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Centripetal Force How Does It Relate To Skiing? - Page 5

post #121 of 141


 

Quote:
Originally Posted by ChuckT View Post

mdf,

 

 In your celebrated rotating FoR, it is rotating relative to WHAT?

 

Well, I don't really need an absolute inertial frame, and that is a good thing, since they are hard to come by in the real world.  (Lots of approximately inertial frames, none that are exact.)

 

So if I let our fun push me into a rhetorical corner, I would say they are all  rotating.  The inertial ones are the ones where the rotation rate is zero.  And operationally (or at least thought-operationally) you determine if you are in a zero-rotational-rate frame by measuring the Coriolis force.  You can't use the centrifugal force, because locally you cannot tell the difference between it and the reaction to a linear acceleration.

 

post #122 of 141


Fixed it for you.

Quote:

Originally Posted by rcahill View Post For example, looking at things from a frame of reference fixed to the centre of the circle,  if the string is cut when swinging a ball around it is the removal of a real force (centripetal) that causes the ball to fly off tangentially. Centrifugal force if it were physically real, in this fixed frame of reference,  would cause the ball to move in a different direction with a radial component. Since this does not happen the apparent centrifugal force does not physically exist in this fixed frame of reference and hence the term apparent.  

 

Looking at things from the rotating frame of reference, there is a body force being exerted on objects that unless it were resisted for example by applying a force with a string, much ike a hangman's noose holds up a cadaver to prevent itfrom being accelerated to the ground by the body force of gravity.  When you cut all strings and eliminate all other forces acting on the body, you can measure the object's accleration you determine the very real in that frame of reference body force acting on it by using f = ma if you know its mass.

 

To remove any obfuscation, it is accurate to say that even though centrifugal force exists in a rotating reference frame it still has no physical reality in a different frame of reference, that is to say the frame of reference preferred by Chuck and rcahill.

 

 

 

BTW, my skiing qualifications, nill, just been skiing for half a century.   Physics, Ph.D. MSc.. B.Sc. all in Civil Engineering, B.Ed, with physics specialist credentials.  As far as relativity, just read a bit of Einstien's books on it in my spare time out of personal interest.  (aimed at people with a high-school level education according to the introduction of one IIRC)

 

post #123 of 141

Ok, it looks like things are edging towards flamewar, so in the spirit of Christmas, a few concessions:

 

1) my debating position is exaggerated for effect.  I think it is a consistent and valid way to look at things, but it is a bit more extreme than the way I approach things in real life. I would still  encourage everyone to consider the ontological / epistemological / foundational questions of "what is force" and "how real is all that post-Newtonian stuff?" 

 

2) the "fictitious" label question is at best a draw.  While I still think nobody says "fictitious force" outside of an intro-to-physics context, they do say "apparent force" a lot, and that is pretty close to "fictitious."

 

3) on the definition of force as whatever the time derivative of to momentum is equal to... turns out that isn't quite right.  It is more usual to use (displacement)*(force) = work = (delta energy).  This happens, for example, when you set up a complicated dynamics problem in a Lagrangian or Hamiltonian formalism and then want to know "how hard is my motor pushing?"

post #124 of 141
Quote:
Originally Posted by jst4fun View Post

Cool ideas gentlemen, I've skied with Bob a number of times and while some of his details are slightly off at times, he is a pied piper who has helped more than a few skiers learn to learn skiing. I wonder though, does anyone else involved in this thread shares his same skiing credentials. Seems only fair to ask since Chuck brought up credentials.


Hey--welcome to EpicSki, jst4fun! I'm glad you've enjoyed skiing with me--and I'm sure the joy was mutual, although I do not yet know exactly who you are in "real" life.

However, I do not appreciate statements like, "'while some of his details are slightly off at times," without specific references. I work extremely hard to to make sure the details are on, and I welcome specific corrections with supporting argument (not just opinion) always, to help further my understanding. I do not believe that any of my "specific details" in this thread have been refuted by anyone--and continue to maintain that all frames of reference are equally real and valid, even if they are not always mutually understood and may often be inaccurately expressed. Any apparent differences in "the facts" in this thread--at least from the knowledgeable posters--stems from describing motion from different frames of reference. It is very clear: centrifugal force does not exist as a real force in some frames of reference. And it does exist as a real force, that can be measured, felt, and described, in others. For some, those frames of reference appear to have less "reality," but that either stems from a misunderstanding, or from a strong preference for avoiding those frames of reference in conversation due mostly to their propensity for being misunderstood. I suspect that even ChuckT would agree with that, since he has finally insisted that he does accept that any frame of reference can be valid.

