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Post by rick1776 on May 4, 2003 7:16:46 GMT -5
Pabs,
Im willing to concede on symantics and nomenclature.
Angular acceleration = centrafugal acceleration.
But to say it does not point out radially, I cant concede on that one. Tie a ball onto the end of a string, now spin the ball around over you head. Let go of the string. The ball will fly out radially.
Another good experiment I did on the class was to spin a ball bearing in a sort of roulette dish. The dish had a gate in it that I openned and as the ball went past the gate, hey presto it would fly off radially.
Plus I would clean up big time as the roulette whell was rigged in the houses favour.
cheers rick1776
PS what I dont know I make up, most of the time the differences are minimal.
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Post by pabs on May 4, 2003 16:36:49 GMT -5
No Rick, the ball would fly away TANGENTIALLY to its path which is the direction in which the angular acceleration vector points out. Otherwise it would oppose centripetal acceleration.
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Post by rick1776 on May 4, 2003 20:00:22 GMT -5
Sorry Pabs,
Like I said, what i dont know I make up, but most of the time the two are one in the same.
Look up: Fundimental University Physics, Alonso & Finn 2nd edition page 98-99 for explanation.
It also gives my example about net change in gravity on page 118. And my memory wasnt too bad. the change in gravity is about 0.0339 at the equator. I said about 0.03.
Now back to the confusion with with regards to angular acceleration and centrafugal acceleration. You are correct the two are not the same. I concede on semantics.
tangential acceleration = angular acceleration
a = R.dw/dt
If the circular rotation is constant then there is NO angular acceleration. ie dw/dt = 0, Therefore a = 0
Centrafugal acceleration = normal acceleration (radially outward pointing).
a = w^2.R
This is opposed by centripetal acceleration radially pointing inwards.
Both examples are on page 98. But any first year physics book should have the example.
Now Im not sure if Im reading your post correctly. Are you saying that if you spin a ball on a string above your head and let go of the the ball it will have a trajectory tangential to its motion prior to letting go of the string??? Sorry, no way!!
If the circular motion is CONSTANT (ie no angular acceleration) prior to letting go of the string the ball will fly outwards radially once the string is let go.
If the we do not have constant motion (ie we have angular acceleration) then YES I concede that there will be a tangential component to the motion once the string is let go. Is that what you meant?
If you dont believe me just try it. Grab anything laying around in the lab. A pen, pencil, rubber and tie a piece of string to it and spin it (with constant angular velocity) above your head. Then let go of the string. See what happens and write back.
cheers rick1776
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Post by raptor22 on May 4, 2003 20:08:13 GMT -5
Just catching up on this thread and I have to agree with Pabs, any body held captive in a circular, or thereabouts, orbit will leave that orbit tangetiallyb] and not radially
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Post by rick1776 on May 4, 2003 23:08:16 GMT -5
I am wrong. I slink away into a corner and hide. For constant angular velocity there is no tangetial force but there is a normal force. However as soon as you let go of the string the normal force is zero. Ball continues tangentially.
A thousand lashes for me.
cheers rick1776
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Post by CFF on May 4, 2003 23:23:49 GMT -5
errrr - Rick, sorry buddy, I'm going to have to cast my vote with Pabs & Raptor on this one. I think semantics (verbage) got a lot of this misunderstanding going *I'm referring to someone suggesting centrifugal force was a 'fictional' force that doesn't exist. I DON'T agree with that statement - it seems many a Engineering course has taught its students that as a way to avoid some errors in basic mechanics, and of course it doesn't take into account different (inertial) frames of reference*. However on the question of a ball (mass) moving about a circle of constant radius, at a constant SPEED (velocity of course is a vector, so is changing), the ball will continue in a path that is tangent to the circle inscribed at the point the force is released. After searching high & low for an explanation on the web, have just found one .....: physics.webplasma.com/physics11.html#ucmCFF
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Post by CFF on May 4, 2003 23:25:25 GMT -5
*Slapping myself Rick - After all that typing and sourcing an article, I find you beat me to the punch !!!!! CFF
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Post by Pabs on May 5, 2003 12:21:11 GMT -5
CFF,
I don't know if I misread your post, but are you saying centrifugal force does indeed exist?
There's a very easy experiment to demostrate the existance of the centripetal force. That experiment also shows that there is not centrifugal force in a body in uniform circular motion.
Basically the experiment goes like this. Grab a plastic bottle of coke and drink the coke. Fill it up with water. Then attach a ball of wood to a thread or nylon. Then fix the Nylon and ball to the inside of the cap of the bottle. Then put the ball/nylon/cap assembly onto the water bottle and close the cap.
