Centripetal Vs. Centrifugal

[helmet fire]

Glad you lept back in Lu.

You have accurately described the situation in your post. We have said all along that centrifugal is essentially a tool to easily explain what is going on when talking about another complex subject. In Nick's case, that was blade lag, so he used centrifugal because it simplified the explanation of blade lag. He even put a joke in there for those who know centripetal is real & centrifugal is not.

Same same with gyroscopic precession. Easy to explain and use as a tool for day 1 knowledge, but not what is really going on.....but lets NOT go there again!! <img src="eek.gif" border="0"> <img src="eek.gif" border="0">

[helmet fire]

Ressurected for two reasons: we have reached the twentieth example of the same thing, and I note that Flight Safety is amongst us again: perhaps he/she would like to address some of the questions above.. .. .Now for the current list:1. David with his rock in a sling Vs Goliath.. .2. A girl holding a chain on a merry go round.. .3. A marble on a rotating disc.. .4. An Alcohol pump.. .5. Droop stops on a rotor disc.. .6. A car going around a round about.. .7. A steel ball on a chain.. .8. A tractor on a chain around a stump.. .9. Liquid dye on a rotating disc.. .10. Sattelites and orbits.. .11. A dairy farmer's centrifuge.. .12. a science lab centrifuge.. .13. NASA & USAF centrifuges.. .14. a man on a cornering bus holding a helium balloon.. .15. a pilot's bodily fluids.. .16. a jar full of water.. .17. ANOTHER cornering car.. .18. a 35,000 Kg truck and cardboard Britteny Spears cuttouts.. .. .The new ones:. .. .19. Main rotor blade seperation.. .20. A Korean war Fighter pilot whom ejected from his rapidly rolling aircraft using centrifugal to carry him clear of the airframe.. .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[Flight Safety]

HF, I was hoping this thread would get resurrected. When I have time later this weekend, I have a rather long post to add...

[helmet fire]

FS:. .I look forward to your contribution.. .. .Would it be possible to stick to the questions about your last points before launching another detailed explanation?. .. .Cheers,. . <img border="0" title="" alt="[Smile]" src="smile.gif" />

[Flight Safety]

