The centrifual force is not a pure force, it's a psuedo force felt do to an object being in circular motion. It is related to the inertia of a body. The orbit will only be stable if the tangential velocity squared, times the mass of the body divided by the radius of its orbit, is equal to the radial force due to gravity acting in the direction of the centre of gravity of the body this object is rotating around, namely mv^2/r=GMm/(r^2) There is no such thing as a centrifugal force, as an observer outside this frame of reference will see that it is the rotating body inertia which alters its motion. Same goes for the coriolis force.
James, It seems that you switched gravity and inerta around. It used to be that an object would continue to move at a constant speed and in a straight line unless acted upon by an outside force. Now you claim that an object will only move at a constant speed and in a straight line if acted upon equal external forces, and that gravity is the result of unequal external forces. Either, way you still have to have a force present. You just replaced the force of gravity on an object with the forces of empty space on that object. I don't see how this curved spacetime model is any simpler than the classical model for gravity. Tom
A body in free fall does not "feel" gravity. Gravitational force is manifest through constaints. One particular constraint is that we are constrained to the surface of the earth. If we wish to be no longer constrained to the surface of the earth, we must supply a force greater than that of gravity, in the direction opposite to it.
Equivelence ryans, ANS: So you disagree with Einstein and his equivelence principle. Seems like a lot of hollow talk and words about "Constraints" etc., and no useful physical underpinnings. What causes these constraints that moves bodies around without using force?
I am not disagreeing with einstein, but agreeing with him. And Mac do you regard the centrifugal force as a pure force which is manifested through the exchange of what particles or fields. I'd like to see you to come up with field equations for the centrifugal force. For those of you who are interested, the centrifugal force is simply due to an observer being in a frame of reference which has angular momentum. An observer who is not on this spinning object, will derive that the accelearation of the observer towards the periphery of the spinning object is simply due to the observer being in a non-inertial frame of reference. You can derive this force by considering newtons equations in spherical polar co-ordinates. Coincidently, this also leads to the 2 observers measuring different values of Pi, c.f. the relativistic merry-go round.Please Register or Log in to view the hidden image!
Re: Equivelence Sorry to answer for you ryan... correct me if I'm wrongPlease Register or Log in to view the hidden image!. Originally posted by MacM ANS: So you disagree with Einstein and his equivelence principle. No. Nothing ryan said contradicted the principle. There are various ways of looking at the system which will make the results easier to observe depending on what you are looking for. I'm not 100% sure on this, but the equation listed most likely is from a simplified force balance which treated gravity as a force (because the result is the same).
Equivelence Persol, His intentions are not my concern. My concern is he states CF is not gravity. Einstein says they are equivelent. CF is a function of F=ma or inertial mass subjected to a force of acceleration. A feature that GR you claim isn't involved.
Statement Persol, ANS:The above is your absolute statement. The folloiwng is from the link chroot provided for my to "learn" Relativity. ANS: Perhaps I should provide you the link. I am at lunch and don't have tiime to find a direct statement from Einstein but I will look later.
Re: Statement Originally posted by MacM These forces are even today sometimes called fictitious forces, but according to Mach's principle they must be gravitational forces. In this sense the centrifugal force, for example, is not any more or less fictitious than gravity. Exactly... gravity is technically not a force either... but is represented as one (or as acceleration) for simplicity. I don't have the link, so I'm not sure how he goes about this... but if you keep reading it will probably go into Mach's fixed stars and that this is an 'appearance of force'. Or it may use the other teaching route and explain circular motion as having a dynamic frame of reference.
Actual Statement Persol, I deliberately didn't post the entire paragraph since the issue was the "Equivelence Principle". They do state that both CF and Gravity are viewed as "Ficticous Forces". I won't argue over that view but I don't agree with it. I know it is the GR view but seems to me pretty silly since weight is a force and it is the result of gravity. The point here was that you want to break that equivelence principle and GR says you shouldn't. I find it also a bit confusing when people keep making different claims. In "Relavistic Pi" ryans argued (rightfully) that the radius would undergo contraction due to GR gravity due to acceleration. He was wrong about Pi changing since both the radius and ruler would undergo the same affects of GR. But now James R., says there is no acceleration and you say that observers inside a rotating frame could tell they were rotating. Let me suggest that is not correct. Obsevers inside a rotating frame would not know they were rotating (unless they had windows. They would feel gravity only. This assume they are in a small cabin and not free to roam and notice the "Zero" g's and reversal of forces toward the ceiling on the other side.