Finally, I find it odd that there is so much emphasis on "credentials" here. Credentials have absolutely nothing to do with being right or wrong (although they may admittedly skew the likelihood). Facts are facts. Laws are laws. I learn more about skiing every day, often from my students, despite having taught the sport for over thirty years and been an avid student of it for my entire life. I don't claim to be "right" because of my "credentials." Nor do I grant inherent "rightness" to anyone with a PhD in any field--even in their field (although I will certainly consider their opinions and words seriously, by virtue of their credentials and the fact that they are likely to have at least given a fair amount of study and thought to the matter). I've seen too many exceptions where highly credentialed individuals were actually mistaken about something--often something as basic as a discussion of mechanics to a highly educated physicist. For many of them, of course, it's been a long time since they've studied basic mechanics, and many of them have moved on considerably in their education toward some highly specialized field of physics. I can describe several actual examples in which PhD physicists found themselves in error about something basic--and admitted (sometimes with embarrassment) that error. Let's not forget that sometimes bridges, designed by the fine, credentialed engineers and physicists, do fall down! And doctors aren't always the best nurses--nor would I want some of them administering basic first aid to me.

In any case, I challenge--and humbly request--anyone to point out a factual error in my arguments here--or elsewhere (and defend your point of view with logically valid argument and based on sound premises--not just opinion and statements of personal preference--not that those are not also worth describing sometimes). I have chosen to avoid getting into the mathematics of the argument for several reasons. Primary among them is that, first, math is not necessary to understand the principles involved, and frankly, math proves absolutely nothing in this discussion. The math can be shown to work equally well for any frame of reference. All math shows is that there's an equal sign between the sides of the equation. It does not show--and certainly does not prove--that one frame of reference is preferable or "more real" than another. Based on math, centrifugal force is as completely real and necessary as any other force in the right frame of reference. Math proves nothing (here). But if it's math you want, ChuckT has done an admirable job of bringing the equations into the discussion.

Finally, for the credentially-persuaded among us here, I find it curious that no one has thought to question either the physics credentials of some ski instructors, or the skiing credentials of some physicists. There are many who have lots of both....

Best regards,
Bob
post #125 of 141
Quote:
Centrifugal force if it were physically real, in this fixed frame of reference, would cause the ball to move in a different direction with a radial component. Since this does not happen the apparent centrifugal force does not physically exist in this fixed frame of reference and hence the term apparent.


whoa....now hold on there, rcahill! What you are describing is a very common, but fatally flawed argument against the "reality" of centrifugal force. The argument is simple: we feel a force pulling directly to the side (that we call "centrifugal force") in a turn--or, we feel a pull directly away from our hand, in the direction of the string, when we swing a ball around on a string--but when the string breaks, the ball does not fly in that direction, therefore the force was not real. I've heard it a hundred times. But the flaw should be obvious with a little thought: the moment the string breaks, or the moment our skis break loose and slip sideways, there is no longer any centrifugal force--of course! Naturally, there is also no longer any centripetal force either, but that never seems to bring the "reality" of centripetal force into question, for some reason.

The force was 100% real--and measurable--until the moment the string broke. At that moment, in most relevant frames of reference, all non-negligible forces (ie., discounting air resistance and so on) on the ball--or the skier--cease, and the momentum of the "body" simply continues in its straight line, constant speed trajectory, just as Newton's First Law says it should. Even there, however, As MDF's elegantly simple little illustrations (post #116 above) show, observers will describe the event in very different ways, depending upon their chosen frame of reference.

Best regards,
Bob
post #126 of 141
Quote:
Originally Posted by mdf View Post


#6 - you [Spooky] still don't get it.  You are arguing that because the centrifugal force does not appear in your favorite reference frame, it is not real.  Of course it doesn't cause radial acceleration in the frame where it doesnt appear!  It does cause radial acceleration in the frame where it does appear!  You guys are mixing up frames and drawing bogus conclusions, so much so that post #109 says something wrong rather than just misguided.  (And to fix it, you are have to resort to, "what the candidate meant to say was...")