Here’s the long post. To HF, I’ll get to the above question, but I feel I need to cover the following topics in order to give the best answers to that question.. .. .After a fair amount of looking into this, it’s become clear to me that the physics community is in a state of serious confusion regarding “real” and “apparent” forces, with physicists and other scientists hotly debating the issues of what forces are “real” and what forces are “apparent”. I think the Coriolis “force” is a good example to illustrate this problem. This following example will also illustrate my personal frustration with the lose use of the word “apparent”, that some people are using as a label for some very real natural forces.. .. .Suppose you have a merry-go-round (turning counter-clockwise as seen from above) that has a putting cup for a golf ball right in the center of the turntable. Suppose there’s a chair on the outer edge of the turntable arranged facing inward, so that a guy sitting in the chair would look directly at the putting cup. Now suppose there’s a platform right next to the turntable, arranged with the edge of the platform level with and nearly touching the rotating turntable. Now imagine there’s a guy with a putter who strokes a golf ball into the cup in the center of the rotating turntable. Assume the turntable’s surface is quite low friction.. .. .To the guy with the putter, the golf ball appears to go straight into the cup. But to the guy sitting in the chair, the golf ball appears to be taking a left turning, inwardly directed spiral path into the cup. To him, the golf ball “appeared” to have been moved by a “force” that directed the ball to the left in a circular motion around the turntable. To this observer, the motion also appeared to have an inwardly directed component that eventually moved the golf ball, in its spiraling path, towards the cup.. .. .Both men observed the same event, but from very different “perspectives”. In my study it seems clear that these observational “perspectives” are referred to in the physics community as “reference frames”. More on this will follow later.. .. .The “apparent” motion that the man sitting in the chair observed is called the “Coriolis effect”. I found an interesting formula for the “Coriolis effect”, caused by the earth’s rotation on its axis, on an oceanographic and atmospheric website. It looks like this:. .. .C = fV. .. .Where “C” is the “Coriolis force”, V is the velocity, and “f” is the “Coriolis parameter”.. .. .The Coriolis parameter is defined as (I have to insert the words for the Greek symbols): f = 2(omega)sin(phi) where,. .. .(omega) is the angular velocity of the earth rotating on its axis (~ 7.3 x10(-5) rad/sec, and. .. .(phi) is the lattitude (either positive or negative, thus left or right visual effects for the 2 hemispheres).. .. .Notice that in the language of this website (and in the formula), the Coriolis “effect” is referred to as the Coriolis “force”. It is not a real force, but is in fact an observed effect on motion, since observers on earth are “observing” oceanic and atmospheric phenomena while sitting on a rotating earth. This is just like the guy sitting in the chair on the merry-go-round. The guys who do oceanic and atmospheric research have to take this “visual effect” (and motion effect) into account, thus the reason for the above formula’s existence. . .. .But I have problems with the language. First of all, it’s incorrect to call this observational effect a “force”, and since it’s not really a force (and we have to correct for this), we now have to call it an “apparent” force. “Apparent” is a word that can mean, “appearing as such but not necessarily so”. References to the Coriolis “force” are all over the Internet and I assume in textbooks also. This is a kind of “double corrective language” that in my opinion is a source of great confusion, which we will see later.. .. .REFERENCE FRAMES. .. .Now to reference frames. There are basically 3 type of reference frames, inertial, non-inertial, and free body. The following definition for an “inertial” reference frame from the Harcourt Dictionary of Science and Technology should help.. .. .inertial frame of reference Mechanics. a frame of reference that will allow Newton's laws to be valid in describing the motion of a system; the frame must either be at rest or be translating with a constant velocity. . .. .Note that an “inertial” reference frame requires a “perspective” where the observer is either at rest, or moving at a constant speed. In a nutshell, “non-inertial” reference frames involve acceleration of the observer. A “free body” diagram is a specifically confined “perspective” whereby all other objects are “removed” from the “environment” so that only the object or body under study is of “visible” interest to the observer.. .. .To illustrate all 3 types, I’ll use an elevator example. Suppose you have a 25-floor hotel with a very large atrium, and glass elevators (visible to an observer within the atrium) that can traverse all the floors. Suppose a 200lb. man (the rider) gets into an elevator and rides it up and down inside the atrium. Suppose another person (the observer) wants to measure some of the forces involved in this process. To a large extent, the force data “the observer” is trying to measure would determine which “reference frame” he chose to use. All 3 types of reference frames are available in this example, and they are as follows:. .. .The “inertial” reference frame – The observation point (“the observer’s” location for taking measurements) could be any location inside the atrium where the moving elevator could be observed, and “the observer” himself is not undergoing any “accelerated” motion. Another possible “inertial” reference frame would exist if “the observer” was riding in a glass elevator next to “the rider’s” elevator, AND “the observer’s” elevator was in constant motion WHILE he was taking measurements.. .. .In the first “inertial” reference frame (standing on the atrium floor), “the observer” can take direct measurements of acceleration and velocity of “the rider’s” elevator without any corrective factors, since neither he himself nor his coordinate system (for measuring), are in motion.. .. .In the second “inertial” reference frame (“the observer’s” elevator in constant motion), the only corrective factor that needs to be applied in order to derive accurate data about the motion of “the rider’s” elevator, is an adjustment for the constant velocity of “the observer’s” elevator. We assume our intrepid “observer” knows the math. . .. .The “non-inertial” reference frame – Suppose “the observer” (in an elevator) chose to measure “the rider’s” elevator WHILE his elevator was either being accelerated from a stop, or was being decelerated to a stop. As most of you know, the acceleration (or deceleration) of an elevator is non-linear, meaning the rate of acceleration changes before either rest or a constant speed is reached. Now the math is much more complicated, and our intrepid observer necessarily requires a COMPLETE understanding of the acceleration involved in HIS elevator, so he can apply the appropriate corrective factors to derive ACCURATE data about the “the rider’s” elevator. His math skills now must be very good indeed. So as a general rule, “non-inertial” reference frames are avoided wherever possible, but sometimes they cannot be.. .. .Some of you may have figured out by now that our man sitting in the chair on the 18th-hole merry-go-round, is observing from a reference frame that involved his constant (directional) acceleration. To him, the golf ball is moving in an inward left-hand spiral towards the cup, while the man with the putter is in an “inertial” reference frame and can observe the true forces involved without visual distortion. The man-in-the-chair’s reference frame is clearly “non-inertial”, and is often called a “rotating reference frame”. But what if the poor guy has only this particular