Re: Actual Statement Originally posted by MacM I deliberately didn't post the entire paragraph since the issue was the "Equivelence Principle". They do state that both CF and Gravity are viewed as "Ficticous Forces". Then please don't take the sentence out of context. The part with cf and gravity being the same is more complicated then I (and probably the book) make it out to be. It basically has to due with the cause of inertia possibly being gravity. This would then relate cf to gravity because gravity->inertia->cf. It is said that while Einstein knew of this theory, he did not use it. I won't argue over that view but I don't agree with it. I know it is the GR view but seems to me pretty silly since weight is a force and it is the result of gravity. You can consider gravity a force, but it is a different way of loking at the system which doesn't answer all the questions. The point here was that you want to break that equivelence principle and GR says you shouldn't. There are different ways of examining different situations (and the same situation). It doesn't really matter how, as long as it is consitent and gives the right answer. Regardless, the equivalence principle does not have to do with centrifigual force as the equivalence principle does not take rotating reference frames into account. He was wrong about Pi changing since both the radius and ruler would undergo the same affects of GR. Sorry, I'm not aware of the exact circumstances here... but it is possible for pi to change. But now James R., says there is no acceleration and you say that observers inside a rotating frame could tell they were rotating. One is a rotating frame, one is not. They are different situations. Let me suggest that is not correct. Obsevers inside a rotating frame would not know they were rotating unless they had windows. They would feel gravity only. Observers in a rotating frame CAN know they are rotating. All they need to do is drop a ball. Now observers in constant and straight acceleration will not know. I explained this earlier using my guy in a box who releases an object. In gravity it continues to orbit. In forced rotation it flies off on a tangent.
Equivelence Persol, The following extract is from one of several link I have to Relativity. *************** Extract ********************** Principle of Equivalence Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field. ************************************************** ** ? Do you interprete the term "uniformly" to exclude rotary motion? I don't. You can have uniform rotation producing uniform acceleration. If your position is the correct one then I think these sites should state "Linear uniform" acceleration. The above does not differentiate between linear or angular acceleration. Are you saying ryans was incorrect stating that GR causes contraction of the radius? And finally my arguement was that neither SR at the rim nor GR along the radius could result in a change in Pi since a ruler in motion is subject to the same affects. If the radius contracts then so does the ruler. No measurement change hence no change in Pi. ******************** Extract ********************* One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass. One of the implications of the principle of equivalence is that since photons have momentum and therefore must be attributed an inertial mass, they must also have a gravitational mass. Thus photons should be deflected by gravity. They should also be impeded in their escape from a gravity field, leading to the gravitational red shift and the concept of a black hole. It also leads to gravitational lens effects. Index ************************************************** * ANS: I brought this forward in that I noticed they claim an inertial mass and gravitational mass for a photon. Is not "Inertial Mass" and "Rest Mass" one in the same? This runs counter to everything I have seen about photons.
Re: Equivelence Originally posted by MacM Do you interprete the term "uniformly" to exclude rotaary motion? I don't. You can have uniform rotation producing uniform acceleration. I disagree. Acceleration is a vector quantity. In 'rotary motion' the vector is changing direction. Are you saying ryans was incorrect stating that GR causes contraction of the radius? As I said... I don't know the conditions/ And finally my arguement was that neither SR at the rim nor GR along the radius could result in a change in Pi since a ruler in motion is subject to the same affects. If the radius contracts then so does the ruler. No measurement change hence no change in Pi. pi is determined along 2 axis. If only 1 axis on 'contracts' then pi changes . Is not "Inertial Mass" and "Rest Mass" on in the same? 'rest mass' is now just called 'mass'. 'inertial mass' refers to relatevistic mass 'mass' is a constant. 'relativistic mass' depends on velocity This runs counter to everything I have seen about photons. It shouldn't. I'm sure you knew that photons were influenced by gravity... as per lensing/blackholes.
