Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 25/12/2010 22:52:05
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1. Is motion around a circle keeping a constant speed an accelerated motion?
2. Which description would you use, centrifugal force or centripetal?
2a And when do you use them, respectively, or does one cover it all?
2b. According to whose definitions?
2c. And does the 'frame' chosen have anything to do with it?
3. Can you define a non-rotating frame versus a rotating frame using the example (& descriptions) above.
4. Can you name any 'effect' created by the centrifugal force on Earth?
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You need to get over this.
1. NOTE (from Wikipedia)—“First law: Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant speed in a straight line.” An external unbalanced force causes acceleration. So, is a body that moves in a circular motion accelerated? What frame of reference is this?
2. Depends, see below.
2a. Centripetal covers it all in Newton's frame of reference, but engineers who design, for example, high rpm disk drives find that calculations and creative thinking are simplified by using centrifugal force.
2b. Newton defined them both.
2c. The frame chosen is all important and in a physics discussion someone who doesn’t define, and justify, the frame within which they are talking is disruptive because they cause confusion and arguments.
3. The frame of reference of a man on an athletic field who swings a weight around while watching it sees the weight as stationary and that it is pulling his body with centrifugal force. Another man, standing nearby, seeing the first man rotate recognizes that the man is applying centripetal force to accelerate the weight in a circle.
4. If you want this answered, please define your frame of reference.
5. Do you get it?
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Steve I'm surprised?
I actually made them in fun?
Apparently not.
Never mind. You are perfectly correct in saying that it was from a Newtonian perspective I asked. I was hoping to get alternative views also from a relative viewpoint too, but here it is as I understands it.
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1. Is motion around a circle keeping a constant speed an accelerated motion?
Yes, it is.
Why?
Because that even though its speed is 'uniform' its direction of motion is in constant change. Newton said that "only if a body moves with constant speed along a straight path are no forces needed to maintain that motion."
2. Which description would you use, centrifugal force or centripetal?
"To clearly distinguish between the centripetal and centrifugal forces:
* The centripetal force is used when we calculate motion, using coordinates, velocities and accelerations of a non-rotating frame of reference.
* The centrifugal force is needed to describe the same motion, using coordinates, velocities and accelerations of a rotating frame of reference."
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3. Can you define a non-rotating frame versus a rotating frame using the example (& descriptions) above.
Just read 1 and 2 and then you will see that you can define it both ways, only the choice of frames differing. Both will, as I understands it, be correct. As for 4 I was open for suggestions :)
Myself I was thinking of the bulge of the Earth's equator, due to centrifugal force, but with the Coriolis effect added in. But it wasn't meant to make that stomach ache, I made it for fun.
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To make the distinction another way.
You're on a carousel spinning. To stop yourself from gliding, getting thrown out, you take hold of 'something' bolted to the carousel. That's your centripetal force applied, otherwise the centrifugal force would rule, throwing you out.
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* The centrifugal force is needed to describe the same motion, using coordinates, velocities and accelerations of a rotating frame of reference."
Hmmmm? But would a rotating frame of reference actually have any sense of rotation? If there is no sense of rotation, there will be a force, but is there any reason to refer to it as centrifugal, or anything else for that matter?
Put another way, if I'm being forced against the inside surface of a rotating cylinder, but I have no knowledge of what's going on outside the cylinder, all I know is there is a force pushing me against the wall of the cylinder. I cannot know that the cylinder is actually rotating without access to some external non-rotating reference.
Therefore, I posit that centrifugal force is baloney [;D] It's simply a reaction to centripetal force.
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Well. I agree with you both there Geezer, that is how I understands it too. The centripetal force is the used one. But to me it became interesting to see what differed them. I must admit that I grew up with Einsteins ideas, not Newtons, mostly as I'm what they call an avid reader. And I never liked school that much, so I used to avoid it :)
That makes what some of you guys take as the 'natural' manner of looking at it (order of things) quite new to me. But Newton make a whole lot of sense to me the more I read about it.
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"But would a rotating frame of reference actually have any sense of rotation?"
As long as you have a universe with 'matter' in it I would expect you to feel a 'centrifugal force' though, if you're the one riding the carousel I mean. I think that's what Mach's principle (http://en.wikipedia.org/wiki/Mach%27s_principle) wondered about too?
