is incorrect, then the analysis is incorrect. That would not be the fault of the frame of reference.

It does not surprise me when I hear a "lay person" say something like, "When you drive fast around a curve in a car, you feel like there is a force pressing you sideways against the car door--perhaps even pushing the door open, if you go fast enough (and are very unfortunate--please wear your seatbelt!). But what is actually happening is that you continue going straight, while the car door, which is moving in an arc, moves toward you, not the other way around....." Not surprised at all. But I am always surprised to hear a trained physicist say something like that. The problem is not the description of you going straight while the car turns and runs into you--that is certainly a legitimate way to look at it. The problem is the suggestion that this description is somehow more "actual" than the description that you are pressing sideways against the car door. The only "really real" truth is that the distance between you and the car door closed. The rest is just two equally accurate ways of explaining it.

Obviously, it takes real force to make a car door open. That's something I suspect that most physicists would agree with. Car doors do not just fly open all by themselves. (Yes, I know, you still want to describe it from another frame of reference, in which the car does NOT "actually" fly open, it just goes straight while the car itself moves away from it (and let's not even count the rotational force or torque that makes it pivot on its hinges). But come on, Chuck--to have attained a degree in physics, you clearly have the mental agility required to recognize that all frames of reference are equally valid. (This is not to suggest that they are all "the same," or that there are not "nuances" of difference--or vast polar chasms of difference, for that matter. There are--for example, centrifugal force is very real in some frames of reference, and does not exist in others.)

You have a personal bias toward a particular frame of reference, Chuck--you have repeatedly and eloquently said so. I do not. That's all. I do not believe that we disagree on at least most of the facts--we just don't always choose to look at them always from the same frame of reference, As you say, it is not about opinion. The bottom-line critical question where the rubber meets the road (or is it the other way around?), especially considering the title of this thread, is this: do you believe so strongly in your contention that the phenomenon known as centrifugal force is "fiction" and "not real" that you are willing to make a high speed turn on skis without inclining toward the center of the arc? If so, please have someone video you--I would not want to miss it!

I don't think you believe that at all. And I'm sure that, if you were so inclined, you would accurately point out that "that is not what I've been saying at all--it's just that the explanation for why I need to incline is not "actually" the way you have described it...." I hope you can see that I do "get" that already, and that a repeat of your explanation is not going to change anything.

Best regards,
Bob

PS--Do you know how many ski instructors at some of New Mexico's resorts have a "day job" as physicists at the Las Alamos National Labs near Santa Fe--which employs some of the world's top physicists? Well, neither do I, in fact, but I have met and skied with quite a few of them. You'd be surprised how many physicists also teach skiing, and consider skiing to be a wonderful way to experience and experiment with applied mechanics, having fun playing with gravity (and centrifugal force). I have a ton of respect for physicists--including you. But I also know that they are no more infallible than anyone else. And they do not all always agree--as you can see here in this very thread. But the accurate descriptions of motion from two different reference frames will not "agree" at all--and might well sound completely opposite and contradictory. That does not make either of them necessarily wrong!

Hi Bob,

I admire your writing and enjoy your posts. Saying you were wrong, as I did, is probably inappropriate. What I should have said is that your view of the nature of centrifugal force as I understand it from your writing is not that of the physics community as reflected in the text books that generations of physicists have learned from. I presented the standard physicist's reasoning that I am sure you get it. Of course, I agree that one can certainly explain the phenomenon of skiing just as accurately, and perhaps more intuitively, with centrifugal force in the appropriate frame of reference as you stated. That this force is "unreal" in the physicists' view is largely academic in the context of the OP's question. Sorry I was being unnecessarily dogmatic.

I am currently holed up in a Motel 6 at Mammoth anticipating the pleasure of being on skis again tomorrow. I think it will be very icy. We will see if I can finally manage to turn without skidding in such condition.

Regards,

Chuck

Bob you' re reaching a bit here. We aren't on a train, or in a car, we are sliding down a snow covered hill. So it is us in motion not the trees, or the rocks and the snowpack (although a hydroligist friend of mine would remind me that the snow is creeping along very, very slowly down the hill. But that's a lesson for another thread.)

