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# Centrifugal force - Page 2

Quote:
 Originally Posted by Baja I know that some of you folks are aware of this, but some of you may not be. Just FYI: There is no such thing as "centrifugal force." And in the manner in which some of the folks here are using this term, that force they are describing does not represent a true physical force. "Centrifugal force" is a pseudo-force, an illusion that is the product or result of inertia. Newton's First Law informs us that mass, once moving in some direction with some velocity, will continue to move in a straight line in the same direction with the same velocity unless some external force acts upon it to bend its path, speed it up, or slow it down. This is inertia, the underlying principle that produces the "centrifugal force" effect. If you are in a car that turns a corner, inertia is what causes your body to try to continue in a straight line. Forces exerted by the seat, the seat belt, the doors, tires, car frame, etc. are required to push your body in the new direction. Thus, you have the illusion (because your point of view is in a reference frame that is being accelerated in a circle) that some external force is "building up" to "push you away" from the center of the turn. There is no such force. What you actually experience is inertia trying to continue your motion in a straight line while the structure of the car around you is moving in the circular path of the turn. As I mentioned, I realize that at least some of you folks know this already... but it's possible that some of you may not... so I offer this post in that spirit. It may seem like splitting hairs, but I do think it's important to be clear on certain concepts if we're going to teach them... and that includes knowing the difference between "centrifugal force" and "inertia." Thanks for your time and attention! :::: running and ducking for cover ::::
Baja:

Notwithstanding the bit of a shitstorm that you've unleashed by bringing up this never ending controversy in the rarified world of pure physics,
you might be interested to know that the new CSIA ( Canadian Ski Instructors Alliance ) manual NOW uses the term "inertial force" instead of "centrifical force" in their chapter on physics and forces. How about that, eh?

cdnguy ( stirrin' it up! )
Quote:
 Originally Posted by cdnguy Baja: Notwithstanding the bit of a shitstorm that you've unleashed by bringing up this never ending controversy in the rarified world of pure physics, you might be interested to know that the new CSIA ( Canadian Ski Instructors Alliance ) manual NOW uses the term "inertial force" instead of "centrifical force" in their chapter on physics and forces. How about that, eh? cdnguy ( stirrin' it up! )
Introductory physics texts should do the same. Using the term centrifugal force tends to lead to a general confusion on the subject of the fundamental role of intertia in the subject of dynamics. Textbooks don't do a good job of seperating the notion of an inertial force from the notion of the commonly known 'fundamental' forces of nature..gravity elctromagentism , ect. This is why many students become confused when the concepts of inertial and gravitational mass are discussed in relativity theory. They never understood what classical dynamics was saying to begin with.
Quote:
 Originally Posted by JackFrost The force is real - you feel it. What I am saying is there is not a 'force' that initiates the displacement. The force of centripital acceleration is a result of the inertial properties of the mass resisting the change in direction. Your body wants to keep going straight and your skis are turning so your body mass presses into the ski giving the resulting feeling of being 'pulled' towards the center of the arc. In other words your body wants to keep going straight but the positioning on the skis prevents it from doing so and the result is you get 'mashed' into the ski.
So if one of my students tell me that there is no such force as a centrifugal force I can quickly reply:
- in simple terms in ref. to the snow we are skiing on there is....
or:
- sorry, I ment the resisting of the inertia....
Quote:
 Originally Posted by tdk6 So if one of my students tell me that there is no such force as a centrifugal force I can quickly reply: - in simple terms in ref. to the snow we are skiing on there is.... or: - sorry, I ment the resisting of the inertia....
See the post I just made...this is really turning into semantics. IN everyday words any 'push' or 'pull' is a force.

There is often confusion on this subect and as I said it is in a large part due to the how the concept of inertia is presented in introductory textbooks(if it is presented at all). Students need to learn from the start the concept of inertia and the inertial force and how this differs in theory from the notion of a fundamental force such as gravity or electromagentism. Even many high school educators I believe are confused on the subject.