vantage point available to him, and he still wanted to understand the exact forces involved in moving the golf ball from the edge of the merry-go-round to the cup in the center. What corrective factor could he apply to his “observed” data to derive the correct data?. .. .The answer would be a modified version (and formula) of the Coriolis “effect” that the atmospheric and oceanic observers have to use. Suppose the surface of the merry-go-round were laid out with radial lines (extending from the cup to the outer edge) and radius lines (equally spaced concentric circles extending out from the cup), which could provided the man-in-the-chair observer with a coordinate system to measure the motion of the gold ball. His coordinate system for measuring then, would be in accelerated motion just as he is, and would be part of the “non-inertial” reference frame. This of course is just like the circumstances the atmospheric and oceanic observers find themselves in, and their coordinate system that moves in “accelerated motion” with them, is the earth’s latitude and longitude system.. .. .The Coriolis “effect” works in “rotating reference frames” and is quite useful as a corrective factor here. If the man-in-the-chair observer applied this “effect” as a (should I dare say it) “apparent force” in a motion-force vector arithmetic formula, he could then derive the correct path and force applied to the golf ball. The man with the putter however, can derive this information much more directly, since his “inertial” reference frame contains no acceleration distortion. As I said, some “non-inertial” reference frames cannot be avoided. Except for the privileged few that travel in outer space, the A&O observers have no choice but to use the “Coriolis effect” (or corrective factor) when observing atmospheric or oceanic phenomena, since they cannot get off of the earth’s merry-go-round.. .. .The free body diagram – Suppose “the observer” in our hotel was only interested in measuring the forces being applied to “the rider’s” body while he was riding in the elevator. “The observer” could chose to isolate the man (in a “perspective” sense) from all of the other objects in his environment, and concentrate solely on what’s happening to “the rider’s” body, while forces from the elevator ride are being applied to it. In a free body diagram, all other objects are removed, and only the forces (represented by arrows, vectors, and values) are applied to the “body” under study. However to accurately understand a “free body’s” reaction to external forces being applied to it, those external forces MUST be accurately understood and quantified. As Nick said, this is how rotor blades (and other helicopter parts) are designed. This method is commonly used in FEA (Finite Element Analysis) of structural parts, where a part’s reactions to the various external forces (expected to be applied to it), are under computer study.. .. .THE CENTRIFUGAL FORCE. .. .There are all kinds of forces operating in the natural world. Among them are inertial forces, which constitute the resistance of all types of matter to acceleration (expressed in Newton’s first law of motion). Let’s look at a “free body” diagram familiar to all pilots, the airplane with the 4 basics forces of flight shown on it (gravity, lift, thrust, and drag). The drag force is an inertial force, as it consists of the collective resistance of all the air molecules near the plane, that must be moved out of the way as the planes passes through that local area. In order for an air molecule to be moved, it MUST be accelerated so its state of motion can be changed, and the resistance to this acceleration by the molecules is the force called “drag”. You’ll note that in most cases the “drag” force does not exist if the “thrust” force does not exist.. .. .But some in the scientific community are arguing that “inertial” forces are “apparent” and therefore non-Newtonian. Where on earth they are getting this I don’t know, but it’s clearly wrong. Do you guys remember that old desktop artifact from the 70s with the 5 shiny steel balls suspended on “V” shaped strings all lined up in a row? Remember that if you lift up a ball on one end and then drop it, the energy from that ball is transferred to the ball on the opposite end, and it flies up. It’s the “inertia” (or momentum) of the first ball that caused the opposite ball to move. Tell me this not a real force as work, and that’s its not demonstrating Newton’s laws of motion!. .. .Here’s a quote that Lu gave us from Robert McFarland of Sikorsky in the first post of this thread…. .. .“It took awhile, but the response from our Rotorhead Design group is as follows: . .. ."When you set up a free body diagram of a rotating mass such as a blade, one force is in the direction toward the center of rotation (centripetal force), the other force is in the direction away from the center of rotation (centrifugal force). They are the same force, centrifugal being the more common term." . .. .We hope that this is helpful. . .. .Regards,. .Robert McFarland. .WCS HelpDesk. .Sikorsky Aircraft Corporation. .. .Notice that just like in the “free body” diagram of the airplane, we have 2 opposing forces being applied to the rotor blade, one towards the hub, and the other towards the tip. We know that one is the centripetal force, and the other is the centrifugal force, and one does not normally exist without the other. Just like “drag” in the airplane diagram, centrifugal force is also an inertial force, but this time resisting “directional” acceleration instead of “linear” acceleration, but “resisting” all the same. Just as “thrust” (the acceleration force) is different from “drag” (the inertial “acceleration resisting” force), so the “centripetal” force (the acceleration force) is different from the “centrifugal” force (the inertial “acceleration resisting” force).. .. .One last example, then I’ll close this very long post. Suppose you have car with a passenger sitting in the back seat next to the right rear door. Suppose the window of that door is rolled all the way down, and he has an ice cream cone with a very large scoop of ice cream on top. Suppose also that the scoop is very loosely attached to the cone. Now suppose the car suddenly enters into a hard .5g left hand turn around a roundabout, and the ice cream scoop suddenly detaches itself from the cone and goes flying out of the window.. .. .To the man sitting in the back seat of the turning car, what path to the ground does the ice cream scoop seem to take from his perspective? Keep in mind that his frame of reference is “non-inertial” since the car is turning and he with it. He would see the scoop moving away from the car in an arc that travels towards the rear of the car, and down to the ground. But to a guy standing outside the car on a curb, the path of the scoop will appear to travel in a straight line away from the car and arcing towards the ground only (due to gravity). The only “apparent force” here is the one that seems to make the scoop arc towards the rear of the car. This particular arc is caused by the visual distortion of the car’s directional acceleration in the left-hand turn. . .. .The centrifugal force that caused the ice cream scoop to detach from the cone is real (an inertial force that caused the scoop to resist its acceleration around the roundabout). But the “apparent force” that seemed to cause the scoop to arc towards the rear of the car is not. A corrective factor could be applied to what the car passenger saw (and measured), then the true path of the scoop could be known to him.. .. .I hope this clears things up. If not, I tried my best.. .. .HF, I hope this discussion sheds some light on your question regarding steps 13 and 14 in your logic chain. This explanation would be the clearest answer that I think I could possibly give to that question.. .. .(edited for clarity). . . . <small>[ 18 March 2002, 06:30: Message edited by: Flight Safety ]</small>