Along with the rest of your post, MDF, I particularly like and agree with this point.

Spooky--where on earth (oops, sorry, your profile suggests that earth is not necessarily your "Milky Way" home--sorry to make unfounded assumptions) did you come from? It sure appears to me that you are guilty of exactly what you decry in your very first EpicSki post: a poseur who thinks he can impress by spouting big words and mathematical formulae he may have read in a book, but has very little understanding of. Please read the thread before you make such hollow boasts, and bone up on your understanding of relativity. Please!

---

To counter the "big words and math formulae" smoke screens presented here as "argument," first, I suggest that anyone truly familiar with logic and argument theory would immediately recognize the (informal) fallacy of it. Then, I want to repeat the ideas that I described in an earlier post: The fact that you can describe the motion with complete accuracy with a mathematical equation does not, in any way, extrapolate to defend the position that some forces are absolutely "real" while others are absolutely "fictitious." The real forces in the analysis from one frame of reference in which there is acceleration become fictitious forces in another frame of reference in which there is no acceleration. And vice-versa. The math will--must--work either way. So, to be short, mathematics, no matter how daunting, impressive, and accurate, proves nothing in this discussion about any "absolute" nature of centrifugal and centripetal force.

Absolutes, as Einstein might have said, are for people--including physicists and mathematicians--with inflexible minds. Motion is relative, not absolute. Both centrifugal and centripetal forces are, in the "right" frames of reference, "fictitious." And both can be real in other frames of reference. (Although, as we've discussed, it is true that both are not real and fictitious simultaneously in the same reference frame.) If you want to argue that centrifugal force is necessarily (absolutely) a "fictitious" force, you must also accept the point that what we call "centripetal force" is likewise a fictitious force in the reference frame in which there is no centripetal acceleration (that is, no turn).

Best regards,
Bob Barnes
post #127 of 141

I tried to stay away from this thread because my ski qualifications are completely non-existent, and gravity is not my field of concentration but at this point we're hardly arguing about skiing and mostly arguing about physics, thus I have a few things to add.
 

Quote:
Originally Posted by Bob Barnes View Post

I do not believe that any of my "specific details" in this thread have been refuted by anyone--and continue to maintain that all frames of reference are equally real and valid, even if they are not always mutually understood and may often be inaccurately expressed. Any apparent differences in "the facts" in this thread--at least from the knowledgeable posters--stems from describing motion from different frames of reference. It is very clear: centrifugal force does not exist as a real force in some frames of reference. And it does exist as a real force, that can be measured, felt, and described, in others. For some, those frames of reference appear to have less "reality," but that either stems from a misunderstanding, or from a strong preference for avoiding those frames of reference in conversation due mostly to their propensity for being misunderstood.

 

You can probably, with most clarity, define a real force as a force that is not an apparent, sometimes termed fictitious, force. If a definition of a "real force" exists, it's this. I am not aware of any other definition of a real force that is not equivalent to this one, and it, by definition, excludes any force not necessary in an inertial reference frame since that is the definition of a fictitious/apparent force. The centrifugal force is such a fictitious/apparent force, termed such because it is not necessary to explain how physics works in an inertial reference frame.  While it is probably possible to describe physics in inertial reference frames by inventing a centrifugal force, you also need to invent other forces (or also keep inertia) for other situations to describe the effects of inertia. For example a breaking car: what centrifugal force also explains why when a car breaks and if your body is unrestrained it will keep moving forward. Can you give us a formulation of such a force that describes both effects? Alternatively, you can keep only inertia and describe motion in inertial reference frames with just one idea, as it is currently done. Are you not satisfied with the explanation of circular motion with just inertia? If so, what aspect is unsatisfying?