I would just tell the students when they ask that any time you change direction or oreintation there will be a reactive force felt somewhere in the body. When undergoing circular motion or rotation this will be felt as 'centrifgual' or an outward pushing force as your body's inertia keeps trying to maintin its present momentum on a linear path. YOur students are basically feeling the force of working to keep the body on the same trajectory as the skis.
Your student is probably seeing things from a frame of reference fixed to the snow. You should keep it simple and say, "Correct, there is no centrifugal force, just momentum that you have to resist by applying a centripetal force to make you turn."
Note: you can apply a centripetal force before you are turning, and thus use it to initiate a turn before the existence of any centrifugal force.
Quote:
 Originally Posted by Ghost Your student is probably seeing things from a frame of reference fixed to the snow. You should keep it simple and say, "Correct, there is no centrifugal force, just momentum that you have to resist by applying a centripetal force to make you turn." Note: you can apply a centripetal force before you are turning, and thus use it to initiate a turn before the existence of any centrifugal force.
Brilliant Ghost, thanx . Yes, I got it , but lets keep a low profile so that we dont get squashed by gravity!
Just semantics, Jack? Well, of course! It is a question of definition. Centrifugal force is something a person either understands, or doesn't. Yes, it is an inertial force--by definition--so I don't see why preferring "inertial force" over "centrifugal force" clarifies anything. Centrifugal force is a specific type of inertial force, an inertial force that results from turning as opposed to other forms of acceleration (slowing down or speeding up). It is (when understood) more specific, and thus more explanatory, than the general term "inertial force." But one would have to comprehend the definition of either term.

For some reason, few ever question the "reality" of centripetal force, yet it is no more a "fundamental force such as gravity or electromagnetism" than centrifugal force is. Centripetal force refers to any motive force--gravity, magnetism, wind, a string attached to a ball, snow pushing on skis, or any combination, that causes a body to accelerate (move) in an arc. And centrifugal force is the result. To be picky, as I noted before, from my accelerated frame of reference as a skier, it is centripetal force that doesn't "really" exist. No frame of reference is any more correct or real or true--any frame of reference is arbitrary--so it is folly to simply proclaim that centripetal force is "real" and centrifugal force doesn't exist, a "pseudoforce." It could just as truthfully be the other way around!

By the way, the common argument that "if you cut the string, the ball will not fly directly away from your hand--it will fly in the direction it was moving the moment the string was cut, in a straight line tangent to the arc," which is so often used to "prove" that centrifugal force doesn't "really" exist, proves no such thing. It merely reinforces the definitions. The moment the string is cut, centripetal force ceases to exist, so the ball stops accelerating in its circular arc and travels at constant velocity in a straight line (disregarding gravity and other forces that may act on it). Because the ball is no longer "turning," centrifugal force--that measurable pull at right angles to its direction of travel--also ceases to exist at that moment. It's not that the force isn't real. It is gone!

Best regards,
Bob Barnes
Just my opinion that most students are not taught the concept of inertia or presented with the idea of an inertial force in a comprehensive manner. This is whay there always seems to be confusion later on regarding this issue.

The point I made about differentiating inertial forces from the commonly known 'fundamental' forces of nature is not that inertia is any less fundamental. It is never taught as a fundamental force like Gravity or E&M to begin with.

For instance as in this case many think 'centrifugal' force it is not real or is "pseudo" because firstly they never understood the notion of an inertial force and secondly they think a force of nature is synonymous only with gravity, E&M ect.. - The Inertial force does not lay on their mind as a fundamental force and in their mind does not jive conceptually with a force such as gravity. It kind of takes a back seat. Thus the inertial force termed 'centrifugal' becomes some type of "pseudo" force or doesnt really exist.

Anyways it has been an interesting discussion and sorry to hijack the thread.

Quote:
 Originally Posted by Bob Barnes/Colorado Just semantics, Jack? Well, of course! It is a question of definition. Centrifugal force is something a person either understands, or doesn't. Yes, it is an inertial force--by definition--so I don't see why preferring "inertial force" over "centrifugal force" clarifies anything. Centrifugal force is a specific type of inertial force, an inertial force that results from turning as opposed to other forms of acceleration (slowing down or speeding up). It is (when understood) more specific, and thus more explanatory, than the general term "inertial force." But one would have to comprehend the definition of either term. For some reason, few ever question the "reality" of centripetal force, yet it is no more a "fundamental force such as gravity or electromagnetism" than centrifugal force is. Centripetal force refers to any motive force--gravity, magnetism, wind, a string attached to a ball, snow pushing on skis, or any combination, that causes a body to accelerate (move) in an arc. And centrifugal force is the result. To be picky, as I noted before, from my accelerated frame of reference as a skier, it is centripetal force that doesn't "really" exist. No frame of reference is any more correct or real or true--any frame of reference is arbitrary--so it is folly to simply proclaim that centripetal force is "real" and centrifugal force doesn't exist, a "pseudoforce." It could just as truthfully be the other way around! By the way, the common argument that "if you cut the string, the ball will not fly directly away from your hand--it will fly in the direction it was moving the moment the string was cut, in a straight line tangent to the arc," which is so often used to "prove" that centrifugal force doesn't "really" exist, proves no such thing. It merely reinforces the definitions. The moment the string is cut, centripetal force ceases to exist, so the ball stops accelerating in its circular arc and travels at constant velocity in a straight line (disregarding gravity and other forces that may act on it). Because the ball is no longer "turning," centrifugal force--that measurable pull at right angles to its direction of travel--also ceases to exist at that moment. It's not that the force isn't real. It is gone! Best regards, Bob Barnes