How our eyes relate to our traveling is that our eyes are the window to that external world and like a camera they can be fooled but only to a point. That doesn't mean trees that are firmly rooted in the ground are actually moving towards us, in spite of my colorful but fictitious stories about trees that move and trip us as we ski, they are quite stationary. I understand the PoV and FoR argument from both sides quite well but I want to be there in your next class when you tell them they aren't moving at all and it's the trees that are actually moving towards them. Maybe we should film their reactions.

Back to the OP's question though, Centripetal forces are forces that accelerate us into a turn, without them we would simply travel in a straight line. Period. End of story, question answered. It really doesn't take a doctorate in physics to understand that if you don't turn you will go straight.

Edited by justanotherskipro - 12/17/11 at 3:08am
Quote:
Originally Posted by ChuckT

I'm here Bob - I was going to stop, but since you asked, I thought it would be impolite not to respond. I will once more address what I see as your incorrect interpretations (the highlighted parts of the quote below)

Hiking last weekend in the late afternoon with my young kids, I estimated the time before the sun "went behind" the mountain as if the sun moved around the earth in a circle in a day. I had to caution my kids that it's just a model with apparent motion to derive a simple solution, not that the sun actually moved like that lest they share our fun with their friends or teachers and cause an uproar in school that their supposedly highly educated dad is a flat earth guy. That apparent motion appeared extremely real to them.

It seems obvious from reading the above that you are devoted to your belief that one particular frame of reference is "real" and the others are not "real".  You can see how the less familiar model is "only a model", but you can't see your own "reality" as a model too.

BTW and IMHO, even Galileo and Urban were sufficiently aware of the nature of natural philosophy to know better, and their much reported feud and resulting house arrest had more to do with the politics of power and ancillary impacts of a preferred system than actual belief in one system being more "real" than the other.

@ Bob,

I never said it wasn't important!  It's one of the most important tools we have.

BTW, Even the ancient Greeks knew the earth was not flat; they calculated it's radius.  It's only the uneducated who believed Earth to be flat.

Quote:
Originally Posted by Ghost

I'm glad you included"former" in the sig.  What is forcing the object around in that circle (eg. string pulling a tether ball, Earth pulling the Moon, snow pushing skier's ski base)?  The object is exerting an equal in magnitude and opposite in direction force on whatever is making it travel in that circle (eg. ball is pulling back on the string, moon is pulling the Earth, ski is pushing the snow).

You're right and I'm wrong, of course. How mortifying. I knew I'd forgotten stuff over the past two decades, but not to that extent! :(

I thought Bob was working to get Chuck to say "I'm here."  And then he doesn't deliver the punchline!  Or does Bob just have a lot more self control than the rest of us?

As for real physicists believing in centrifugal force...

I have never seen anyone label anything as "real" or "fictitious" beyond a freshman physics textbook.  It is not an agreed convention of the physics community -- but rather a convention of the "Intro to Physics" author's community.

You will see discussion of whether forces are "fundamental" or not, but never "real" or not.

I can't find my copy of Goldstein, but I am interested now to see what he says in the introductory paragraphs before we get generalized forces in generalized coordinates.  In a classical mechanics sense, force is whatever appears on the other side of "dp/dt =".

Another example, Landau and Lifshitz, "Fluid Mechanics" (vol 6 of "Course of Theoretical Physics"), section 1.14, "Waves in a Rotating Fluid".

"Another kind of wave can be propagated in an incompressible fluid uniformly rotating as a whole.  These waves are due to the Coriolis forces which occur in rotation. ...The centrifugal force can be written as grad((1/2)Omega x r)^2, where ... the Coriolis force is 2v x Omega, ..."

Just very straightforward handling of these forces.. no hand-wringing over there metaphysical bonefides.

--------------------------------------------------------------------------

Different topic - I do find JASP's point of view somewhat persuasive.  The difference between us and the rest of the world, is that there is so much more of it than of us.  And vision gives us a large-scale average sense of where the world is.  (There is a reason generaly relativity thought experiments are set in an elevator without windows.)  And as a point of physiology, that vision does affect balance a lot.  Consider the fact that sea sickness can often be avoided by looking at the horizon.