[heedm]

Flight Safety,. .. .I'm sorry, but you're wrong.. .. .There are not three distinct frames of reference. A frame of reference is something you purposely or inadvertently choose when you make a measurement. That reference frame is either inertial or non-inertial. There are an infinite number of choices of each. It has nothing to do with what reference frame you put the observer in. You could observe someone putting on a merry go round while doing aerobatics...makes no difference to the reference frame of the golf ball.. .. .A free body diagram is not a reference frame. It is a simplification of reality chosen to illustrates something. When you make some sort of measurement on your free body diagram, you are selecting a reference frame. That frame is inertial or non-inertial.. .. .Centrifugal and centripetal forces are the same magnitude and they point in opposite directions, but it is wrong to call them equal and opposite. Very very wrong. Tell that guy at Sikorsky to stop embarassing his company. The fact is, centrifugal forces can be observed in one reference frame where a centripetal force is produced in a different one. Because they can't both be observed equally and oppositely in one reference frame, they are not equal and opposing forces.. .. ."You’ll note that in most cases the “drag” force does not exist if the “thrust” force does not exist." Not at all. If this were true then gliders wouldn't have any drag. Drag is a force that is dependant on velocity, not on thrust.. .. .Finally, you claim that "the physics community is in a state of serious confusion". No, physicist understand fully the difference between real and apparent forces. It's others that get it wrong.. .. .If a physicist went to a biologist and said, "That's not an amoeba, it's a jelly bug." and then insisted that all amoebas should now be called jelly bugs, does that mean the biology community is in a state of confustion?. .. .Take expert advice from experts. Not people who use a little bit of the knowledge, not people who answer the phone at big companies, not self proclaimed experts who have too much time on their hands so make long posts on the internet ( <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> am I the kettle or the pot). Take advice from those who study in that area.. .. .Last point. Why is an apparent force called a force at all? I guess the simplest answer is "if it talks like a duck and walks like a duck, call it a duck." When my copilot turns too sharply and pins me to my seat while I'm trying to drink some coffee, I don't think "hmmm...that felt a lot like a force but because I know some physics it totally wasn't". It feels just like a force, much of the physics we can study on that force work the same as real forces. Force is a good word for something that accelerates you, even when it's observed only because the seat you're sitting in gets accelerated.. . . . <small>[ 18 March 2002, 19:29: Message edited by: heedm ]</small>