### Another force enters the picture

Quote:
 Originally Posted by Bob Barnes/Colorado By the way, the common argument that "if you cut the string, the ball will not fly directly away from your hand--it will fly in the direction it was moving the moment the string was cut, in a straight line tangent to the arc," which is so often used to "prove" that centrifugal force doesn't "really" exist, proves no such thing. It merely reinforces the definitions. The moment the string is cut, centripetal force ceases to exist, so the ball stops accelerating in its circular arc and travels at constant velocity in a straight line (disregarding gravity and other forces that may act on it). Because the ball is no longer "turning," centrifugal force--that measurable pull at right angles to its direction of travel--also ceases to exist at that moment. It's not that the force isn't real. It is gone!
And what else happens after the rope is cut? As seen in the rotating reference frame, the ball starts out moving directly away from the observer, but then its path starts to curve. In the non-rotating frame, this is because its speed isnt fast enough to "keep up" with the longer length of an arc at a larger radius.

So what makes it curve in the rotating frame? Another force, Coriolus.

If you've ever looked at numerical weather prediction, the models are integrated in an Earth-fixed (therefore rotating) frame and these "ficticious" forces are pretty darn real.
In the "rotating" frame of reference, when the string is cut, the ball does accelerate straight away from the observer in the direction of the centrifugal force. In the "non-rotating" frame of reference the ball does not accelerate, but travels in a straight (tangential) direction because there is no centrifugal force to accelerate it radially (there never was), and no longer any centripetal force accelerating it towards the centre of the circle.

The failure to accept centrifugal force lies in the fact that most people do not fully adopt the "rotating" frame of reference; if they did adopt it, it wouldn't be rotating to them.
Quote:
 Originally Posted by mdf So what makes it curve in the rotating frame? Another force, Coriolus. If you've ever looked at numerical weather prediction, the models are integrated in an Earth-fixed (therefore rotating) frame and these "ficticious" forces are pretty darn real.
Dang, so that is why I can turn easier on skis to the left than the right. I'm in z northern hemisphere.
It's become apparent to me , I'm too stupid to ski and comprehend what's taking place. At least its still fun!

The faster you go and sharper you turn, the more you will need to move your Center of Mass inside of your feet to remain balanced. Attribute it to whatever you will.
Hear, Hear, Rick!
No hijack there, Jack! Misunderstanding runs rampant about the nature of forces, acceleration, motion, position (that's relative too!), and relativity, as you suggest. But it's all fascinating.

For what it's worth, I think that few people without at least a little background in physics really understand what a "force" is in the first place--real, "pseudo," or otherwise. How can a wall or a chair be a force, I've often been asked? How can snow be a force--it's just. . . snow! I suspect that many people mistake "force" for "energy," or "effort." Force is not "The Force" of Star Wars fame. It need not make noise, glow, or move. It is just a push, or a pull, and anything, from any source, that pushes or pulls on us, from any frame of reference, is a real, measurable force in that frame of reference.

As I have often said, our bodies know how to deal with it, whether our minds can define or comprehend the concept or not. Call it whatever you want--a force, an effect, a pseudoforce, whatever--but don't deny that you've got to lean into a turn to balance and, as Rick says, that how much you must lean depends on the speed and radius of your turn.

That's just skiing!

Best regards,
Bob Barnes
Quote:
 Originally Posted by Rick Here's all new students of the sport need to know about this topic. The faster you go and sharper you turn, the more you will need to move your Center of Mass inside of your feet to remain balanced. Attribute it to whatever you will.
Amen.

Think about riding a bike. Turning a bike is actually more complex than turning a ski as a matter of analysis, and in fact people still aren't really sure how a bike turns. But most people have no idea that this complexity of whys & hows exists, and learned to ride perfectly fine at an age when this stuff would go over their heads anyway.

### Always time for another turn with the Centrifugal Force Monster?

I just joined the forum and found what is now beginning to look like an old stale thread, but since it’s physics I can’t help adding another physicist’s point of view.

In my experience, the idea that “centrifugal force” is a pseudo-force (we used to call it the Centrifugal Force Monster) is indeed covered in high school as well as university physics. This concept often causes some initial confusion, but most students eventually seem to get it. And it is certainly not a topic of debate within the professional physics community.