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Ah I think I see what you was thinking of. If we assume a universe with stars you will have a reference telling you that you spin, but if you don't? How will you ever define your 'motion' and the 'force'?
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"But would a rotating frame of reference actually have any sense of rotation?"
As long as you have a universe with 'matter' in it I would expect you to feel a 'centrifugal force' though, if you're the one riding the carousel I mean. I think that's what Mach's principle (http://en.wikipedia.org/wiki/Mach%27s_principle) wondered about too?
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Ah I think I see what you was thinking of. If we assume a universe with stars you will have a reference telling you that you spin, but if you don't? How will you ever define your 'motion' and the 'force'?
You got it. You will be aware of the force, and you can measure it and it is real to you, but you have no way to establish that it's caused by rotation within your frame of reference because there is no way to tell if you are rotating or not.
To you, it is indistinguishable from a force field that surrounds your cylindrical universe.
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A man in a cylinder that is relatively small and/or rotating fast enough would easily be able to tell what his situation was. I don't believe that the experiential viewpoint is useful to prove anything.
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A man in a cylinder that is relatively small and/or rotating fast enough would easily be able to tell what his situation was.
How would he be able to confirm he is rotating? (Remember, he has no reference points outside the cylinder.)
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You have to set your parameters before this gets interesting. For example, if you were in one of those carnival rides where you get into a cylinder and when it rotates everybody sticks to the walls, it it easy. The people across from you feel gravity in the opposite direction that you do.
Consider a cylinder that is one of those science fiction habitats that might be 10 kilometers in diameter and gravity is generated by rotation. For simplicity sake also assume that the habitat is far away from any other gravity sources and the interior consisted of a series of concentric floors that were broken up into rooms so that you couldn't see the upward going curvature spinward and anti-spinward, but not at 90 degrees to this. If you run or throw a ball in the spinward direction the gravity on the moving object will increase, while in the opposite direction it will decrease to a noticeable degree. Throwing the ball across a room in the direction orthogonal to the spin direction would be very strange. You would always be innacurate to the left from one end of the room, but off to the right when on the opposite side. Even stranger yet, if the floor was actually open clear around the cylinder, and discounting air resistance, if you threw the ball fast enough in the antispinward direction it would appear to go into a sort of inverted orbit where it would go clear around the circumference without dropping to the floor, and would hit you in the back. In this instance the frame of reference of an observer, outside the habitat, would see someone throwing a ball so fast that it completely counteracted the velocity of the spin so that it hung motionless while the whole enormous habit continued to rotate.
Suppose you were on Larry Niven's Ring World where the diameter of the cylinder is as large as a planetary orbit. The personal viewpoint would be very poor at detecting what was going on, but with a little cleverness and a very accurate high powered rifle the same effects as above could be demonstrated. If you are just a guy working on his hunting rifle centripetal or centrifugal are not pertinent and in a somewhat smaller cylinder what would be important is developing an inertial compass that would give automatic distance adjustment to the scope depending upon what direction it pointed.
When being scientific one should be very clear as to what frame of reference is being used, and if necessary justify it. Steve
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You have to set your parameters before this gets interesting. For example, if you were in one of those carnival rides where you get into a cylinder and when it rotates everybody sticks to the walls, it it easy. The people across from you feel gravity in the opposite direction that you do.
But how does that tell them they are rotating? All they know is that they are being forced against the walls.
They might infer they are rotating of course, but as long as they are pinned to the walls, all they know is that something is pinning them to the walls.
If they could pry themselves away from the walls and get to the axis of the cylinder though, I suppose they would rotate relative to the cylinder. If so, that might be hard evidence.
EDIT (A bit later.)
Here's one reasonably straightforward way to do this, and it might even allow the rotational speed to be quantified fairly easily.
If there are two people (of not too dissimilar weight) in the cylinder and they have a rope, they could start pulling each other towards the axis from diametrically opposed positions. As they reduced their distance, they would speed up relative to the cylinder. This would confirm rotation and direction.
I think they will be able to determine the rotation rate of the cylinder by observing how their speed, relative to the the cylinder, varies with the distance between them, but I'm leaving that as an exercise for Yoron [:D]
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Next year :)
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Next year :)
What's your phone number? I'll call you shortly after midnight to remind you.
