centrifugal force


by asi123
Tags: centrifugal, force
cabraham
cabraham is offline
#109
Jul3-08, 08:27 PM
P: 997
Quote Quote by tiny-tim View Post
Hi Claude!

I won't repeat what others have been saying about the difference between Newtonian and Einsteinian frames.

I'll just say that you're trying to understand centrifugal force, which appears only in rotating frames, by using the demon drop example, which is not a rotating frame.

The extra force in the demon drop is the R-double-dot force.
Yes I'm well aware that the demon drop is not a rotating frame. But in formulating centrifugal "pseudo-force" the "inertial force" concept was introduced. I figured that where a rotating frame in ucm has constant speed and changing direction, I used its counterpart "demon drop" with constant direction and changing speed. Demon drop doesn't accelerate in a rotational sense, but in a translational sense. It was mentioned to discuss the concept of inertial force.

By "R-double-dot" I presume you mean acceleration, the 2nd time derivative of position. If so, I agree. The gravitational force vector is countered by what you call r dot dot, or more precisely "ma". No other forces are present. Agreed.

Doc Al: "The point of my comment was that the outward force that you feel when twirling the rock is not what we call "centrifugal force". That outward force that you feel is a "real" force caused by tension in the string--nothing "pseudo" about it."" end quote

Of course! I've never thought otherwise. When I pull on a rope with the other end attached to a heavy object, the tug that I feel is what I've always referred to as tension. Tension is real. Likewise with twirling a rock in ucm on a string. The outward force the string exerts on my hand is tension and nothing more. I don't regard it as centrifugal.

I'll do some research. Sorry to annoy anyone, but the teachings of my profs still seem invincible.
D H
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#110
Jul4-08, 10:09 AM
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cabraham, what exactly are you arguing here? That centrifugal force is an inertial force, or that it does not exist?

Suppose two spacecraft meet such that one spacecraft sees the other as stationary but rotating, and rotating about an axis normal to the line connecting the spacecraft. The first spacecraft sees this condition persist for some time. The other spacecraft will see the first spacecraft as rotating, but with the axis of rotation directed opposite to the axis seen by the first spacecraft. The other spacecraft will not see the first spacecraft as being stationary. Instead, the other spacecraft will see the first spacecraft as undergoing uniform circular motion. Which point of view is "correct"?

The answer is, both points of view are correct. Just because the first spacecraft's description of the motion is simpler (rotation only) than the other spacecraft's description (rotation plus translation) does not make the first point of view more "correct" than the other. Both spacecraft might be completely passive (no active controls) with the first spacecraft in inertial attitude and the other spacecraft rotating. Alternatively, the first spacecraft might well be continuously firing its thrusters to place itself in a forced orbit around the other spacecraft and continuously rotating so it is always pointed at the other spacecraft. I have not told you which, if any, scenario is the case. The crew on the other spacecraft will conclude the alternate explanation if the gyroscopes and star trackers on that spacecraft indicate zero rotation with respect to inertial.

If, on the other hand, the other spacecraft's inertial navigation system says that they themselves are rotating and the first spacecraft is completely passive. The crew on the other spacecraft can still explain the first spacecraft's motion from the perspective of a frame fixed with respect to the other spacecraft. In this frame, the first spacecraft will have an velocity of [itex]\mathbf v = \mathbf r \times \mathbf {\omega}[/itex]. This velocity will appear to change due an apparent centrifugal acceleration of [itex]\mathbf {\omega} \times (\mathbf r \times \mathbf {\omega})[/itex] and an apparent Coriolis acceleration of [itex]2 \mathbf v \times \mathbf {\omega} = 2 (\mathbf r \times \mathbf {\omega}) \times \mathbf {\omega}[/itex]. While there is no real centripetal force causing the first spacecraft's apparent uniform circular motion, there is an apparent centripetal force equal to the vector sum of the centrifugal and Coriolis forces.
cmos
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#111
Jul4-08, 04:37 PM
P: 367
Quote Quote by cabraham View Post
I'll do some research. Sorry to annoy anyone, but the teachings of my profs still seem invincible.
Have you ever considered that you misunderstood your professor? You have said yourself that you are an EE, which means that the only formal physics you've done is at the elementary level. I have a feeling your professor said something along the lines of, "the centrifugal force is not a real force."

Well, that's more or less what we've been saying all along.

The centrifugal force is NOT usually taught at the elementary level. That's because non-inertial frames are not usually taught at the elementary level. That's because, at this level, we are still trying to get students accustomed to what exactly is a force and the formal concepts of Newton's laws.