There has been much discussion of different reference frames being equally valid, but this is only true for “inertial reference frames”, which by definition are not accelerating. As was pointed out, you can tell whether you are in an inertial or noninertial reference frame by simply first assuming that your reference frame is not accelerating and then performing any test of Newtonian mechanics (such as the previously suggested tossing a ball up). The results of the experiment will only agree with Newton’s laws if the observer is indeed in a nonaccelerating (ie inertial) reference frame. If you are in a free-falling elevator when you toss your ball, then you will incorrectly deduce that there is no such thing as gravity. If you are sitting in a bucket that is being spun around over the head of some very large person when you toss the ball, then your observation will tell you that there is some magical force that pushes away from the center of rotation without making any contact with the ball. In the elevator case, you cannot detect the force of gravity because your reference frame is accelerating downward at the acceleration of gravity, while in the bucket case what you observe as the ball accelerating towards the side of the bucket under the influence of the imaginary Centrifugal Force Monster is actually the bucket accelerating towards the ball under the very real centripetal force exerted by the rope that is being pulled on by some giant hand. Giant Man is using this inward-directed centripetal force to keep turning the bucket into a circle, when what the bucket really wants to do is continue along a straight line at constant speed (neglecting gravity and collisions with air molecules).

This is indeed analogous to the skier that uses friction between ski and snow to exert an equally real centripetal force in order to turn rather that continue straight downhill. The skier’s centripetal force is always pointed “inward” towards the center of the circle he is making.

If you want the Centrifugal Force Monster to be a real force, then you have to explain what is exerting the force. This is difficult for the ball tossed in the bucket because nothing is touching it, it has no electrical charge, it’s not radioactive, all of its quarks are securely bound into nucleons inside nuclei that are many nucleus diameters away from other nucleons, and gravity always pulls down rather than sideways. If you consider the bucket itself, which also experiences an apparent outward centrifugal force, then perhaps you will argue that the centrifugal force is exerted by the rope (What else?). But that means that Giant Man is pushing on the bucket with a rope. This idea will be difficult to accept for the numerous household experimenters that have tried unsuccessfully to push things around with rope.

Yes, a force is something that pushes or pulls, meaning that it’s anything that causes an object with mass to undergo an acceleration in an inertial reference frame. As a side note, Newton actually formulated his force law slightly more generally by saying that a force is something that causes an object's momentum to change in time. The distinction between momentum change and acceleration is important for a rocket, for example, where mass changes as fuel is expended. But the Centrifugal Force Monster does neither pushing nor pulling as viewed from an inertial reference frame. There really is such a thing as a fundamental force. In fact, as far as anyone knows, there are only four of them (at most): gravity, electromagnetism, and two nuclear forces. At the most fundamental level, all of the nongravitational pushing and pulling that we experience day to day or when skiing (some of us are not fortunate enough to have “while skiing” and “day to day“ be equivalent expressions) is actually the effect of electromagnetism. The “real” force observed when pushing a chair results from electrons in the hand electrically repelling electrons in the chair. So the student that protests the idea that “a chair is a force” is justifiably indignant. A chair is indeed not a force, and neither is snow. But both are made out of matter that can exert forces on other matter in very close proximity (this is sometimes called “touching“ or “physical contact“) via electromagnetism. Centrifugal force cannot so be explained as a manifestation of a fundamental force because it is not a force at all. Really, truly, and unambiguously independent of reference frame.

So what does understanding this distinction between real and pseudo forces mean for the skier that is executing the turn? I can’t imagine it matters much at all. The turning skier is in a noninertial reference frame, and in this frame there is an “apparent force” pushing outward away from the center of the turn. If the skier does not lean over to counteract the effect of this apparent force, then he or she will probably fall over. When the turn is initiated and the skier seems to feel the Centrifugal Force Monster start pulling, I expect that he will naturally feel that action must be taken to push back on the monster. From a practical standpoint, it doesn’t really matter that the skier actually has to lean over because the centripetal force is applied at his feet rather than to his center of mass and that all monsters are pretend.

Finally, in the previous discussion I refer to an “apparent force” rather than a “pseudo force” only because I think it sounds more natural and would allow the ski instructor to make a statement that is technically correct but less likely to raise time-consuming (albeit interesting) questions and start contentious debates. If I was an instructor, that’s the language I would use: “Yes, Learning One, when you are turning you feel an apparent centrifugal force pushing you out, and this will make you fall down if you do not take certain actions that we will now discuss and practice”. After a little while I would quit saying these things because all of my students would realize that I am astonishingly unqualified to teach skiing. They would head off to look for a real instructor, and I would have the rest of the day to ski and have the first of several nice dinners courtesy their nonrefundable instruction fees.
amiles....