Quote:
Originally Posted by mdf

As for real physicists believing in centrifugal force...

I have never seen anyone label anything as "real" or "fictitious" beyond a freshman physics textbook.  It is not an agreed convention of the physics community -- but rather a convention of the "Intro to Physics" author's community.

You will see discussion of whether forces are "fundamental" or not, but never "real" or not.

I can't find my copy of Goldstein, but I am interested now to see what he says in the introductory paragraphs before we get generalized forces in generalized coordinates.  In a classical mechanics sense, force is whatever appears on the other side of "dp/dt =".

Another example, Landau and Lifshitz, "Fluid Mechanics" (vol 6 of "Course of Theoretical Physics"), section 1.14, "Waves in a Rotating Fluid".

"Another kind of wave can be propagated in an incompressible fluid uniformly rotating as a whole.  These wav1es are due to the Coriolis forces which occur in rotation. ...The centrifugal force can be written as grad((1/2)Omega x r)^2, where ... the Coriolis force is 2v x Omega, ..."

Just very straightforward handling of these forces.. no hand-wringing over there metaphysical bonefides.

--------------------------------------------------------------------------

Different topic - I do find JASP's point of view somewhat persuasive.  The difference between us and the rest of the world, is that there is so much more of it than of us.  And vision gives us a large-scale average sense of where the world is.  (There is a reason generaly relativity thought experiments are set in an elevator without windows.)  And as a point of physiology, that vision does affect balance a lot.  Consider the fact that sea sickness can often be avoided by looking at the horizon.

mdf, are you a physicist?

You don't seem to understand the education of physicists. In physics we teach the basic stuff at the freshman level. But we don't teach students the wrong things or ambiguous interpretations, ie things that have to be unlearned or corrected later on in more advanced courses. We don't teach metaphysics. So this stuff is considered basic, taught and learned as freshman or sophomore. This concept, though technical, is nothing deep. Landau and Lifshitz is an advanced series. The fact that L&L don't talk about it being real or unreal should be an indication to you that there is no controversy whatsoever about this stuff. "Intro to physics" texts are not written by quacks, unvested for use, and taught by the unqualified (for the most part any way, the future of public university in America is a concern)

I don't know you and therefore what I say next may be completely inapplicable to you. I just see a striking similarity in your line of reasoning with what I often encounter.

In my experience, it is very common to find people throwing smoke and mirror, especially at parties before they learn about the background of their new party mates, with fancy terminologies, a named equation or such devices when they don't really know what they are talking about. For example, you throw out a differential quantity dp/dt. Your statement " In a classical mechanics sense, force is whatever appears on the other side of "dp/dt =". is of course correct, but only in that form when p, the momentum = mv, is determined in an inertial frame of reference (any inertial frame of reference). Nevermind my mumbo jumbo. Let's do the math for a skier. In the earth frame, you get the centripetal force. No problem here, right? In his "skier here" frame, he is not moving  by definition, therefore p=0, dp/dt=0. No centrifugal , no centripetal, no forces at all. It doesn't compute! So, what are you saying with dp/dt here? How do you dig out the centrifugal force with your math?

The problem, of course, is nothing deep. It's just a faulty handling of non-inertial frame of reference.

In an earlier post, you implied a that the important thing in physics is the math. Well, I just did your (wrong) math for you and the results are physically absurb: You could simply turn forces on and off by changing a human, arbitrary choice of reference (????!)

In my bantering on the concept of centrifugal force here, I realize and said that I was being unnecessarily dogmatic and not helpful i the context of skiing. Whether centrifugal force is real or not is not important to discussion with non-physicists as PhysicsMan said. I wholeheartedly agree. Physics is not a subject that can be learned haphazardly, by reading bit and pieces or engaging in casual conversation.

You are of course completely entitled to your opinons, thoughts, and believes on here. This "discussion" has lost its fun factor for me.