When you flip the bottle upside down, the wooden ball will float, but since it is attached to the nylon which is fixed to the cap, then it will not float to the top right? Now grab the upside-down bottle and start spinning. You will see that the ball moves towards you as opposed to away from you (the ball is deflected towards the center of the circle). This is due to the fact that there is a centripetal force acting on the ball.
Again, I don't know if I misread your post but just in case I understood you correctly, this experiment should show that centrifugal force is indeed a fictional force, although it is sometimes a nice convenience to have when explaining certain concepts.
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Post by CFF on May 5, 2003 13:15:12 GMT -5
Pabs - Sorry, but I must disagree (and I'm sure Rick as a physicist will back me up on this one). First, let me say (clarify) that centrifugal force IS an inertial force - it only appears in an accelerating frame of reference. The classic rollercoaster problem is viewed from 2 points of reference at the following site: www-istp.gsfc.nasa.gov/stargaze/Sframes2.htm ... it even states early on there that "centrifugal force is not a "real" force, in the sense that in any motion calculated "in the frame of the universe" (or in one moving uniformly with respect to the universe) it does not appear at all. In such a frame, if an object moves around a circle, a centripetal force is needed to maintain that motion--otherwise it flies off at a tangent, with constant velocity along a straight line." But if you follow thru the discussion, I think you'll have to agree that there is more than one point of reference, and the roller coaster can be viewed from each. If we treat the motion in the coordinates of the accelerating frame--e.g. if your coordinates are those of the inside of the coke bottle --they must be included. Using the coordinates of an outer frame of reference, which does not accelerate or rotate--e.g. if your coordinates refer to position in the outside world--they do not exist. This can be quite confusing. Some textbooks avoid inertial forces altogether or call them "fictional forces". As we will see, however, they can be quite useful. One thing you should remember: be sure you know to which frame of reference your coordinates belong. With accelerated frames, all frames are not all equivalent. The best known example, of course, is a rotating frame of reference, where the centripetal force pushes inwards, and the centrifugal force pushes outwards. Which to use? The centripetal force is only used in a nonrotating frame of reference. It is the force holding the rotating mass to its curved path, and is directed towards the axis of rotation. The centrifugal force is only used in a rotating frame. It must be added to other forces in that frame, in order to take into account the frame's rotation, and is directed away from the axis of rotation. CFF
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Post by CFF on May 5, 2003 13:51:56 GMT -5
Further to whether or not centrifugal force exists or doesn't (is fictional), might I suggest browsing the following post over at AtlasBB (you don't need to be a member to view posts). Originally this post was about 'What is the wooden plank under an F1 car for?", but at about post #14, someone mentions that centrifugal force is fictional, and the argument begins.
See post here :http://www.atlasf1.com/bb/showthread.php?s=&threadid=51305
If I might C'n'P a rather lucid answer by one of the people over there (DOHC - Post #41), I think he clarifies the whole argument:
The best post so far on centripetal/fugal effects.
There's a lot of confusion on this issue. A lot of people claim that there is no "centrifugal force." Even universities teach that, so that students will avoid making certain well-known standard mistakes in mechanics. There's a number of traps and pitfalls. Everybody makes them.
But centrifugal force certainly exists. To turn the car, a force must be applied to it (from the ground, via the way you turn the front wheels); this force corresponds to the centripetal acceleration. The car moves in a curved trajectory, hence even at constant speed its motion is accelerated -- acceleration is change of velocity; velocity is speed + direction; direction changes; hence velocity changes even if speed is constant in a corner; because velocity changes there is acceleration: centripetal acceleration. According to Newton's second law, there is a force corresponding to that acceleration. For simplicity we might call it lateral force.
But according to Newton's law of action and reaction, the car exerts the same force, but in opposite direction, on the road. This is the centrifugal force. It exists in a relative, accelerated (hence non-inertial) coordinate system attached to the car. It is perceived as a force, and exists as a force, only in this moving coordinate system. (Ask a driver, or tell him that centrifugal force doesn't exist -- he will laugh at you, he spends an hour and a half being thrown around in the cockpit because of centrifugal force.)
The statement that centrifugal force doesn't exist comes from the view that motion can only be described in an inertial frame of reference, or in some "magical" absolute coordinates. That's nonsense. There is (for practical purposes) an inertial frame of reference called the track, but there is an equally valid reality in a moving frame of reference called the car. As seen from the track, there is no centrifugal force, only centripetal acceleration, but as seen from the car, there is only centrifugal force.
CFF
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Post by pabs on May 5, 2003 14:09:52 GMT -5
CFF,
Thanks for the clarification. I must concede that when I typed my post, I wasn't considering a moving frame of reference and that indeed if you center your coordinate system on the car, then the centrifugal force must be included as it is the reaction force to centripetal acceleration.
I guess I am a victim of that education system in which I always drew only the centripetal force in my free-body diagrams.