APPLAUSE!!!

"Centrifugal Force Monster." LOL!
Amiles - Nice comments and explanation. Thanks. One question tho -- If forces are only "real" in an inertial frame of reference, what causes the banked ruts in the snow left by a skier in a high G turn. The snow is stationary in a reasonably inertial frame of reference, so it should be a fair and unbiased "observer" of what happened when the skis went by.

In fact, when they did go by, they pressed downwards and sideways and caused the compaction and the banked ruts that we see. So, it seems to me like the force the skis exerted on the snow was every bit as real as any other force and was caused by the electron clouds in the ptex repelling the electron clouds in the snow. I'd call that pretty darn good evidence of an absolutely real force directed to the outside of the turn, and that force is usually called centrifugal force.

????

YOT
Actually the strong nuclear force isn't real either; it was just made up because physicists have a pathetic model of electrical forces, (propping up the chemical model). And the existence of gravity is only appearant too.
Ghost:

### More on skiing with the monster

YOT - It’s not that forces cease to be real in noninertial reference frames. Some have argued above that centrifugal force must be just as real as any other force because there exists some reference frame (the turning skier‘s, for example) from which the observer would measure an outward acceleration resulting from circular motion. The argument is that any reference frame is just as valid as any other, so this apparent centrifugal force must be just as real as any other force. My point was that the “all reference frames are equally valid” argument only holds for inertial (nonaccelerating) frames. In a noninertial frame, real forces continue to be real, but you can no longer identify and describe these forces by simply measuring accelerations relative to yourself.

For example, in the “guy in the falling elevator” referred to above, the observer measures zero acceleration relative to his reference frame, so if all frames are equally valid then he must conclude that falling either makes one’s mass equal to zero or causes gravity to cease to exist. In fact, since any accelerating object is not accelerating relative to its own accelerating reference frame (how‘s that for a truism…?), you must arrive at the exciting conclusion that if a force is applied to an object, then no force is applied to that object!

But for an even more entertaining example, remove the elevator and have yourself thrown off the cliff without anything to obscure your vision. Now look around and take some measurements. You will observe a small rocky planet accelerating directly toward you at about 10 m/s^2. Since you know Newton’s second law and you happen to have recently committed to memory the mass of the Earth, you are able to state with confidence that some gigantic force of about 6x10^25 Newtons is pushing the Earth directly at you as you casually hover at a fixed (though unfortunately chosen) point in space. If all reference frames are equally valid, including noninertial, then this force must be just as “real” as the force of gravity a ground-based observer claims the Earth is exerting on you. As far as I can tell, the argument that centrifugal force is just as real as centripetal force because it corresponds to an acceleration measured relative to some noninertial reference frame is exactly equivalent.

So what force accelerates the snow ahead of the turning skier as measured from the entirely valid inertial reference frame of the hill? The skier does. I suppose that’s why your legs (or my legs anyway) get tired in those high-speed turns - from pushing so hard on all that snow. In fact, you will of course also throw snow if you throw your skis sideways and skid to a stop. Similarly, when you jog in the sand you make narrow backwards “ruts” about the width of your shoe. In these cases there’s no turn, so the monster can’t help you. And you don’t need to invoke him in the turning case either.

Ghost - (With apologies to everyone who finds this part is way irrelevant) The strong nuclear force might not represent a complete theory, but it‘s an integral part of a very successful (in a predictive as well as descriptive sense) Standard Model. Quantum electrodynamics is arguably the most successfully predictive model in all of physics, and I don’t see how either theory is “propping up” the other. Your point about gravity is of course valid from a classical (GR) point of view, but if you like gravitons then you might have to call it fundamental again someday. I admit that either way I still think of it as one of the big four. Or the big three or two or part of the big one…
Amiles -
I was going to let this go, but I find I cannot.
Clearly you, I, and Ghost (and others) understand the equations similarly but disagree on what they "mean."

My point, and I suspect Ghost's, is that insisting on an inertial reference frame is counterproductive. For many real-world complicated computations a suitable accelerating frame is the natural setting to work in. (That is why I brought up Numerical Weather Prediction -- doing the computations in an inertial frame would be absurd.)

A skier's natural reference frame is the path of his (or her) own body -- the centrifugal force is real and is felt by the skier -- it is trying to pull him over!