[Flight Safety]

heedm, thanks for your reply.. .. .You said...There are not three distinct frames of reference. A frame of reference is something you purposely or inadvertently choose when you make a measurement. That reference frame is either inertial or non-inertial. There are an infinite number of choices of each.. .. .I generally agree with you, and there are an infinite number of choices for each type of reference frame. It might be fair to say however that there are 2 measurement types of reference frames, and one discreet object reference frame.. .. .You said...A free body diagram is not a reference frame. It is a simplification of reality chosen to illustrates something. When you make some sort of measurement on your free body diagram, you are selecting a reference frame. That frame is inertial or non-inertial.. .. .One HAS to take measurements of the forces acting on a discreet body, from either an inertial or non-inertial reference frame. But once in the free body "perspective" those forces are summarized as vectors and magnitudes (and sometimes with changing vectors and values over discreet periods of time, if that's part of a computer simulation).. .. .You said...Centrifugal and centripetal forces are the same magnitude and they point in opposite directions, but it is wrong to call them equal and opposite. Very very wrong.. .. .I beg your pardon, can you say that again?. .. .You said...The fact is, centrifugal forces can be observed in one reference frame where a centripetal force is produced in a different one. Because they can't both be observed equally and oppositely in one reference frame, they are not equal and opposing forces.. .. .Both forces exist due to acceleration, centrifugal does not exist with centripetal. What prevents them from both being observed and measured in a rotating reference frame?. .. .You said..."You’ll note that in most cases the “drag” force does not exist if the “thrust” force does not exist." Not at all. If this were true then gliders wouldn't have any drag. Drag is a force that is dependant on velocity, not on thrust.. .. .Alright, I knew when I used the powered airplane example that I was referring to level flight, and the statements I made are valid in regards to maintaining constant speed level flight. The glider diagram has only 3 forces, but that wouldn't have been helpful in my explanation. In the glider, gravity provides the force to accelerate the air molecules out of the way of the passing airplane. Just to further clarify, I was referring to the aerodynamic drag, not the induced drag.. .. .You said...Last point. Why is an apparent force called a force at all? I guess the simplest answer is "if it talks like a duck and walks like a duck, call it a duck.". .. .Here's my whole problem. If if "feels" like a force (walks, talks, and quakes like a force) it must be a force. But wait, it only seems like a force (even though it walks and talks and quakes like one), but really it's not, so in reality its only "apparent". Honestly, is there confusion here or what?. .. .(edited to add the last point). . . . <small>[ 18 March 2002, 23:04: Message edited by: Flight Safety ]</small>

[heedm]

Flight Safety,. .. .I'll try clear up the centrifugal vs centripetal discussion.. .. .Centrifugal force is felt by someone in the rotating reference frame. The need for a centripetal force is realized in a non-rotating reference frame.. .. .Why can't you measure them both in one frame? Because you can't have a frame that both rotates and doesn't rotate at the same time.. ._________. .I enjoy these kind of discussions, but this stuff isn't opinion. Physics is written down, precise definitions. I'm just echoing what I spent five years studying after high school, with frequent cross checks to my dusty text books to make sure. Granted many physics texts don't explain rotational dynamics very well, but then it's not a simple subject. Do lots of exercises and experiments, study vector calculus and classical mechanics, and five years later it should all come very clear.

[lmlanphere]

guess what?