Terminology Persol, ANS: I had never seen this switch. It used to be that F = ma was a function of inertia or inertial mass. ANS: Yes but the arguement was curved space and I've seen arguements agains photon mass being the cause of the bending. Running counter was considering photon to have rest mass, which was the way I was readding this based on my antiquated terminology.
Tom: <i>It seems that you switched gravity and inerta around. It used to be that an object would continue to move at a constant speed and in a straight line unless acted upon by an outside force.</i> No, I haven't switched anything. Objects in curved spacetime move at constant speeds in a straight line (i.e. on geodesics) unless acted on by an outside force. <i>Now you claim that an object will only move at a constant speed and in a straight line if acted upon equal external forces, and that gravity is the result of unequal external forces.</i> Please don't put words in my mouth. If you don't understand something, ask. <i>Either, way you still have to have a force present. You just replaced the force of gravity on an object with the forces of empty space on that object.</i> Wrong. <i>I don't see how this curved spacetime model is any simpler than the classical model for gravity.</i> It's not. It's more complex by far. But it is also much more accurate.
Re: Equivelence MacM: <i>They do state that both CF and Gravity are viewed as "Ficticous Forces". I won't argue over that view but I don't agree with it. I know it is the GR view but seems to me pretty silly since weight is a force and it is the result of gravity.</i> What you are saying here is: "It seems pretty silly to me because I believe in Newtonian physics." Well, experiments show us that Newton was wrong and Einstein is right, MacM. The universe doesn't care what you think is silly. GR is the way it works. <i>In "Relavistic Pi" ryans argued (rightfully) that the radius would undergo contraction due to GR gravity due to acceleration.</i> Yes, and he was right, although it could have explained it more clearly. <i>Let me suggest that is not correct. Obsevers inside a rotating frame would not know they were rotating (unless they had windows.</i> Have you ever been on a ferris wheel, or a merry-go-round, or in a car going around a corner. Can you tell you're rotating without looking out the window? I think you can. <i>You can have uniform rotation producing uniform acceleration.</i> No. The acceleration in a rotating frame is continuously changing direction. It is not uniform for that reason. <i>If your position is the correct one then I think these sites should state "Linear uniform" acceleration.</i> "Linear" is what is meant by "uniform" in this context. <i>And finally my arguement was that neither SR at the rim nor GR along the radius could result in a change in Pi since a ruler in motion is subject to the same affects. If the radius contracts then so does the ruler. No measurement change hence no change in Pi.</i> That's what you've been repeating over and over again, without bothering to look elsewhere for actual facts. You are wrong, MacM. I'm not going to argue with you on this point because frankly I don't think you'll understand the explanation. If ryans wants to take it further it is up to him.
No I do not wish to take it further. Go to the thread and have a look at my arguements, they still hold. If I take it further, I will be simply hitting the same brick wall known as Mac's "intuition"
Basis James R., ANS: Neither do I care to pursue the issue further. I do however want to make one thing clear. You accuse me of blindly repeating the same thing over and over without any evidence. The fact is that happens because replies here have repeated the same thing over and over without any evidence. That is to say when it is all said and done on this topic, none of you have addressed the root question. There is and was no question regarding SR, GR, MR , RR or JR views. The question was and is (left unanswered) to explain how any affect of any theory causing contraction at the rim, at the radius and/or both could cause a change in calculated Pi based on measurement by a ruler on the moving frame. That is explain how such a process changes either the rotating platform and not the ruler or changes the ruler and not the platform, such that there becomes a measurement change. If the affect changes the platform it must alsochange the ruler and there can be no measureable change, hence no change in Pi. The response to this question is to state simply "You just don't understand" but appartently nobody here can answer this simple question over a simple process without appealing to authority and saying because Relativity says so. The fact is this issue goes beyond Relativity, that is has nothing to do with Relativity. It has to due with if the m-g-r is constructed of rulers, it cannot be denied that both the m-g-r and the measuring ruler change equally and there can be no measureable change hence no change in Pi. Don't bother responding if you don't have a direct answer. Thank you.
That's right Mac, when the entire world is shouting "you just don't get it, do you? God, it's so simple!" it must be that the entire world is wrong. - Warren