(Edit: When I say elementary level, I mean first/second-year physics)

Another thing; it is oft said that one cannot learn nor understand physics without doing physics. Have you tried doing a simple problem (maybe one of the examples myself or others have come up with) yourself to see how much simpler life becomes when you invoke a non-inertial reference frame? Do this and you'll see.
cmos
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#112
Jul4-08, 04:50 PM
P: 367
Quote Quote by cabraham View Post
I visited MIT physics site, and found a lot about bodies in ucm. Centripetal is discussed in every lecture note, but centrifugal is not mentioned. I asked 2 ME people here at work, BSME education, about it, and they said that centripetal acts inward, and velocity is tangential, but centrifugal is something they don't use. I'll find a Ph.D. physics prof at the university I go to, but school is out for the summer, so it may take a while.
I check out the MIT OCW out of curiosity. They do not have ANY lecture note on intermediate or advanced mechanics anywhere. This may you cannot find notes on non-inertial frames. But check this out:

http://ocw.mit.edu/OcwWeb/Physics/8-...ings/index.htm

This is schedule for an intermediate/advanced mechanics class. Look at session 15. Non-inertial frames!

Why don't you pick up one of those books, or any book on classical mechanics at your library?
cabraham
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#113
Jul7-08, 07:40 AM
P: 997
Quote Quote by cmos View Post
I check out the MIT OCW out of curiosity. They do not have ANY lecture note on intermediate or advanced mechanics anywhere. This may you cannot find notes on non-inertial frames. But check this out:

http://ocw.mit.edu/OcwWeb/Physics/8-...ings/index.htm

This is schedule for an intermediate/advanced mechanics class. Look at session 15. Non-inertial frames!

Why don't you pick up one of those books, or any book on classical mechanics at your library?
I'm going to do just that. No offense, but maybe all of us should do likewise.

As far as misunderstanding my professor goes, that is a possibility. Often I hear people quoting something from a lecture, or a highly esteemed source like Newton, Maxwell, Einstein, etc. and they misunderstood the source. I'm always careful regarding that. However, here are my prof's exact words:

"There is no such thing as centrifugal force."

I guess I must have misconstrued the above statement.

In the final analysis, maybe the viewpoint today, vs. the 1970's, is to give fictitious, or "pseudoforces" due respect as if they were "real", whatever "real" means. If that be the case, well, I don't want to argue that point. I just completed a semiconductor physics course in EE. I took the prerequisite class in Spring 1979 as an MSEE grad student. Back in those days, an electron was regarded as an actual particle, and a hole was "virtual". Anyway, in my Spring 2008 sequel class, the prof mentioned that holes are injected into the base lead of a bjt, and I remarked, "uh, isn't it more like electrons are extracted from the base lead as holes are virtual and do not exist outside the semiconductor?" He rebuked me with "You are using the older viewpoint that holes are not true particles. Today they are viewed as such." I guess I can't argue with whatever the science community regards as "real". It's sort of like Pluto. In my youth Pluto was a full planet. Now, poor Pluto is but a dwarf, 2nd class, reduced in stature. Oh well. Best regards.
D H
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#114
Jul7-08, 09:04 AM
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Quote Quote by cmos View Post
I check out the MIT OCW out of curiosity. They do not have ANY lecture note on intermediate or advanced mechanics anywhere. This may you cannot find notes on non-inertial frames. But check this out:

http://ocw.mit.edu/OcwWeb/Physics/8-...ings/index.htm

This is schedule for an intermediate/advanced mechanics class. Look at session 15. Non-inertial frames!

Why don't you pick up one of those books, or any book on classical mechanics at your library?
Quote Quote by cabraham View Post
I'm going to do just that. No offense, but maybe all of us should do likewise.
No offense, but many of us already have. It is obvious that at least cmos and jospuur have taken the equivalent course. So have I. The texts for the MIT class are Goldstein, Classical Mechanics, 3rd Edition, and Marion & Thornton, Classical Dynamics of Particles and Systems, 4th Edition. Those are exactly the books I recommended in post #50. Jostpuur obviously used the Classical Dynamics of Particles and Systems, 5th Edition. Marion died between the 4th and 5th editions and lost his place as first author but kept his place as a posthumous author. cmos must have used Fowles and Cassiday, Analytic Mechanics. These three books are the standard set of texts for the advanced undergraduate classical mechanics. These texts are so widely used that they are referred to by author rather than name.

As far as misunderstanding my professor goes, that is a possibility. Often I hear people quoting something from a lecture, or a highly esteemed source like Newton, Maxwell, Einstein, etc. and they misunderstood the source.
Stop generalizing. It makes you come off as a pompous whatever.

However, here are my prof's exact words:

"There is no such thing as centrifugal force."
Freshman physics instructors sometimes go overboard because the intent of freshman physics is to introduce basic concepts. Things like centrifugal and coriolis forces are distractions in freshman physics. Freshman physics instructors also say that gravity is a real force. So go figure.