[helmet fire]

FS:. .Thank you for the time and effort of your post.. .. .I am trying to see which part of the logic chain is the part that you do not accept. I believe you are making the subject unnecessarily harder to understand with the introduction of frames of reference. Heedm hit the nail on the head – any measurement occurs in a frame of reference, but that frame of reference does not, in itself, change the reality of what is occurring. The forces do not change because the observer changes his frame of reference. For example, your seated observer in the golf ball merry-go-round example. Although the hitter and the observer have different frames of reference, the forces acting on the golf ball are the same – i.e. the reality of the situation is not altered just because you view it from a different perspective. Thus, the frame of reference is irrelevant to the reality of the situation, and I believe, irrelevant to this discussion.. .. .But, I believe you may be considering frames of reference in an attempt to rationalise the term “apparent”, as in an apparent force. If so, it is here that we seem to have the disagreement. What you are really proposing is that we concede that centrifugal is a real force because of your belief that it changes from apparent to real depending on your frame of reference. I go back to my logic chain in which I structure the vector analysis of the forces on a straight line accelerating car to begin with, then examine the turning car. I specifically did this to address the fact that they are the same set of forces. Nothing changes just because the car turns (i.e. the frame of reference is different). Centripetal is simply a name given to an acceleration that is causing the car to turn, just like other accelerations are named specifically. But, a simple acceleration is all it is. . .. .You have still yet to prove that the “force” you feel pressing you back into the seat of a straight line accelerating car (or flinging you out through the back of the van, etc, etc) is a force at all.. .. .I have repeatedly said throughout this thread that centrifugal is a useful term, it is a useful tool in the explanation of rotational dynamics. It makes rotational dynamics easy to understand for people whom don’t need to understand the intricate reality. It is a simplification used to describe other more relevant topics to the learner, e.g. “the coning angle of the blades is the result of the balance of lift and centrifugal forces.” Obviously, this is not accurate (because centrifugal is not a real force), but it makes it very easy for the average pilot/mech to understand why the coning angle is what it is. Similarly with gyroscopic precession. US pilots get taught centrifugal force and gyroscopic precession, whilst the UK pilots get taught centripetal and aerodynamic precession. The English course takes far longer (and is thus more expensive), but is technically more accurate than the typical US course. BUT, does that make them better manipulators of the flight controls, better pilots/mechs, or better aviation businessmen. Absolutely not. It has nothing to do with these skills - it is simply an easy way of explaining the scenario, but that doesnt make it fact.. .. .You bring yet another example (21 now!) to the table. The ice cream cone. You are correct about the apparent paths of the ice cream, but as per above – why does that change the reality of the situation? How can it? The ice cream cone is not flung outward from the car. At the moment it comes free of the cone, it is no longer turning toward the centre of the circle, i.e. it no longer has an acceleration applied to it to keep it turning, thus it travels in a straight line. Which way in relation to the car? At a tangent from its release point, NOT flung outward!. .. .I cant believe I am going to introduce another example her, but here goes number 22… <img border="0" title="" alt="[Roll Eyes]" src="rolleyes.gif" /> . .. .Everyone whom reads this thread need only carry out one simple experiment to see if the world’s physicists are correct in believing centripetal is real and centrifugal is not. Please carry this experiment out before posting again on this thread.. .. .Get a large easy to see object with some weight, such as a full soft drink can or water bottle. Go out side with a friend. Face a very obvious marker, such as a distant white building or even use your friend. Hold the object out at arms length from your body, DIRECTLY TO YOUR FRONT, so that as you turn your body, your arm stays straight out in front. Spin around. When you and your hand are exactly facing your marker, open your hand.. .. .If the object flys straight at your marker, Nick, heedm, the worlds physicists and I are wrong, and your friend gets hit. This would prove that centrifugal is real because the object would no longer be restrained against that “force” that is pulling the object outwards.. .. .FS & Lu, which way did your object fly? Did your friend see the object fly in any other manner than you? <img border="0" title="" alt="[Confused]" src="confused.gif" /> <img border="0" title="" alt="[Confused]" src="confused.gif" />

[pants on fire...]

I can settle this for once and for all.. .. .A centipede has 100 legs, (and some of them are very big and very poisonous).. .. .A centrifuge doesn't and isn't!

[Lu Zuckerman]

To: All. .. .Some of the explanations made in this thread to either support centrifugal force as a real force or to say it is an apparent force make my head ache. This thread has made me both unhappy and happy at the same time. Unhappy for having started the thread with limited understanding of the subject and happy, that I majored in Industrial design as opposed to engineering.. .. .Each of you has expressed your opinions and from your individual perspectives you are all right. Helmet Fire summed it all up in explaining the difference in how the subject is taught both in the UK and the States. The Brits explain it in engineering terms and in the States they use (by comparison) a more simplistic approach. The pilot in the UK understands how his helicopter flies and the Yank pilot understands how his helicopter flies. Then they both go to work and completely forget what they were taught. That is until a Troll like me opens the subject on an open forum.. .. .What I would like to say is that the words of Helmet Fire about different teaching methodologies between the Yanks and the Brits are exactly what I was trying to say relative to gyroscopic precession Vs aerodynamic precession. Both are are correct it just depends on where you are from.