 

There are two main reasons that inertial reference frames are preferred and given a sort of special status. The first (and main) is that the laws of physics are simplest in such a frame (for example, there is no centrifugal force necessary). Occam's razor. The second is that there is a absolute cosmological rest frame which appears necessary (this absolute frame is completely special and the best, most preferred frame. It's also inertial).

post #128 of 141
No arguments, Hmpph (except that I assume that you mean "braking" a car, not breaking one). If you prefer to work and think exclusively in an inertial frame of reference, you do not need--and in fact cannot have--a centrifugal force. No one here that I'm aware of has ever argued otherwise! But whether you prefer to think in that reference frame or not is not the point. The point is simply that other reference frames exist, are also valid, and are, in fact, very commonly inferred by "lay people" as well as physicists, in every day use, and every day language. You'll have to read the thread if you want the specific examples and support for this point.
Quote:
There are two main reasons that inertial reference frames are preferred

By whom?

Are the Laws of Physics always simplest in an inertial reference frame? I've described several instances where I submit that an inertial reference frame not only muddies the description of what happens (mostly involving the description of ski technique--body part movements), but also--and very much the significant point here--an inertial reference frame is not the most commonly used frame, whether motion is described by a physicist or just a lowly skier trying to describe what movements he's making. You have obviously not read the entire thread, so I'll just re-ask a question from long ago: can you describe the simple idea of "hold your hands still" really more simply from an inertial reference frame, in which you'll need to describe that "stillness" as the hands and the skier move through the surrounding landscape, rather than referring to "still" compared to the skier's own body? Of course it can be done, but can you do it more simply? Let's hear it!

Best regards,
Bob
post #129 of 141


Yes, braking of course :). English is not my first language, although admittedly I've been speaking it for a very long time now.

 

Quote:

Originally Posted by Bob Barnes View Post
If you prefer to work and think exclusively in an inertial frame of reference, you do not need--and in fact cannot have--a centrifugal force. No one here that I'm aware of has ever argued otherwise! But whether you prefer to think in that reference frame or not is not the point. The point is simply that other reference frames exist, are also valid, and are, in fact, very commonly inferred by "lay people" as well as physicists, in every day use, and every day language. You'll have to read the thread if you want the specific examples and support for this point.


It's true that non-inertial reference frames exist, are valid and commonly useful. What I mean to stress is that "real force" by definition means (if anything) a force that is not fictitious/apparent, which by definition means not necessary in inertial reference frames. Thus simply by definition centrifugal force is not a "real force."

 

Inertial reference frames are preferred by a majority of physicists. (I mention inertial vs non-inertial because this is where real vs fictitious/apparent comes from: by Occam's razor, the simplest explanation is usually the right one and inertial frames are simplest. Thus if those forces are "real", and if it's fully possible to explain accelerating reference frames from the outside without any new forces then it sort of makes sense to say that other forces are apparent/fictitious). Not preferred in the sense that we like them best but in the sense that they seem to have a special place: the laws of physics are always the same (most important [I forgot to explicitly state this before and implied it as simplest, they are very related]) and simplest, also the cosmological rest frame seems necessary. The laws themselves are always (or, at the very least, almost always) simplest in the inertial reference frames, but to do an analysis on a situation is definitely not always easiest in inertial reference frames (which is why people switch to others). That inertial reference frames have a special place and that laws there are generally simplest is a commonly known fact among physicists, probably goes back to Galilean invariance. Here is a quick citation, or the second sentence here, before other physicists weigh in with their thoughts.

 


Edited by hmpph - 12/26/11 at 9:13pm
post #130 of 141
hmppf--plenty of people have already weighed in with plenty of thoughts here--physicists and non-physicists alike. You may think you are furthering the discussion, and I appreciate that, but you have simply repeated what has already been said and discussed, as you would see if you were to read the thread. I'm not suggesting that you have nothing to add, but please do not just repeat the same old stuff that has already been discussed ad nauseum.

If you prefer to think in inertial reference frames, go ahead--knock yourself out. It's fine. It works. If you do it right, it's perfectly accurate. But I'm really getting tired of pointing out to self-proclaimed physicists who ought to know this that other frames of reference are not only equally valid, but often considerably more useful for simplifying the understanding and description of movements--and are commonly and naturally used by lay people and physicists alike for just that. Whether you "prefer" a particular frame of reference or not is your own personal preference and really has no bearing in this discussion. You can prefer anything you want! But there is nothing inherently more real or universally preferable about an inertial frame of reference over any other, and I remain quite surprised to have to try to convince a "real" physicist of this elementary truth of relativity.