Happy skiing,

Chuck

The time rate of change of momentum is equal to the sum of the forces, of which the centrifugal (if the frame of reference is accelerating in a curved fashion) force is one of the forces, which together may well add up to a sum of zero.  That the sum is zero, does not mean the centrifugal force is zero.

Just say'n.

Quote:
Originally Posted by justanotherskipro

Back to the OP's question though, Centripetal forces are forces that accelerate us into a turn, without them we would simply travel in a straight line. Period. End of story, question answered. It really doesn't take a doctorate in physics to understand that if you don't turn you will go straight.

Yep, I guess the simplest way to look at it is that we have momentum carrying us in a straight line and when we deflect our direction of travel with our edges, centripetal forces are created that cause us to turn.  That much is certain.

And technically speaking, inclination is required only because the centripetal forces are pushing the skis towards the center of the arc, which if we are not perfectly in "balance" (don't forget gravity in the balance equation), the skis would be pushed around us instead of directly at our COM, (resulting in a low side or high side).

And in some ways, when we are thinking about our ski manipulations (ie, edging, bending, steering angle etc), That is exactly what we are trying to do, we are trying to create centripetal forces and get them to zoom our skis this way or that.

Fine so far and useful to think about it that way!

But when we start talking about balance, the truth is that its highly useful to think in terms of our accelerated frame of reference, as Bob defined it, because that is what we feel and experience when we ski in terms of maintaining our balance.  And its simpler for most of us.  At least conceptually, its a lot easier to think about what direction our COM is trying to go, and then balance it on the BOS, thinking of the BOS more like a stable platform, instead of something that is pushing back at us; and that we are trying to make sure it pushes exactly into us, and not around us, causing loss of balance.

In terms of balance, is our COM a tightrope walker or a catcher's mitt?

If we had to think about balance in terms of centripetal forces and the earth frame of reference instead of the "legendary" centrifugal force, then it becomes quite hair brained and unintuitive.  Just imagine, centripetal forces are pushing away from the direction inertia is going and gravity is also pulling in yet a new direction that is close to the momentum, but quite different then the centripetal direction.

Yet if we think in terms of centrifugal, then gravity is pulling down into the ground (and somewhat down the hill), momentum is carrying us down the hill also, same general direction.  The sensation of centrifugal force is also generally going broadly in the direction we are traveling.  It all adds up to a so called "resultant force vector" as LeMaster and others are fond of calling it in the ski world.  Its very easy to understand this and intuit what you need to do to remain in balance, since it feels more like a singular G force resulting from several vectors that are all generally pointed in the same general direction.

Thinking of balance in terms of centripetal forces and the earth frame of reference, while perhaps more accurate according to some, is definitely more complicated to understand and intuit, in terms of balance.

In short, I am saying that when it comes to talking about skiing, its beneficial to talk in terms of centripetal forces for some things (ski manipulations) and its more convenient to talk in terms of the mystical centrifugal force for other things (balance).  Does it really matter which one is the correct one or not, or which frame of reference is preferable?  I find both frames of reference useful for thinking about skiing, it just depends what aspect of skiing we're talking about.

Chuck -

I'm sorry if this discussion is getting too serious.  But I find I cannot let this go without a few more points.

I find it aggravating that you assume I am a newb who doesn't know what he is talking about rather than considering my arguments.  I assure you I understand the physics at least as well as you.  I was giving you the courtesy of assuming that you would recognize and understand the examples without a ton of background, and then use them to consider my points.  We can trade resumes by PM if that will make you happy.

You seem to think we are arguing about equations of motion.  In fact we are arguing about language.

I still claim that the phrase "fictitious force" is a pedagogical invention of intro-to-physics authors that did not come from the broader community.

And of course we teach things that need to partially un-learned later, or at least seen as a simplification of a more sophisticated real story. Think wedge skiing giving way to parallel.  Think Newtonian mechanics giving way to special relativity and quantum mechanics which in turn give way to general relativity and relativsitic QM which then gives way to field theory.  We don't usually describe that progression as "oh, what we just taught you was wrong" but strictly speaking it would be an accurate thing to say. (There is more disagreement about whether teaching the wedge is useful pedagogically than my other examples, of course.)  Think of your first calculus teacher telling you that thinking of dx as an infiinitely small displacement is handy but not really legit, and then later finding out that infinitesimals can be made rigorous and it is possible to develop calculus that way.  Or English teachers teaching not to begin a sentence with a conjunction.  But Hemingway does it a lot.  There are lots of things we teach freshmen that later have to be given caveats.