[helmet fire]

Well summed up Lu.......but....... .. .my point was actually that of the two teaching methodologies, the UK one is scientifically substantiated, the US one is not (in regard to the topic). The UK one is long & expensive and offers no real advantage over the simplified and user friendly US approach as it applies to doing the job, other than you will not find a physicist who disagrees with you in the bar!! <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .But we digress. Lu, did you do the experiment? . .. . <img border="0" title="" alt="[Cool]" src="cool.gif" />

[captaincrack]

I get a little bored of reading these threads that are so long they end up going around in circles. As a result I havent read all the replies, so I apologise if I am repeating what someone else has already said.. .. .I am going on what I was taught in O level physics. I am not a professor in Newtonian physics, but I am a trained engineer.. .. .As far as I can make out this force is usually termed cenripetal by physics teachers and engineering lecturers. Everyone else uses centrifugal. Whatever you use, it is a reaction force. . .. .As a blade, wheel or your underwear spins round, its velocity constantly changes, (velocity being defined as rate of change of distance with time in a straight line). If velocity is constantly changing, then there must be acceleration (acceleration being rate of change of velocity with time). Now Newton steps in with his third law, stating that a body that has mass and is being accelerated, must have a force associated with it. That force being centri******.. .. .Nothing to do with inertia, momentum etc, just the simple fundamentals of mechanics. Dont use intuition to understand, use basic facts.

[Dave Jackson]

Last night on TV, they said that every galaxy is paired with a black-hole and they rotate about each other.. .. .Which way does the centrifugal go and which way does the centripetal go?. . <img border="0" title="" alt="[Confused]" src="confused.gif" /> <img border="0" title="" alt="[Confused]" src="confused.gif" /> <img border="0" title="" alt="[Confused]" src="confused.gif" /> . .. .And where do you suggest this posting go?. . <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" />. . . . <small>[ 22 March 2002, 00:49: Message edited by: Dave Jackson ]</small>

[heedm]

Dave centrifugal go in centripetal go out.. .. .This posting go to that twin galaxy.

[Dave Jackson]

Heedm,. .. .said &gt;".... centrifugal go in ...."&lt;. .. .Does this mean that 'centrifugal force' does not exist?. .. .'Cause everything that goes into a black-hole ceases to exist.. .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[Flight Safety]

One final question regarding the question of whether the "centrifugal" force is real or not.. .. .Take the device known as a "centrifugal" clutch as an example. When the spin rate of the clutch causes the friction pads to move out from the hub (overcoming the centripetal force of the springs), what force is causing the friction pads to press firmly enough against the clutch drum to cause it to turn and transfer power?. .. .P.S. I hope some of you still remember what a "centrifugal" clutch is.. .. .(edited for word omission). . . . <small>[ 25 March 2002, 22:11: Message edited by: Flight Safety ]</small>

[heedm]

The friction pads are being moved in such a way that they are trying to move away from the center. This is evidenced by the state of the clutch prior to the pads contacting the drum. Once the pads contact the drum, then the drum pushes on the pads to prevent them from continuing their motion. The force is generated by the cohesion of the material of the drum. To convince yourself of this, replace the drum with a strong elastic band. You will see that the elastic band will stretch to a point and then if it's strong enough, will hold the pads.

[Flight Safety]

Heedm, a very interesting answer.... .. .You are correct, cohesion of the material of the drum, does create a force to cause the drum to "push" against the pads to create the necessary friction to cause the clutch to transfer power. However, this clutch design (like many different clutch designs) operates by pressure creating friction between 2 or more surfaces.. .. .You've accurately described how the clutch drum applies pressure to the pads to create the necessary friction for this design, but my question deals specifically with how the pads apply pressure to the clutch drum to create the necessary friction. These two surfaces MUST apply pressure against each other to create the power transfering friction, so specifically, what force is causing the pads to apply pressure to the clutch drum?