Humber, if you are serious about trying to resolve the ongoing differences of opinions, then I think there really has to be some "back-to-basics" discussion (from both sides). I say this because it seems pretty clear that often a lot of people don't understand what you are trying to say, and perhaps the opposite is true also. You may already understand everything I'm about to say, but in the interests of trying to establish and confirm some level of common understanding let me now spell out how I see it.
My understanding of a frame of reference is NOT (in the strictest sense) a physical object but rather an imaginary but well defined coordinate system (including a clock). In that "frame" we use those coordinates and that clock to determine position, velocity, and so on. An inertial frame of reference is a frame of reference that isn't accelerating (and that includes not rotating).
Good, I can get over the obstacle of the hanging post. Thanks for the effort and patience, by the way. Sometimes its easier to make short posts on the spot, but yours demanded more attention.
Yes, I agree. Some may equivocate, but for the purposes of Newtonian objects, that definition is certainly good enough.
In other words, an object that is not subject to a net force is seen to have a constant velocity in an inertial frame of reference. When an inertial frame is used with Newtonian mechanics (which I believe most or all of us agree is essentially all that is required to discuss the cart going downwind to a reasonable degree of accuracy), we are using a single frame of reference at a time and using that for all the measurements of position and time and other derived values such as velocity and acceleration.
Yes!
We can optionally also use other frames of reference but if we do that we can't always directly compare values from one frame to another in any generally sensible way, and we therefore need be careful to specify which frame is being used if there is any chance of confusion. Finally, with Newtonian mechanics as our basis, all clocks are assumed to run at the same rate regardless of the frame of reference being used.
Yes, again. Indeed, taking a new frame is to take another perspective. All links to the other objects, considered or not, remain in place. Force, energy friction, remain the same. There is symmetry between frames. Time is invariant in our case.
So, using this general approach, we choose a suitable (hopefully also convenient) frame of reference, make the appropriate measurements and then use Newton's laws and so on (via mathematical calculations) to predict what will happen in certain circumstances and so on.
Yes, a common practice of convenience. I have said that equivalency is just a reiteration of Newton's laws. Rather, it confirms that Newtons laws do not change with perspective, or from where you take the measurements.
Yes, strictly speaking there are possibly no true inertial frames of reference, but once again we can hopefully agree that things like the curvature of the earths surface, it's rotation and so on, are not likely to be significant issues when it comes to predicting the cart's performance.
Where do we disagree? I have said that, too. The effects are small, or common to all objects under consideration.
To say that the ground is a reference is considered to heresy, but of course equivalency allows that.
It does not mean that is is "absolute" (though that is not so easily dismissed for acceleration), but a datum. If I have a GPS, I can set my home co-ordinates to (0,0), and take relative measurements from there without insisting that my house is the center of the Universe, or that everyone else must do so.
We all understand that in the real world things are often more complicated than in some simplified model that we may choose to use. This, plus other similar factors, all combine to mean that the predicted results from the formulas are almost certainly not an *exact* representation of "reality". But if reasonable assumptions are made to start with, then we expect the discrepancies to be relatively small, and so the predictions to still be "useful".
Yes. These are limits of the model or measurement. The modeler rarely has the freedom to reproduce everything, so simplification removes the superfluous, or leaves an approximation that is deemed to be useful, even if not 100% accurate.
Given all this, we can choose the origin of our frame of reference to be moving at the same speed and in the same direction as the top surface of the treadmill belt.
Yes, but now we are about to disagree.
In that case it is quite easy to verify that we will measure the various speeds and so on as being identical to what we would measure if the cart was in fact rolling along the ground at windspeed (in a "real wind") and the frame of reference used was instead at rest relative to the ground (and was also aligned in the appropriate way). In this way those two frames of reference produce exactly the same measurements and therefore generate the same predictions.
No. The treadmill "works" only for one specific case; windspeed. Even then, it is wrong. It fails quite obviously for all intermediate values.
Treadmill wind, is not like real wind. I have an idea how I can show that.
We can also choose our frame of reference to be at rest relative to the body of the treadmill. Note the treadmill itself is not actually *the* frame of reference.
That is consistent with your views, and mine. I can accept that the treadmill observer is traveling at windspeed. It can be notional or literal. However, the results must be in accordance with my ground side, (or any other) view. That is, I no longer take the windspeed view, but that of someone on the ground. The treadmill is not consistent in this respect.
When someone refers to the treadmill as a frame of reference (or similar) to me that's a kind of shorthand for a notional coordinate system that is at rest when compared to the body of the treadmill and aligned in some sensible and agreed fashion.
I say its a model, but I can accept that interpretation too. It is not actually important to my criticism of the treadmill. I think that it is impossible to create a frame of reference for something that is moving wrt the ground, freeze it at standstill, yet claim it is the same. The dynamic properties of the object are missing, but let's say that is an acceptable simplification.
The real question is whether treadmill wind is like a real wind. No it is not.
It is a conclusion that
must be made to support the assumed position that widspeed travel is possible. There is
definitely a difference, it is significant, and falsifies the claim.
This will in fact produce the same values for velocity and so on as a frame of reference that is moving at the same speed as the "real wind" for a case where the cart is rolling over the ground instead of on a treadmill.
Same velocities perhaps, but the work is wrong, at the very least.
Try an object that moves at 10% windspeed, then the same at 20% and so forth. Something odd happens.
In any of these (close enough to "inertial") frames of reference , we can then do the various calculations of velocity and kinetic energy and momentum, etc., etc. and we "know" that the results of that will be correct, at least in the same sense that we believe "Newtonian mechanics" to be correct.
If the model or frame is accurate enough, yes.
Now, my training in physics is not much beyond high school level, so perhaps I've not been 100% clear or accurate in what I've said above, but hopefully I'm at least "close", and I'll invite others to make appropriate corrections if they believe I've made important errors or omissions.
More than enough, Clive.
Questions:
1. What exactly do you see as being wrong or invalid with this approach? Where does the "nonsense" creep in?
I must go right now. I was working on a reply to your previous post yesterday when on the train. (Avoiding looking out of the window, of course)
I still have the same problem. If I work through it, I reach the same conclusion as all of the other similar experiments, just more convoluted. The models that you cite are also not the problem. The "quibbles" are not about details, or permissible but imaginary devices. I appreciate the effort that you expended to make the details clear, but I still see that it is not like the treadmill, or if I take another view, like it, but similarly flawed. If I answer it as I intended, I see that we would indeed be "back to basics".
I have a thought experiment that I hope will convince you. Later today.
2. If your approach is different (and it certainly seems to be) please spell it out as clearly and concisely as you can, treating me as a rank beginner (which is all I really am) so that there is minimal chance of misunderstandings.
No, they are the same. There is nothing complex. They ideas are that every rational scientist or engineer will tell you, and yours seem quite solid to me. That is why there
is no academic support for the treadmill.
(Please ignore any other posts from parasitic namesakes)