Again, thanks for setting me straight.
As a side note, that's one of the things I love about this forum. You can come here and talk about all sorts of things, and even be wrong without having to be crucified when you screw up...
Rick, can I borrow the whip once you're done with it?
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Post by Henrik on May 6, 2003 3:27:53 GMT -5
Another great thing about this forum is the things we can learn from the other posters!!! Man, I haven't studied physics since 12th grade when I took my physics International Baccalaureate exam. Having been fairly good in physics (considering that I have a very logical mind) I decided to take the exam a year early, so as to free up some time in the 13th year for other subjects. Well, as luck would have it, the examiner produced an exam that was way to difficult (university level), and was consequently sacked from the examiner's board. Unfortunately for us students, we had to either accept our grades or re-take the course/exam the following year. I decided to keep my 4/7 score and move on..... If only I would have had you guys around back then I'm sure I would have aced that exam!!!! Then again, some of you were not even born then...
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Post by OT on May 6, 2003 4:33:10 GMT -5
....yeah ....and then again ....some of us were getting our CHILDREN started in school!!!
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Post by El Sid on May 6, 2003 5:48:26 GMT -5
It's interesting to note that all of the bodies in our solar system 'rotate', including the sun. With the exception of Venus & Uranus, all the planets (and the sun) rotate counterclockwise when viewed from their northern pole. In the case of Venus & Uranus (retrograde rotations), it's hypothesized that collisons with a large asteroid or planetoid caused the axis of the planet to be changed, in Uranus's case to 90º normal, and in the case of Venus, 180º to normal (Venus rotates clockwise when viewed from it's north pole). CFF This is indeed an interesting thread and very indicative of the sort of mindset of the posters here. Shamu rulz! CFF, the reason for quoting you above is to bring a further thought into the picture. You may have noticed that I have a sort of fixation with this "Magnetism" thing in the "Perpetual Motion" thread. The Earth spins (BTW, as far as I know the moon does not and neither does it have a magnetic field) and has a magnetic field. I am sure that all the other planets in our solar system also have magnetic fields in the same sort of orientation/configuration as earth. Would it be far fetched to imagine that all of the heavenly bodies have these magnetic fields overlapping each other in varying degrees as a result of a dynamic cosmos. My question is this: Does the cosmos not generate energy in this way? (don't ask me what kind or what happens to it!) Just as a further side thought: I think the earth "wobbles" because it is not a balanced object. Things like tides, varying weather patterns and even previous collisions by asteroids surely must have an effect on how smoothly it turns. If thinking is the best way to travel then I'll have to renew my passport and update my visas one of these days.
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Post by daSilva on May 6, 2003 10:07:04 GMT -5
A MYSTERY OF EARTH'S WOBBLE SOLVED: IT'S THE OCEAN
The century-old mystery of Earth's "Chandler wobble" has been solved by a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The Chandler wobble, named for its 1891 discoverer, Seth Carlo Chandler, Jr., an American businessman turned astronomer, is one of several wobbling motions exhibited by Earth as it rotates on its axis, much as a top wobbles as it spins.
Scientists have been particularly intrigued by the Chandler wobble, since its cause has remained a mystery even though it has been under observation for over a century. Its period is only around 433 days, or just 1.2 years, meaning that it takes that amount of time to complete one wobble. The wobble amounts to about 20 feet at the North Pole. It has been calculated that the Chandler wobble would be damped down, or reduced to zero, in just 68 years, unless some force were constantly acting to reinvigorate it.
But what is that force, or excitation mechanism? Over the years, various hypotheses have been put forward, such as atmospheric phenomena, continental water storage (changes in snow cover, river runoff, lake levels, or reservoir capacities), interaction at the boundary of Earth's core and its surrounding mantle, and earthquakes.
Writing in the August 1 issue of Geophysical Research Letters, Richard Gross, a JPL geophysicist, reports that the principal cause of the Chandler wobble is fluctuating pressure on the bottom of the ocean, caused by temperature and salinity changes and wind-driven changes in the circulation of the oceans. He determined this by applying numerical models of the oceans, which have only recently become available through the work of other researchers, to data on the Chandler wobble obtained during the years 1985-1995. Gross calculated that two-thirds of the Chandler wobble is caused by ocean-bottom pressure changes and the remaining one-third by fluctuations in atmospheric pressure. He says that the effect of atmospheric winds and ocean currents on the wobble was minor.
Gross credits the wide distribution of the data that underlay his calculations to the creation in 1988 of the International Earth Rotation Service, which is based in Paris, France. Through its various bureaus, he writes, the service enables the kind of interdisciplinary research that led to his solution of the Chandler wobble mystery. Gross's research was supported by NASA's Office of Earth Science, Washington, D.C.
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