I guess I did cheat a little on the dp/dt = point, because one would actually say "generalized force" not just "force."  But my point is that the definition of force is that it appears in an equation with change of momentum on the other side.  If you want to have forces at all in a more general context (e.g. QM or general relativity) you have to define them that way.  Of course, it would also be valid to say that "force" is a technical term of Newtonian mechanics, and the term just doesn't apply outside of that context.  I like my way better.

Last night I dug out my copy of Misner, Thorne and Wheeler's "Gravitation" looking for a good quote to use.  I stumbled on a section on "what does Newtonian physics look like in the language of spacetime geometry."  And of course, they get the usual equations of motion, but organized quite differently.  Then they discuss how a hypothetical observer could determine whether or not he is in an inertial frame experimentally.   And here is my point: you verify that you are in an inertial frame by measuring the Coriolis force and verifying that it is zero. Obviously, you would describe the goal of the experiment quite differently, and you are free to do so.  Nonetheless, here is an example where "real physicists" find the most natural way to talk about Coriolis is as a real force on the same footing as any other.

BTS, that's where mixing FoR's is confusing, your talking about moving your CoM in a direction but is that really possible in that AFoR. Many here, even Bob admit that isn't possible. So while it's valuable to use that FoR, perhaps it's not as valuable when you start talking about  moving in a particular direction. Feeling your shins press against the tongues, feeling pressure across the bottom of the foot, feeling your legs flex / extend, all of these do not need an external FoR. Start talking about moving in a direction,or towards something external to your body and you do. Turn to the trees, turn across the hill, Slow line fast, all have an external FoR, or as Bob called it coordinate points.

Quite Correct MDF. If you are not a park and ride skier, you will of course want to accelerate, no matter your frame of reference (FoR).  If that frame is moving with the skier, then you are changing the acceleration of the FoR, and that is no longer a simple discussion, except to view it for an infinitesimal dt, but some physicists or math teachers  might take umbrage at that .

MDF,

Quote:
Originally Posted by justanotherskipro

BTS, that's where mixing FoR's is confusing, your talking about moving your CoM in a direction but is that really possible in that AFoR. Many here, even Bob admit that isn't possible. So while it's valuable to use that FoR, perhaps it's not as valuable when you start talking about  moving in a particular direction. Feeling your shins press against the tongues, feeling pressure across the bottom of the foot, feeling your legs flex / extend, all of these do not need an external FoR. Start talking about moving in a direction,or towards something external to your body and you do. Turn to the trees, turn across the hill, Slow line fast, all have an external FoR, or as Bob called it coordinate points.

First of all, are we really talking to most students about physics and Frames of Reference?

I think its only confusing to switch FoR for physicists and cerebral people like some of us on this forum who are always trying to think about everything so much and have everything make absolute sense.  For most people, its quite easy and natural to think about balance in terms of falling down or not, falling high side or low side or not, etc.  Generally, that is the accelerated FoR for them.

On the other hand when thinking about turn shape, I also think it is quite easy and natural for them to feel the centripetal forces pushing them out of a direct line of motion, talking about centrifugal forces when talking about turn shape would be utterly confusing as well.

I think people can and do switch their frames of reference on the fly without thinking about it all the time, and while it would drive a physics professor to the nut house, I see it as perfectly acceptable to do so.  We aren't trying to perform any physics experiments or calculations, we are trying to ski.

Quote:
Originally Posted by Ghost

Exactly.  Now I know why the true experts get so riled up on the ski technique threads!

New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: Ski Instruction & Coaching
EpicSki › The Barking Bear Forums › Ski Training and Pro Forums › Ski Instruction & Coaching › Centripetal Force How Does It Relate To Skiing?