Indeed, your post really exemplifies the problem, I would think. The fact is that anyone who understands the skiing instruction to "keep your hands still" has no problem thinking in terms of the skier's accelerated frame of reference. It is only a problem, and only becomes complicated, when someone tries to impose another frame of reference onto the same description. Yes, you're right--those hands are anything but "still" in your inertial frame of reference. But they are (or can be) very still in the skier's accelerated frame of reference. Whether you "prefer" it or not, if you want to understand simple instructions like that, you must let go of your stubborn insistence that "inertial" reference frames are "preferred" or assumed by anyone else. Ever! As a scientist, I would like to think that unverified assumptions are something you would have trained yourself to recognize and shun anyway. Aren't they?

I know--and it has been stated by many here including me many times--that centrifugal force does not exist in your preferred inertial frame of reference. No one here has suggested otherwise. Tell us something we don't know!

Best regards,
Bob Barnes
post #131 of 141

I once stumbled upon some old engineering texts from the early 20th century that included a "momentum force" to represent the effects of inertia.  As I was just learning the basics at that point in time it was a little confusing until I realized what they were on about.  Apparently in some circles it was quite common at that time to model momentum this way.  If you go back far enough you will find texts on natural philosophy explaining that objects seek their natural position without the aid of forces, which are only needed to change their natural tendancy.  This apparant lack of understanding didn't prevent the building of some fine catapults and amazing structures.  It is amazing what you can find in some text books.  I bet you can find texts and internet souces to back up any postion you please.  Just say'n. 

post #132 of 141

 

Quote:
Originally Posted by Bob Barnes View Post


If you prefer to think in inertial reference frames, go ahead--knock yourself out. It's fine. It works. If you do it right, it's perfectly accurate. But I'm really getting tired of pointing out to self-proclaimed physicists who ought to know this that other frames of reference are not only equally valid, but often considerably more useful for simplifying the understanding and description of movements--and are commonly and naturally used by lay people and physicists alike for just that. Whether you "prefer" a particular frame of reference or not is your own personal preference and really has no bearing in this discussion. You can prefer anything you want! But there is nothing inherently more real or universally preferable about an inertial frame of reference over any other, and I remain quite surprised to have to try to convince a "real" physicist of this elementary truth of relativity.
Indeed, your post really exemplifies the problem, I would think. The fact is that anyone who understands the skiing instruction to "keep your hands still" has no problem thinking in terms of the skier's accelerated frame of reference. It is only a problem, and only becomes complicated, when someone tries to impose another frame of reference onto the same description. Yes, you're right--those hands are anything but "still" in your inertial frame of reference. But they are (or can be) very still in the skier's accelerated frame of reference. Whether you "prefer" it or not, if you want to understand simple instructions like that, you must let go of your stubborn insistence that "inertial" reference frames are "preferred" or assumed by anyone else. Ever! As a scientist, I would like to think that unverified assumptions are something you would have trained yourself to recognize and shun anyway. Aren't they?



Bob, you've misunderstood what I was indicating. Inertial reference frames are not just my preference (where preferred is a special term, one not synonymous with the every day usage of the word preferred), they're the preference of the entire scientific community. They are a preference not because we're biased but because there is something special about them that you cannot deny. This was my main point, I did not see anyone point out these reasons: the laws of physics are both always the same and simpler in any inertial reference frame, and in addition there appears to be an absolute inertial reference frame (which, if truly real, would be the best possible reference frame out of any reference frame. Your idea that relativity indicates any reference frame is equally valid would immediately falls apart here because this truly is the best reference frame (this is also not what relativity states as I point out further).

 

Relativity does not indicate that all frames are equally valid. What it indicates is in fact exactly what I stated (this is known as the principle of relativity): the laws of physics are simpler in inertial reference frames, and all inertial reference frames should be equally valid. The laws of physics in non-inertial reference frames are more complicated. The laws being more complicated does not indicate that to do an analysis on a particular problem is more complicated (although it usually is but not always, otherwise we would never switch to non-inertial reference frames) - I stated this in my last post but you seemingly ignored it.

 

Now for some quotes regarding the special nature of inertial reference frames (I'll use the fastest source possible, which is Wikipedia. If you'd like more scientific sources I can also provide them but I highly doubt any scientist here will dispute the nature of these quotes as given. The first quote is a direct quote, not written by Wikipedia):

 

First postulate of special theory of relativity (Albert Einstein): "Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K."