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:)
Tip tap Tip Tap, Tipie tipie tiptap tip tip tap.
(Tip-toeing away, in cadence to an old Xmas carol.)
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Had to correct the tip, there was one too much, as well as the toe was wrong..
All Swedes of my generation should know this one. It was very popular in first year(s) of school at Xmas play. 'Santas little helpers' tip toeing sort of. They forced us to do it..
Dam* them for that.
The melody sort of burn itself in:)
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Geezer, if someone is unable to figure out that they are in a rotating cylinder based on the fact that they are in a circular room and the down direction rotates as one goes around the room, then they won't be able to make inferences by moving to the center of the room. One of the points I made was that the information in this particular rotating frame is very different than that in a non-rotating one and an observant and knowledgeable person could figure it out.
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Geezer, if someone is unable to figure out that they are in a rotating cylinder based on the fact that they are in a circular room and the down direction rotates as one goes around the room,
Steve, as I pointed out earlier, they might infer that they were in a rotating cylinder by that method, but they could not prove they were not in a stationary cylinder surrounded by some sort of force field. I'm sure you understand the difference between a proof and an inference.
Surely the only way to prove it is by observing the angular momentum of objects in the room?
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Geezer, you are going to have to help me with this one. In the light of the fact that this is a science site, please tell me what real world "force field" you would invoke in your example. I don't really see how one could reasonably interpret the situation as other than a rotating cylinder, unless we are in the Star Trek universe. Steve
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As I thought of it Steve, it had to be someone born to the situation. And to then infer 'straight paths' I think would be quite hard, although not impossible. I'm sure you and Geezer can come up with a scenario.
But I guess that from their 'universe' they would discuss 'straight paths' much as we do 'centrifugal forces?' As something not entirely 'natural' :)
Quite intriguing.
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And yeah, I too thought of it like a ring world, or even bigger, giving you few clues of a 'outside'.
There would have to be frame-dragging involved too, if we assume that the 'outside' obeyed our laws.
Spiraling in? As the diameter would decide the speed right, the further away from the hub the faster your motion and the more 'gravity' I think. And for it to work the structure would have to be immense, massing, an awful lot:)
Weren't there some guys counting on that for the 'ring world scenario'?
Awh..
Never mind, I refuse diving into a ring world, it's too late for me :)
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Yoron, I perfer to think of it as Doughnut World [:)]
Geezer, you are going to have to help me with this one. In the light of the fact that this is a science site, please tell me what real world "force field" you would invoke in your example. I don't really see how one could reasonably interpret the situation as other than a rotating cylinder, unless we are in the Star Trek universe. Steve
Oh! I admit their inference would would have an extremely high probability of being correct, to the point that it would seem highly unlikely they were in anything other than a rotating cylinder. I was merely interested in experiments they might use to prove the cylinder is rotating if they had no ability to know what was going on outside the cylinder. I'm wondering if there is any method other than detecting angular momentum. I suspect not, but that's really just a guess.
Re. a "practical" method to produce the force field, if the cylinder was made from some remarkably dense, and probably unobtainable, material and the wall of the cylinder was reasonably thick, it would probably do the trick. I suppose it would not even have to be all that dense if it was sufficiently thick, although I suspect that would only work if the internal diameter of the cylinder was large enough. Mind you, it's not something you are likely to find at the local Home Depot.
Slightly off topic, but still related, have you ever been on an amusement ride that was sometimes called "the rib tickler"? It's a remarkably simple, but incredibly effective device. It consists of a fairly large swinging platform with a lot of seats attached to it. Enclosing the swing is a rotatable enclosure which has lights attached to it. The enclosure's axis is parallel with the axis of the swing.
You take a seat on the platform then they close the enclosure so you can no longer see outside. They then rock the swing back and forth. No problem. Then they start to rotate the enclosure. Big problem. It's many years since I was on one, but I can still remember the terrifying sensation that I was upside down and about to fall out of my seat. All I wanted was for it to stop!
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Geezer, with your doughnut hole within a massive structure there wouldn't be the Coriolis effects I described. Regarding your fair ride, this sounds like the barf express to me. Steve
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Geezer, with your doughnut hole within a massive structure there wouldn't be the Coriolis effects I described.
Agreed. The two people at the ends of the rope test would yield a similar result. They would not rotate relative to the cylinder.
