I’m a sceptic and I suggested it. Shame that this has become a “them and us’ debate rather than a persuit of the truth.
ynot, congratulations on your brilliant merry-go-round design. I'm jealous I didn't think of it. It's weird, because my immediate reaction was the opposite, that putting it on a circular path (relatively speaking - getting one's head round the frames of reference stuff when the ground is spinning is more fun -
So now you've got the ground spinning on its axis...!
) tethering it 'sideways' to the centre, the wheels having to scrub or their axes not being perpendicular to the chassis so as to be radial (in fact, even then there's some scrubbing as the real wheel doesn't have a point contact), etc., etc., would just cause more problems and give more reasons for people to dismiss successful demonstrations. But with some of those nice mods, it's berrrrilliant, and might just manage to squeeze itself into that odd position between realistic-but-error-prone (like real world tests, where it's almost impossible to prove that you proceeded accurately in the DDW direction all the time) and equivalent-but-counterintuitive (like treadmills),
and solve the "How long would you like steady state to be maintained?" question at the same time.
Regarding the 'them and us' thing, it's a good reminder, thanks. I've been very guilty of my rational and reasonable scepticism turning into insult, almost without noticing. We all get a bit carried away. I agree completely with your point, and we should all take it seriously. I get it right some times. Above, describing misinterpreted successful tests, it was tempting to use the shorthand of 'naysayers' or 'sceptics', when 'people' was more correct. When someone like spork is a 'believer', that can be taken as reasonable (he's tested it and confirmed the maths from a knowledgable position), but it can also sound like an insult, especially at JREF. 'Sceptical' is also something to be proud of, but spork used it a while back to mean an unreasonably dismissive stance. My first reaction was that it was probably one of those practical jokes resulting from a drunken bet, to see how long the guys could keep up a ridiculous claim, and I consider it scepticism that caused me to keep looking again for clues to support or refute that hypothesis. Scepticism can blow in any direction. I've gone from 'looks very suspicious' to 'looks almost certain to be valid' now, from a combination of having some of the mechanical principles explained and the usual clues we get from how the conversations go.
I think at times my questions or 'objections' to the explanations have been taken as 'naysaying', when they are just part of my sceptical process, from the unfortunate position of relative ignorance on the physics.
JB and I are the furthest thing from "just believe it" types you're likely to ever find. I believed it long before building it. The simple vector analysis I presented was plenty to close the case on this for me.
Unfortunately (because I think you're more sensible than that), these sentences don't express the caution I think you meant them to express. If you were not a just believe it type, you might not have believed it before you built your cart and demonstrated it (to some views - obviously that depends on how confident you are in your mathematics), and "a simple vector analysis" closing the case for you could also be taken as deciding after sketching a partial mathematical model. This did put me off a bit when you said it last time. I'm thinking that the real world might have a lot of complications, and a 'simple analysis' of vectors or whatever else, might not be sufficient. OTOH, it is irrelevant after doing the tests, as long as our analysis of the results is reasonable as well. On which point...
I think JB came around based on any number of approaches as well before we finally built it to convince others. And finally, there's nothing like having to hold the thing back by hand to keep it from running off the front of the treadmill to make you believe.
I'm sure that is absolutely true, and I'm 99% sure it is a correct interpretation, but even so, it is possible to be wrong in situations where apparent proof is staring us in the face. Standing outside and not getting spun into space convinces some that the world isn't spinning on its axis.
But I think the fact that no one has managed to prove it's wrong - combined with the fact that we've proved it's right (through math, relevant engineering analogies, and physical experiments) should be reasonably convincing.
That's it, for me. It's this combination, and in particular the maths, I have to say, which I trust others to do for me most of the time (though I may spend a bit more time trying to catch up on mechanics, as I have found this thread so inspiring - thank you).
You could be right. I'm not going to bother with him any more.
I was close to that several times, and have speed-read much of what Humber has posted. I have made some of the same errors he makes, but I seem to learn quicker. However... no, I'll save that till the end...
What you could use is a simple explaination to help non-Humbers understand it. All that stuff about ice-boats just made me more confused at first. (And still does, a little.)
Here's an explaination that might help people understand it quicker (or would have helped me understand it much quicker).
