What exactly is centrifugal force

sophiecentaur

Haha
I suppose I have to take your point because my suggestion is not falsifiable. But I think that the inverse, - i.e. that Science definitely can establish 'real truth'- probably is falsifiable. So far, we have found this as our experience has been that Science, and its models, continuously changes to fit new evidence.
It has to be true that Science endeavors to avoid saying what things 'really are' because there are so many examples of two or more, equally valid 'realities'. (Note, I write "Science" and not 'Scientists' - who are human and fallible and seldom view things without the distraction of some sort of faith).

Andrew Mason

I have to strongly disagree. It is not reactive centrifugal force that will cause the section to move farther away from the centre. It is the fictitious centrifugal force that would cause that (ie. it is inertia - the absence of centripetal force). The reactive centrifugal force disappears immediately as soon as the bolts are cut. This is exactly why the term "reactive centrifugal force" should not be used. It gets confused with the fictitious centrifugal force.

AM

sophiecentaur

But the other astronaut (sitting in the frame of the wheel) will see the departing astronaut accelerating, initially (during the first 90 degrees of motion, at least) and due to the geometry of the situation. Would he not conclude that there is a force still operating? This perceived force will also be making the departed astronaut perform a spiral outward path - so it would (might) not just be a centrifugal force that he would need in order to explain the guy's path.

stevendaryl

Of course, it doesn't matter what you call them, but the point is that Newton's laws relate motion of one object to vector quantities produced by other objects:

[itex]m \frac{\stackrel{\rightarrow}{dU}}{dt} = \stackrel{\rightarrow}{F}[/itex]

The left-hand side is a fact about the motion of the object, and the right-hand side is about the external situation affecting that motion. In terms of coordinates:

[itex](\frac{\stackrel{\rightarrow}{dU}}{dt})^i = \frac{dU^i}{dt} +[/itex] sum over [itex]j, k[/itex] of [itex] \Gamma^i_{jk} U^j U^k[/itex]

where [itex]\Gamma^i_{jk}[/itex] are the so-called "connection coefficients", which are due to using nonconstant basis vectors. So the full equations of motion, in terms of components, are:

[itex]m(\frac{dU^i}{dt} +[/itex] sum over [itex]j, k[/itex] of [itex] \Gamma^i_{jk} U^j U^k) = F^i[/itex]

What the idea of "fictitious forces" amounts to is moving the extra terms from the left side (where they describe motion) to the right side (where they are treated as forces):

[itex]m \frac{dU^i}{dt} = F^i + F_{inertial}^i[/itex]

where
[itex]F_{inertial}^i = - m[/itex] sum over [itex]j, k[/itex] of [itex] \Gamma^i_{jk} U^j U^k[/itex]

What difference does it make whether you group it on the left side, or the right side? Well, for one thing, when it comes to figuring out the reaction forces (Newton's third law), only the [itex]F^i[/itex] term is relevant. There are no reaction forces to [itex]F_{inertial}^i[/itex]. For another, since real forces are vectors, the components transform in a standard way under a coordinate change: If you change coordinates from [itex]x^i[/itex] to [itex]y^b[/itex], then

[itex]F^b = [/itex] sum over [itex]i[/itex] of [itex]\dfrac{\partial y^b}{\partial x^i} F^i[/itex]

"Inertial forces" DON'T transform that way.

So sure, you can group whatever terms together you want, and call them whatever you want to call them, but when it comes to reasoning about the physics, you have to separate out the "real" forces from the "inertial" forces. You're basically doing extra steps that have to be undone later.

stevendaryl

I tend not to talk about truth, one way or the other. I prefer to discuss the ideas without worrying too much about whether they are truth, or something in our heads, or what.

dextercioby

"Inertial forces" as you (stevendaryl in post #78) mention are a step forward from Newton's postulates (which always have the <with respect to an inertial reference frame> text in them) to Einstein's General Relativity, parallel in a way to the step in which you replace Newton's postulates to Einstein's ones in Special Relativity.

sophiecentaur

Then I guess you are not far from being a Real Scientist.