Re. the "amusement" ride, as I remember (it was a very long time ago and I vowed never to go back on one) it was more about abject terror than nausea. Maybe fear suppresses the upchuck response, but that's probably a matter for a different thread.
EDIT: Now I'm completely confused. I'm not sure whether this proves the existence of centifugal forces or not [:D]
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Geezer. A physicist will tell you that the guy swinging the weight is pulling a moving weight into a circle, the weight is not doing the pulling. Here is an analogous imaginary situation. If you were in an enclosed space on the back of a device that can move slowly and very smooth such that you can't tell that you are moving. You are holding a rope that runs through a hole in the wall that is tied to a heavy weight. Your perception when you are moved by the device is that something is pulling on the rope when, in fact, you are pulling the rope. In your frame of reference the force is real, but it is in fact a fictive force from the frame of reference outside of the imaginary device. Steve
EDIT- I fixed the post to your satisfaction, but you are awfully sensitive. It was only an observation because you keep attributing a lot of importance to the centripetal/centrifugal terminology when it is really no big deal and was laid out by Newton, the inventor of both terms. The Coriolis force is also a fictive force and it is quite useful when trying to understand weather. It is all a matter of ones frame of reference and, for example, the Einsteinian viewpoint of Newton's gravity relegates it to being a fictive force.
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Geezer, you are attaching a little too much importance to this.
Steve,
I think it is rather presumptuous, and extremely patronizing, of you to try to decide how much of anything I am attaching. It's also presumptuous of you to decide what a physicist will tell me, unless of course you are a physicist, in which case you might say "As a physicist......
If you would be good enough to rephrase your post minus the supercilious editorials, I'll try to understand what you're waffling on about.
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Thanks Steve.
Lest you misunderstand, I am only being sensitive on behalf of TNS members in general. Were it not for the rules of decorum here, the boot would be firmly on the other foot.
Believe it or not, some people actually visit this site to learn stuff. They don't expect to get a telling off from some supercilious twit.
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Geezer, you are maintaining decorum by calling me a supercilious twit? One difference between you and me is that I don't take offense at all. I simply don't care what you think of me unless you catch me making an error of scientific information, then I would be very upset with myself and apologize (which I have done here). I am actually a little concerned with this site regarding expertise and accuracy. I have participated in some sites that are run by very high level scientists, and I think you would be much more than a little surprised at how rough communication can get when there is a poster suffering from the Dunning Kruger effect. Steve
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See, there you go again. You inferred that I was calling you a supercilious twit. I didn't.
Meanwhile, returning to the topic, if I understand what you are saying, centrifugal force is merely a name we (and Newton) attribute to the reaction caused by (the real) centripetal force. I think I may have tried to point that out about ten posts ago.
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I don't think that calling centrifugal force just a name, while the real force in this situation is centripetal, is a very good understanding. This is because you would also have to agree that centripetal force is actually just a name because distortion of space time in some instances, according to general relativity, is the real force. They both are just terms in formulas that depend upon ones frame of reference. Further, if one wants to talk about these things it is important to state the frame, hopefully the most appropriate one, and be consistent with it.
As for misunderstanding what you said, it is really quite clear and I don't think anybody would disagree with me. Next you are going to say "if the shoe fits, wear it." I know how to play the "you didn't read what I said" game. You should be ashamed.
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If we want to help people learn and get a better understanding of mechanics and dynamics, I think it's best to avoid the use of terms that can lead to misunderstanding.
I think there is a common misconception that "centrifugal force" is a force that will cause a rotating body to "fly off" at right angles to its direction of travel. Of course, we know that is not true. As soon as the centripetal force is removed, there is no centrifugal force, so why not simply call it what it is i.e., a reaction to centripetal force?
Of course you are right in saying that this is only one model that we use to describe circular motion, I just happen to think it's best to be as clear and consistent as possible within that particular model.
I will refrain from commenting on your obvious reference to Dunning Kruger.
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Geezer, what you say is very sensible because centripetal force is the best and most accurate explanation in our common everyday frame of reference. I tend to correct people who use centrifugal force in everyday usage, especially when it is leading them astray on what they are talking about. But in fact, there are technical uses of centrifugal force that are useful, just like the term Coriolis force is useful in many circumstances. Steve