The caloric and the luminiferous aether do not exist and as such should never be used in describing thermodynamics and light transmission. Similarly, saying "there is no such thing as centrifugal force" strongly implies that the centrifugal force should never be used in describing behaviors. This is what we are objecting to. None of us has said that fictitious forces are "real". There is, however, a big difference between "fictitious" and "does not exist".
cmos
cmos is offline
#115
Jul7-08, 09:09 AM
P: 367
Quote Quote by cabraham View Post
I'm going to do just that. No offense, but maybe all of us should do likewise.
I think most, if not all, of us who've highly contributed to this thread have studied out of one of those books or some equivalent.

I don't want to get too off topic (but it does to serve a point), regarding the case you brought up of hole injection. A hole is what we refer to as quasi-particle (sort of like a quasi- or fictious force). Does it really exist? Not really (sort of like a fictious force). But it makes the analysis much simpler if we talk about about the hole as if it were a real particle (sort of like a fictious force).
cabraham
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#116
Jul7-08, 11:18 AM
P: 997
Quote Quote by cmos View Post
I think most, if not all, of us who've highly contributed to this thread have studied out of one of those books or some equivalent.

I don't want to get too off topic (but it does to serve a point), regarding the case you brought up of hole injection. A hole is what we refer to as quasi-particle (sort of like a quasi- or fictious force). Does it really exist? Not really (sort of like a fictious force). But it makes the analysis much simpler if we talk about about the hole as if it were a real particle (sort of like a fictious force).
Well, what you just stated about a hole being a quasi-particle which does not really exist is what I've believed since the '70's. But, I was just told that holes are now given the same recognition as electrons. The semiconductor physics community, or at least a large number of members, now consider the hole and the electron to be on equal footing. For me that is going to take some getting used to just like considering inertial/centrifugal forces which have always been virtual to now be existant. I'm not averse to accepting new definitions. If holes are now "real" and no longer "virtual", not being a device physics specialist, I am in no position to argue. Likewise with Pluto being downgraded.

As far as freshman physics goes, if centrifugal actually exists, no prof would knowingly say that it doesn't just to keep things simple. They would say that we can neglect it for now, but it appears in more advanced courses. I've never been taught one thing by an engr or physics instructor only to later find out it wasn't so.

The reason I brought up the electron/hole analogy is just to point out that how things are viewed varies as time advances. Maybe Dr. M was putting forward what was the prevailing view at the time, the mid-70's. "Virtual" forces may have been said to be "non-existant" at that time. I'll find out. Incidentally, my 1964 GE transistor manual gives holes as being "virtual" having no existance outside the bulk semiconductor material. My 2005 Sze ref text just used for my recent course makes no mention (or maybe it does but I haven't seen it) about distinguishing "virtual from actual". Holes and electrons are described thoroughly with no mention of which is more significant.

In 40 years, semiconductor behavior didn't change, but how the scientific community views, interprets, and models them did change. That's what I've distilled from this example. BR.

Claude
D H
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#117
Jul7-08, 11:46 AM
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Quote Quote by cabraham View Post
As far as freshman physics goes, if centrifugal actually exists, no prof would knowingly say that it doesn't just to keep things simple.
Sure they do. Even now, freshman physics instructors teach their students that gravity is a real force.

This whole, overdrawn debate hinges on the words "does not exist". That is a very strong phrase in physics. As I mentioned earlier, the caloric and the luminiferous aether do not exist. It would be completely wrong to base any physics education or physics analyses on these falsified concepts. There is a huge difference between "does not exist" and "fictitious". Physicists, meteorologists, engineers, and many others use fictitious forces every day in their work. There is nothing inherently wrong with doing so because there is nothing inherently wrong with the concepts of centrifugal force, coriolis force, etc.

I've never been taught one thing by an engr or physics instructor only to later find out it wasn't so.
Sure you have. You have been taught that [itex]\mathbf F=m\mathbf a[/itex] (Newton's second law) and that [itex]\mathbf v_\text{rel}=\mathbf v_2 - \mathbf v_1[/itex] (Galilean relativity).
cristo
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#118
Jul7-08, 12:05 PM
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Quote Quote by D H View Post
This whole, overdrawn debate hinges on the words "does not exist". That is a very strong phrase in physics. As I mentioned earlier, the caloric and the luminiferous aether do not exist. It would be completely wrong to base any physics education or physics analyses on these falsified concepts. There is a huge difference between "does not exist" and "fictitious". Physicists, meteorologists, engineers, and many others use fictitious forces every day in their work. There is nothing inherently wrong with doing so because there is nothing inherently wrong with the concepts of centrifugal force, coriolis force, etc.
This debate has been drawn out for long enough, and I can see that things will only be rehashed again and again. This seems a good place to end the discussion, thus I am closing the thread here.


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