 

Article on non-inertial reference frames I linked above: "A non-inertial reference frame is a frame of reference that is under acceleration. The laws of physics in such a frame do not take on their most simple form, as required by the theory of special relativity."

 

Article on inertial reference frames: "Physical laws take the same form in all inertial frames. In a non-inertial reference frame the laws of physics depend upon the acceleration of that frame of reference, and the usual physical forces must be supplemented by fictitious forces."

 

As you can see these things are special about inertial reference frames. The laws in them are simpler, a law valid in one inertial frame is always valid in another, whereas this is not true for non-inertial frames, and overall less laws are required in inertial frames (Occam's razor comes in here). You keep trying to indicate that preference for inertial reference frames is a figment of our imagination but you have yet to respond to these reasons for why inertial reference frames take a special place.

 

Quote:
The fact is that anyone who understands the skiing instruction to "keep your hands still" has no problem thinking in terms of the skier's accelerated frame of reference. It is only a problem, and only becomes complicated, when someone tries to impose another frame of reference onto the same description. Yes, you're right--those hands are anything but "still" in your inertial frame of reference. But they are (or can be) very still in the skier's accelerated frame of reference. Whether you "prefer" it or not, if you want to understand simple instructions like that, you must let go of your stubborn insistence that "inertial" reference frames are "preferred" or assumed by anyone else.

I have (I hope) explained why inertial reference frames are preferred, let me also add that they're not preferred in the sense that "we must always use inertial reference frames to analyze all situations." There preferred in the sense that some of their properties are special and indicate that physics laws inside them are  perhaps more fundamental, not in the sense that "we must only use inertial reference frames." Physicists and astrophysicists commonly use non-inertial frames both because some situations are simpler to analyze/model (despite the laws themselves being more complicated) or because some situations are simpler to understand. There is nothing wrong with using non-inertial reference frames, that is not what we're saying when we say preferred.

 


Edited by hmpph - 12/27/11 at 8:39am
post #133 of 141
Quote:
Originally Posted by Ghost View Post

I once stumbled upon some old engineering texts from the early 20th century that included a "momentum force" to represent the effects of inertia.  As I was just learning the basics at that point in time it was a little confusing until I realized what they were on about.  Apparently in some circles it was quite common at that time to model momentum this way.  If you go back far enough you will find texts on natural philosophy explaining that objects seek their natural position without the aid of forces, which are only needed to change their natural tendancy.  This apparant lack of understanding didn't prevent the building of some fine catapults and amazing structures.  It is amazing what you can find in some text books.  I bet you can find texts and internet souces to back up any postion you please.  Just say'n. 


Actually, the momentum force (if I am guessing the details correctly) is a valid alternative way to organize the equations of motion.  Instead of F=m*a, you have F=0 always, with Ftotal = Fapplied + Fmomentum, and Fmomentum = -m*a.  Although it's actually a delta-momentum force, to be precise.

 

If you think about force balance in a tow rope, it is a reasonable way to talk about things.  Where does the resistance pulling back on the tow rope come from?  The towed object's (delta) momentum force.

 

Or were those old texts doing something completely different?

 

post #134 of 141

Hmphh --

 

Let me try to explain a different way of looking at things.

 

If you go way back, to the first few pages of your first physics textbook, you will probably find a statement similar to "a force is a push or a pull."  And the "fictitious forces" are very real, in the sense that they are a real pull -- you can feel them. The fact that they can be transformed away by going to an inertial frame does not show that they are an illusion caused by bad coordinates -- it only shows that they are not fundamental.  

 

But reducing everything to inertial frames with Cartesian coordinates throws the baby out with the bathwater.  The centrifugal "force" and Coriolis "force" capture important patterns about the behavior of objects moving along smooth arcs.  When the situation is naturally described via rotating frames, reducing it to Cartesian inertial coordinates destroys the patterns and replaces them with an arbitrary pile of components.

 

I don't understand why centrifugal force gets singled out for debunking.  Nearly everything in everyday life can be explained in terms of something more fundamental.  That doesn't make it less "real." 

post #135 of 141
Quote:
Originally Posted by mdf View Post

Hmphh --

 

Let me try to explain a different way of looking at things.