DaleSpam

I don't think it is a philosophical claim, I think it is a semantic claim. I.e. "physics" is defined as X, Y is not X, therefore Y is not "physics".

stevendaryl

Okay, but who gets to define what the word means? It seems to me that the meaning is provided by watching what physicists actually do, rather than how they would answer question "What is physics?"

DaleSpam

No, the reactive force is an equal and opposite reaction to the other force in a 3rd law pair. This is Newton's 3rd law, which seems to confuse people in rotating frames for some reason. It has nothing to do with acceleration of a specific object since the acceleration of the interacting objects can be different and the acceleration depends on the net force rather than the individual forces.

Sorry, I guess I didn't my proposed scenario clearly. I specified that the bolts were "suddenly" cut for a very important reason. As the astronaut is standing on the floor the floor is under stress with centripetal forces from the bolts and a centrifugal reaction force from the astronaut. The centripetal force is greater than the centrifugal reaction force so there is a net acceleration towards the center.

When the bolts are suddenly cut the stress is relieved from the outside of the section of floor, but the inner part of the floor (where the astronaut is standing) is still under stress. This sets up a shear wave where the floor material transitions from stress to stress-free. During the time between when the bolts are suddenly cut and when that shear wave reaches the feet of the astronaut the centrifugal reaction force still exists, the feet and floor are still in contact, and the floor is accelerating in a direction away from the center. It may help to think of the floor as being made of a stretchy rubber material.

The centrifugal force is every bit as "centrifugal" as the centripetal force is "centripetal". The centrifugal force points away from the center, the centripetal points towards the center. If either is unbalanced then it will result in acceleration in the corresponding direction. If there are other forces involved then the actual acceleration depends on the net force, per Newton's 2nd law.

I intended to discuss the reactive centrifugal force only from the perspective of the inertial frame in order to avoid any possible mix-up with the fictitious (inertial) centrifugal force in the rotating frame.

DaleSpam

Hi Andrew Mason, obviously my description was poor since rcgldr had exactly the same response. Please see my response to him in the post above.

DaleSpam

Physicists, in particular, the subset of physicists who write physics textbooks. (Maybe we can include Webster and other dictionary writers too)

That would be true if physicists did nothing besides physics. However, since physicists do other things besides physics, the definition must come from the answer to the question you posed. That way you can distinguishing between when they are doing physics and when they are doing things that are not physics.

stevendaryl

Granted. Here's an analogy: sports. Athletes are people who play sports. Of course, an athlete does things besides play sports, but I don't think that an athlete is any better at defining what a "sport" is than anyone else. They can describe what they do when they play sports. I don't think that physicists have any more insight into what "physics" is than an athlete does about what "sports" are.

Anyway, when Einstein, or Newton, or Schrodinger, or just about any other physicist was engaged in doing physics, it certainly wasn't coming up with formulas that make predictions. They were engaged in the struggle to understand the world. That activity is a big part, I would say the center, of what I consider to be physics.

A.T.

So it doesn't matter if you call certain terms "forces" or not. After all it doesn't change the quantitative result of the calculations. That is my point.

stevendaryl

My point was that it doesn't make any difference what you call things, but that for reasoning, it gets in the way to lump things together that are different sorts of objects.

A.T.

I don't see how you can confuse the two. One is an interaction force that exists in every frame, the other is an inertial force that exist only in rotating frames. The diferences are listed in the table here:
http://en.wikipedia.org/wiki/Reactiv...trifugal_force

Calling it "centripetal" as you suggest, despite the fact that it points away from the center, that would be confusing.

sophiecentaur

That assumes everyone has your view of things. I very much doubt that they were as naive as to think they were actually near a conclusion. You cannot have any knowledge of their motives but you must know that anyone who breaks ground in any of this (since the concept of God given laws has ceased to be taken for granted, at least) can only hope to improve on existing scientific models. Models are not 'truth'; they are statements that can be shown to predict the outcomes of certain experiments.
Your three example Scientists were as fallible and human as the next man in many respects and may well have believed at times that the truth is there but they would have been only too aware that it was their models that were the test of their achievements