Now, against my better judgement, I'm going to comment on some of the picky points that were introduced only as debating tactics:
a) gravity - the fact that the "force" is proportional to mass just like all other inertial forces was the key realization leading to the general equivalency principal.
b) strong nuclear force - everyone agrees it is not fundamental, but instead is a second order effect of quantum chromodynamics or string theory or whatever winds up replacing it. Nuclear physics is a phenomenolgical level, working with apparent (fictional) forces.

Your example of the earth "accelerating" towards the person "suspended" in space was a really wonderful teaching aid. However, IMHO, such discussion is more for physicists and serious students of the subject than for practically minded end users of the concepts such as engineers and skiers.

While centrifugal forces clearly don't have the same fundamental status as electrical, magnetic and gravitational forces, from the point of view of a skier in mid turn, I think that the centrifugal "forces" they feel and the analysis they might do in their rotating frame will be important and useful, exactly as such analyses are in the mechanical engineering community who regularly uses rotating frames to analyze rotating machinery such as turbines and helicopter blades. The reason is that transformation to the rotating frame, if done correctly, often dramatically simplifies analysis and facilitates understanding.

I think that 99% of the technical discussions among skiers is about what they feel and what they should do to make their skis work correctly. For example, in the middle of a turn, they might start musing about why their skis, assumed massless, don't suddenly start moving closer or further from their body because of all the centripetal force from the snow pushing radially inwards on the skis and the forces their legs are exerting radially outwards on the skis. If they are modestly inclined and trained, they can quickly mentally form the image of a free body diagram in their own coordinate system, realize that in their coordinate system there is no net radial force on the ski, hence there is no net radial acceleration (in THEIR coordinate system). If they next want to understand purpoising in a turn in deep powder snow, it's a small step from the previous analysis. Analyzing the latter motion from a frame fixed on the hill would be much more complicated and subject to likely error.

Personally, 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.

Anyway, that's my \$0.02 on this subject.

Cheers,

Tom / PM

PS - I just saw the post by MDF and see that we are in agreement.

### Of couse I love the Monster. Deep down everybody loves the Monster.

Hello Tom - I’ve enjoyed reading several of your posts over the few weeks that I’ve been browsing the Forums and wondered how long someone who calls himself PhysicsMan could stay out of a discussion like this. Of course only a physics person would lump engineers and skiers together into one category - but aren’t you afraid that kind of association might offend some skiers?

I agree that that it’s not critical for a skier to understand in detail the relationship between pseudo-forces “felt” by an accelerating observer on the one hand, and on the other hand true forces that don’t violate energy conservation. I tried (perhaps ineffectively) to make that same point in my first post when I said that the skier will feel an apparent force and that from a practical point of view what’s important is that he or she know how to respond to it. I’m also not dismissing the common utility of performing an analysis from a noninertial reference frame that both you and mdf noted. Of course you do the problem in whatever frame is most natural (ie easiest), and when this is a rotating frame I’m not religious about accompanying every use of the term “centrifugal force” with some qualifier such as “pseudo” or “monster”.

In fact the more technically extensive the background of whomever is involved in the discussion, the less pedantic I’m likely to be about the distinction. You might be right that in a setting like a ski lesson it’s better not to attach any qualifiers to the term “centrifugal force”, at least when no issue about it is raised by the student. But it sounds like you’ve found this to be a common source of confusion, and I’m betting that not all of it stems from those little qualifiers. It seems to me that much of this confusion (in all kinds of everyday situations, not just skiing) results from the fact that putting inertial forces on par with “real forces” results in gross inconsistencies that are bound to be noticed by people trying to understand what’s pushing on what and how. When these inconsistencies are completely glossed over, the less technical one’s background, the more likely they will be to sense that something doesn‘t add up, give up trying to sort it out on their own, and leave confused.

When the issue does come up (even if it‘s only because someone vaguely remembers hearing something in a high school physics class they took forever ago), I’m not at all convinced that it will take a devoted mini-course (though that sounds like a great idea too) to get the main idea across. It doesn’t take a specialist to grasp ideas like “moving objects keep on keeping’ on unless something pushes or pulls on them” and “there’s no push if nothing’s pushing”.

I also doubt that a nontechnical skier is more likely to benefit from analyzing a turn relative to his own accelerating frame (in which he‘s not even turning) as opposed to the frame of the hill. In the latter case, it’s just gravity pulling down, snow pushing you into the turn, and friction slowing you down, coupled with the idea that each and every piece of your body is going to insist on continuing straight ahead unless that turning force is communicated from ski to boot to leg to wherever that piece is attached. It’s also not difficult to see how leaning into the turn will facilitate passing the message from the feet to the head that it’s time to turn. Doing a proper analysis from the accelerating frame seems much more complicated and potentially confusing.