 

If you go way back, to the first few pages of your first physics textbook, you will probably find a statement similar to "a force is a push or a pull."  And the "fictitious forces" are very real, in the sense that they are a real pull -- you can feel them. The fact that they can be transformed away by going to an inertial frame does not show that they are an illusion caused by bad coordinates -- it only shows that they are not fundamental.  

 

But reducing everything to inertial frames with Cartesian coordinates throws the baby out with the bathwater.  The centrifugal "force" and Coriolis "force" capture important patterns about the behavior of objects moving along smooth arcs.  When the situation is naturally described via rotating frames, reducing it to Cartesian inertial coordinates destroys the patterns and replaces them with an arbitrary pile of components.

 

I don't understand why centrifugal force gets singled out for debunking.  Nearly everything in everyday life can be explained in terms of something more fundamental.  That doesn't make it less "real." 


mdf,

 

Can you define "real forces" as anything other than forces which are not fictitious/apparent or some equivalent of that? Before we can talk about "real forces" there needs to be a definition of a "real force." I do not think that any physics book I've read has ever said "real force" but as I say, if any such definition exists, it's simply that it's a force which is not fictitious or apparent (or equivalently one that does not exist/is not necessary in an inertial frame). By definition, this obviously excludes the centrifugal force. (I said this in my first and second post.)

post #136 of 141
Quote:
Originally Posted by mdf View Post


Actually, the momentum force (if I am guessing the details correctly) is a valid alternative way to organize the equations of motion.  Instead of F=m*a, you have F=0 always, with Ftotal = Fapplied + Fmomentum, and Fmomentum = -m*a.  Although it's actually a delta-momentum force, to be precise.

 

If you think about force balance in a tow rope, it is a reasonable way to talk about things.  Where does the resistance pulling back on the tow rope come from?  The towed object's (delta) momentum force.

 

Or were those old texts doing something completely different?

 


Yes, you have guessed correctly.   It works fine, but it did seem a bit off at the time I first encountered it.  It was my first experience of looking at it from a different perspective, after having the standard Newtonian law of F=ma (or f=dp/dt if you prefer, p being momentum) firmly planted in my model of the world from high school physics (and calculus). 

 

post #137 of 141
Quote:
Originally Posted by hmpph View Post


mdf,

 

Can you define "real forces" as anything other than forces which are not fictitious/apparent or some equivalent of that? Before we can talk about "real forces" there needs to be a definition of a "real force." I do not think that any physics book I've read has ever said "real force" but as I say, if any such definition exists, it's simply that it's a force which is not fictitious or apparent (or equivalently one that does not exist/is not necessary in an inertial frame). By definition, this obviously excludes the centrifugal force. (I said this in my first and second post.)



I guess I am just resisting using the emotionally loaded term "real" as a technical term.  To me, "real force" means you can measure it  in the laboratory.  

 

Sure, you can distinguish between inertial forces and "real forces" (under your definition) by various technical criteria.  I think your way is a bit circular, though.  You are basically saying "inertial frame = no ficticious forces" and "ficticious force = does not appear in inertial frames".  How would you make the distinction operationally?  They do have different characteristics, so I think it can be done.  But it is not that easy.  It is more or less equivalent to asking, "How do I determine experimentally if I am in an inertial frame?"  

post #138 of 141


 

Quote:
Originally Posted by Ghost View Post


Yes, you have guessed correctly.   It works fine, but it did seem a bit off at the time I first encountered it.  It was my first experience of looking at it from a different perspective, after having the standard Newtonian law of F=ma (or f=dp/dt if you prefer, p being momentum) firmly planted in my model of the world from high school physics (and calculus). 

 

I just learned last night that there is a technical advantage to doing it that way.  Apparently, if you include -m*a in the force balance, so that the total force and total torque is zero, you can choose your origin freely.  In the usual treatment, you have to use the center of mass or an externally pinned point, or do all that messy force couple bookkeeping. The idea goes back to d'Alembert in the late 1700's.
 

 

post #139 of 141

Can we talk about entropy now? smile.gif

 

I wasn't kidding....

post #140 of 141

I thought all the energy had dissipated in this thread.  

post #141 of 141

How about the metaphysics of entrop?  Entropy theory is the result of a pessimistic outlook wink.gif.

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