When the cart is moving at wind-speed...
1. As the cart moves along the ground this movement turns the wheels, producing a backwards force on the cart.
2. The turning of the wheels turns the propeller.
3. The spinning of the propeller pushes back on the air, producing a forwards force on the cart.
4. Since the air is moving in the same direction as the cart, the propeller can push backwards on the air while pushing at a slower speed than the ground is pushing back on the wheels.
5. Gearing exchanges speed for force. (Gearing down produces more force but less speed for the same amount of energy.)
6. By gearing down the speed of the propeller, the propeller is able to push back on the air with a greater force than the ground is pushing back on the wheels.
7. If the cart is pushing back on the air with greater force than the ground is pushing back on the wheels, the cart will accelerate until these forces are balanced.
That's a great pulling together of the bits I'm still ploughing through in the thread. Thanks very much. Of course, I don't dare say it's correct, and will have to wait for verification by cleverer heads than mine. But it does something else, which is demonstrate that non-mathematical types can often get perfectly reasonable explanations of complex issues in English. We might not be able to prove them, and have to rely on our faith in mathematicians, but then we can usually get our heads round most things in an
approximate way - even Relativity or Quantum Mechanics. Of course, when we forget it's approximate, we're in danger of going woo-side.
ETA: Brian, I just wanted to point out that I'm not insinuating that you're one of the non-mathematical types. In fact, that's all relative anyway. I'm lumping myself in there, but clearly I'm not as bad at this as I could be. It's just I'm dumb on aero and have forgotten most of my physics and maths. I did used to love them. I remember a brainteaser from an applied maths lesson of about 30 years ago - I wonder if this is on your list spork - about what you have to do to turn left on a bicycle, assuming you're going straight and perfectly balanced. Most people refuse to believe the answer. Another thread.
Now, the bit of Humber I'm interested in is this: although I absolutely and utterly accept the physical reality of the equivalence of intertial frames of reference (I
think I understand that, but may be wrong), there do seem nevertheless to be actual physical differences in this case between the treadmill test and a reasonable reading of the claim. This isn't anything to do with "but there's no wind blowing", but is more akin to considering the limits within which the relevant conditions are equivalent. For instance, to translate correctly the treadmill situation back to real life, would there not be, a few feet beyond the cart, on each side, and some distance ahead and behind it, an indefinitely large piece of ground moving backwards at windspeed? Now, this may be irrelevant or it may not, but it seems to my mind to be true.
I am hardly qualified, but will risk comparing this with another objection of Humber's - whether there are real mechanical differences concerning the aerodynamic properties of the cart, that show up when it is on the road and on the cart (which I think he said were about the wind whistling past). I am confident that this is - ignoring those spatial limits I'm on about - not valid, since the treadmill is modelling the near-windspeed conditions, and the same near-windspeed conditions apply when the real cart is 'travelling' (as unenlightened observers in the carpark will consider to be an absolute description). Clearly, this is why, in either case, a passenger won't be "sucked into the propeller" if he steps out of the cab (I hope, for my sake as well as his).
Another 'real' difference might be that the 'carpark' would have to be folded on itself and travelling backwards at twice windspeed, if I've got my head round it properly, a great river of tarmac and grass, a few meters below. There may be others. My gut instinct is that they're probably of no true consequence (mainly because clever people tell me that math stacks up anyway in ideal situations).
The circular track spinning backwards might be considered to have similar differences. The true equivalent would be JB (having installed brakes) riding round the edge of a big circular mesa (probably undercut), pulling a radial beam round a central pole at the same time), in a perfectly radially symetrical cyclone, I think. (Please don't try this at home.)
Maybe CERN can help.
I'm not sure whether I want to find a video test of Humber's variation, where the prop is turned upwards, or some other means is used to demonstrate that all that is happening is that the energy of the belt is being dissipated, and hence the model ends up staying 'still', thanks to Newton (if I've misunderstood the argument, Humber, pardon me)...or not. Really. Oh, go on then.
Oh, but I was thinking it would only be a minute's job, but maybe you'd have to rig up a different kind of cart. Down to the hardware store
again!
Newton is trying to spin in his grave, but just.... can't... seem to... get ... uuurrhhhgggghghhg...