I would be interested to hear more detail about your experience using various approaches (or frames) to try to impart the minimal necessary amount of physics to learning skiers, but I know this has been going on for a while and am perfectly willing to let it die here.

mdf - Sorry for driving you to violate your better judgement. I too hesitate to reply - mostly out of fear that the reportable category of “bad posts” might be interpreted by some to include those that clearly have nothing to do with skiing. I’ll just say I don’t know of any law demanding that any force appearing to act on the property of matter we call gravitational mass cannot be fundamental, regardless of whether or not that property really is exactly equivalent to inertial mass. But then this is admittedly not my field (or in other words I own the big thick black book but I haven‘t opened it in a very long time). Ok, I’ll also say that I realize (and said) that QCD is not fundamental, but there’s something real and fundamental holden’ them nucleons together, and for the time being QCD doesn’t make for a bad placeholder. Uh… so in conclusion, the point of this paragraph is that skiing is cool and learning to ski better is good...
As a fellow owner of the big black book, peace.
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 Hello Tom - I've enjoyed reading several of your posts over the few weeks that I've been browsing the Forums and wondered how long someone who calls himself PhysicsMan could stay out of a discussion like this.
Hi Amiles - Unfortunately, for various non-skiing reasons, I haven't been very active on Epic lately, but when I got the email about this thread, I had to see what was going on.

Quote:
 Of course only a physics person would lump engineers and skiers together into one category - but aren't you afraid that kind of association might offend some skiers?
- sputter ... sputter ... why, some of my very best friends ...

Quote:
 I agree that it’s not critical for a skier to understand in detail the relationship between pseudo-forces “felt” by an accelerating observer on the one hand, and on the other hand true forces that don't violate energy conservation. I tried (perhaps ineffectively) to make that same point in my first post when I said that the skier will feel an apparent force and that from a practical point of view what’s important is that he or she know how to respond to it.
I did notice that comment, but I felt that it lost much of its impact after the much longer subsequent discussion about "pseudo or not".

Quote:
 I’m also not dismissing the common utility of performing an analysis from a non inertial reference frame that both you and mdf noted. Of course you do the problem in whatever frame is most natural (ie easiest)...
Same here. I'm not religious about one or the other. In fact, if you look back at previous posts by me, you will see that for some analyses/explanations, I prefer an earth-centric frame, while for other analyses/explanations I will use a skier-centric frame.

Quote:
 I would be interested to hear more detail about your experience using various approaches (or frames) to try to impart the minimal necessary amount of physics to learning skiers, but I know this has been going on for a while and am perfectly willing to let it die here.
30 years ago, as a young pup, I taught a few of my colleagues (all Ph.D. physicists, chemists & engineers, and no, W. Heisenberg was not one of them ) how to ski, and I can clearly remember two of them making absolutely dreadful progress when I was using the typical visual or kinesthetic teaching approaches, but making a huge (and almost instant) improvement in their skiing after I got them off their skis, into the bar, and drew some free body diagrams for them. Following this success, for the next couple of years I believed that absolutely everyone would ski better if they only understood the underlying mechanics. I "pushed" simple physics/mechanics on anyone that seemed a likely "victim" ;-) Unfortunately, since then, I have *never* found anyone who profited from this type of understanding as much as my original set of friends/colleagues.

Fast forward to the last few years when I was teaching the public at Whitetail. Occasionally, I would get a (skiing) student who would bring up centrifugal/centripetal forces. These folks came in two varieties. The first type would be quite content to learn that what they were feeling was essentially the same as what they experienced while riding a bicycle around a curve. We could then immediately move on (if appropriate for their skiing level) to discuss related ski techniques (eg, "banking a turn" vs angulation, etc. ). Such folks would usually progress quite nicely.

Unfortunately, the second type of individual who brought up centrifugal/centripetal forces was probably 10x more common than the first type. This type of person clearly brought up these issues not to help his own understanding or that of his classmates (in a group lesson), but to show off and "prove" that he (and it was always a "he") knew more about skiing than anyone else in the class. This type of person would love to try to engage me in a long on-the-snow discussion of these issues, and would not care in the least that he was completely derailing the lesson for the rest of the class. While tempted to whip out an omega cross (omega cross r) or two , my usual response is to say that I would love to discuss this with him, but we should do so during my break, in the lodge. Almost none of them ever show up.

When I really have to bite my tongue is when the person leading a PSIA clinic or exam that I am attending falls into the second category and doesn't know what he (again, it's always a "he") is talking about on this subject. :

Basically, in spite of being a physicist and being known for my long-winded detailed explanations on every subject under the sun, when it comes to ski instruction, my take is that the amount of physics that most ski students need to hear on the hill is minimal and is exactly what Rick said in post #43 of in this thread: The faster you go and sharper you turn, the more you will need to move your center of mass to the inside of the turn to remain balanced.

Epicski.com is quite different in that it is off-the-snow discussion, there are many folks here who are quite interested in such issues, and who have a technical background. OTOH, even here, I still feel that the use of modifiers such as "pseudo" hinders general understanding because some of the readers of such a thread will (very reasonably) go directly to a dictionary, equate "pseudo" with "false" or "sham" (because of the common, non-technical usage of the term) and conclude that Coriolis and centrifugal forces are not real, even in the non-inertial coordinate system.

Again, just my personal \$0.02 on the subject.

Cheers,

Tom / PM

PS#1 -

Quote:
 As a fellow owner of the big black book, peace.
I'm staying out of this one, but does having Hans Bethe as one's minor advisor in theory count for anything?

PS#2 -

BTW, I forgot to say this before - a hearty welcome to Epic. Please stay around and contribute. There are a bunch of extremely nice folks on this forum. What part of the country are you in?
A few more thoughts came to mind after I wrote the previous post:

1. There is a difference in perspective on this issue that is probably worth pointing out: "fundamental vs. pseudo (or fictitious or coordinate dependent, etc.)" pertain to the source of the force, not the application and use of the force. IMHO, most ski students need to know much less about the source of the force, than they do to apply it.

2. If we are going to talk about the source of the force (in a particular coordinate system), I vote for discarding terms such as "real", "psuedo", "fictitious", "apparent", etc. in favor of "fundamental vs coordinate system dependent forces". The latter is sufficiently techno-geek sounding to scare off the physics poseurs and not convey an impression that any skier that experiences centrifugal force must be delusional because it clearly such forces are not "real".

3. About on-the-snow physics instruction, my feeling is that if a class of ski students knows that their instructor is a physicist, not only is it almost completely irrelevant to this situation, it actually often is detrimental to learning because it always draws out the tedious and show-off physics groupies in the class (as I described in my previous post). Because of this, as theRusty can attest, I essentially go into stealth physicist mode when teaching or in the locker room. For effective ski teaching, IMHO, it is much more important to be a kindly and fun father figure to the little kids, a good demonstrator and listener to the adults, etc. than it is to know physics.

4. OTOH, in the university classroom, the opposite of #3 obviously holds. I can assure you that very few of my seniors or grad students would ever accuse the evil PhysicsMan of being kindly and fun and glossing over the differences between coordinate systems. :evil grin:

Cheers again,

Tom / PM
Tom -

Your description of teaching your physics colleagues to ski reminds me of when I learned to sail. I was in college and had never been in a sailboat in my life. One of my friends had local parents with a cottage on the lake and a couple of small boats (sailfish? sunfish? I can never remember what they are called).

So, both being so inclined, my friend drew the force diagrams with the wind on the sail and the water on the keel and the forward resultant. I said, cool, this makes sense.

So I took the boat out, jibed, and promptly capsized.
(As my friend stood on the shore and laughed.)
Quote:
 my take is that the amount of physics that most ski students need to hear on the hill is minimal and is exactly what Rick said in post #43 of in this thread: The faster you go and sharper you turn, the more you will need to move your center of mass to the inside of the turn to remain balanced.
Sounds good to me.

Quote:
 If we are going to talk about the source of the force (in a particular coordinate system), I vote for discarding terms such as "real", "psuedo", "fictitious", "apparent", etc. in favor of "fundamental vs coordinate system dependent forces".
But where does that leave velocity-dependent forces such as drag and jxB? How about differentiating based on energy conservation?

Quote:
 welcome to Epic. Please stay around and contribute. There are a bunch of extremely nice folks on this forum. What part of the country are you in?
Thanks. I finally ended my days of strictly browsing and officially registered in order to ask some equipment-related questions, but then got a little distracted by this thread. When I finally went to bed last night, I dutifully bemoaned to my wife the evening’s “waste of time” and expressed outrage that in this day and age computers don’t come with warning labels that online content might contain sites that combine discussions of physics and skiing…

Only after your location question did I notice that you and many others automatically include their general whereabouts in their posts. I live in Livermore, Ca (SF Bay area, which I guess means I’m currently a whole lot closer to a whole lot more snow than you are), but I went to grad school at College Park. In fact, I remember at least a couple of Toms on the faculty - one of whom was on my dissertation committee, but then there are several schools in that area and yours isn‘t exactly the least common of names…
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