Yet you've said several times that wheel motion is very tricky. So again, this demonstrates your conceit. Wheels, like every other subject we touch on, is much more complicated and subtle than anyone else could possibly understand, but you don't need to check anything out with anyone else, do empirical testing, or think further on the subject. I guess you must have been born knowing everything.
Need I remind you, that I raised the subject, and now there are "experts" appearing from wormholes.
That is what I said, the frictional force from the road is from the rear. You are ambiguous there, however. There is the other force, that from the wheel, which is from the front. First year physics,...
Should you do first year. No matter, but
what a bromide, John.
....I'm fairly confident, will explain this: a wheel drives by "pushing" backwards against the ground, basically, and the friction of the ground "pushes" back. Now, while "pushing" might give you confidence that the wheel's contact position must therefore be rearwards of the axle (from the principle that we can't push a rope), it is wise to be cautious about that. Friction, as we all know, has some kind of stickiness about it, and we know that we can pull things with an adhesive surface, no push required. We can sit on a wheeled trolley on a flat surface, put out our hands and pull ourselves forward with just the friction of our hands on the surface, well in front of our bulk and centrre of mass.
Double-think. If you want to push a car forwards, where do you stand?
Ahh.. no enough for the wily fox, so where are your feet? Correct.
The force is between the car and your feet, which are to the rear of the intended direction of travel. The force is between the contact patch and the axle. You can keep this up, John, or consult a physics or engineering book. The intertubes do not seem to cover this topic well.
Ok. Good. Something we agree on. I would say that a completely solid, non-deformable wheel is an ideal, a mathematical abstraction, but that there is a gradation from soft objects through to very hard ones.
Such a wheel would slip 100%. Slide, but not rotate.
We haven't considered the surface, except in your suggestion of soft sand, but I didn't understand what you were trying to say. The mathematical ideal is very informative, actually. If a wheel and the surface did not deform at all, the contact point would be one of those odd things in maths, a kind of zero that isn't zero, an infintessimally small location, a point (or line if we give it the sideways dimension). Whether such a wheel could be driven is almost into the realms of philosophy, depending on whether you arbitrarily decide that forces can be transmitted (particularly frictional forces, laterally!) via an "area with one dimension infintessimally small or zero", a "contact line with no width", or however we conceive it. What is clear is that as we approach the "ideal" (in the sense of "in the mind") the usefulness for locomotion becomes less than "ideal". As a rough rule of thumb, I'd say that being deformed at the contact surface is part of how wheels work; without being somewhat elastic they would just spin; with a point contact in the limit, no friction can exist.
Yes, deformation is necessary. "one bit must move faster than another bit" is the way I spoon-fed it it to Spork.
The deformation is simply the equivalent of a toothed gear, though some care is definitely needed with that analogy. Some continuous drive gear boxes use smooth transmission wheels in direct contact, but use a special oil to do much of the 'deformation'. Wheels
cannot work without friction.
Perhaps? Is that your conceit talking?
I'll ask The Dalai Lama.
If there was a "driving plough", I'm not sure what it is. Let me assume it is a compressed area, since you say it is further back, and imply, I think, that its presence pushes the wheel forward. But you say that it is the bit of tyre stuck to the road further back.
No, the deformation is forward of the axle, a visible effect in soft tyres or ground. The friction is behind the axle. The wheel rotates the tyre, so it distorts against the friction. Like a pushed string. Part of the tyre must be in tension to maintain the load.
If we take friction literally as a stickiness, then the contact patch behind the wheel would slow it, as the rubber was peeled off the road. It takes a lot of energy to unstick sticky things.
A car may loose 60% of its energy by that means, though typically it is but perhaps 2%. It depends on the wight of the vehicle, speed, road conditions.
Tyres are useful, even if not the most efficient, because the can adapt to conditions, and cushion etc.
But more importantly, if there is a compression of material behind the wheel axle position, I don't see how it can be sustained: that part of the tyre is being pulled away from the road.
Yes, the tyre is continuously flexed. They get hot. At 60kph, the same part of the tyre meets the road at perhaps 16 times a second.
Well, that's all rather confusing. The "frictional force", if by that we mean what the road applies to the tyre, will be with the direction of motion, not against it. But we might (I would say should) consider two opposite forces when something acts in this way, as per Newton's 3rd: the tyre applies a frictional force to the ground against the direction of motion. The bottom of a wheel doesn't exactly go backwards (or may slighly perhaps), but it has to be pushing backwards for the wheel to move forwards, just as a fish must push water backwards to move forwards or your feet must push backwards on the ground when you walk forwards (the usual way). These two opposing forces cause relative motion between wheel and ground.
Friction is complex and dynamic. It is no "supplied by the road" but a reaction to the applied force. But, yes, like walking. What is confusing?
I think that's right, although you have of course reversed the condition from driven wheels to driven surface, and specified a cart on a belt, where I thought we were just discussing wheels, but no matter.
No that is the point. They are not "reversible". The implied symmetry of the treadmill is false. Spork just says the usual garbage in response. The treadmill is based upon ideas of equivalence, that may be or are true, but they are not applied in the treadmill. That is a design disaster. A dropped brick.
You're obsessed, man! First understand wheels, then get back to the cart on a treadmill.
I think you need to do yout home work, before that point.
That's perhaps why the ladies say, 'Yes, it was good, John'.
Well it does a very good impression of a wheel clawing its way up a curb.
And a rear wheel drive with it's front wheels at the curb? A caterpillar track?
George Jetson has a treadmill.
I don't understand any of that, but what wheel is circular?
It does not have to be. Check it out. However, if undistorted, the part of the wheel that could provide a leading and effective contact patch, would not first hit the ground until directly below the axle. Next stop is behind the axle.
Well it's time to get your rubber pencil eraser out again. Holding it tight in your hand will simulate a tyre on a wheel with the brakes on full. That would be unwise for a proper emergency stop, but it's the extreme case to start with. Rub it along your desk, and you'll find that its contact area is pulled to the rear of the motion. A wheel does the same thing and, with some rotation of the wheel instead of brakes locked, that force is still in the same direction, causes the same kind of distortion of the tyre, but reduces it. Even if we skid, the same is true.
Is the cart on the treadmill braking? If you can't make your case for drive, why should I even consider your case for braking? That is also a quite appalling analogy, and the equivalent of the "scrubbbing" I described, in anticipation of your likely response. Seems you missed the barn door.
Your vision is almost of a tyre that has to jump ahead of the wheel to provide a push force against the wheel...or you just realise that you have to say that, because we can reverse these motions and prove the first case wrong by it, which was why I asked the question, as you will already realise.
The car cannot get to where is is going, and pull itself from there. Caterpillar tracks are capable of this within the bounds of the track, but not ahead of it.
You would by changing frames of reference, of course, see that braking and accelerating are (almost?/absolutely?) identical-but-opposite conditions:
Braking is dissipative. So many mixed and wrong ideas in one sentence.
"Frames of..."
the momentum of a car requires that motion can't just disappear, ...
Wow. Not like the cart on the treadmill?
...the relative motion of ground and tyre can be thought of as the ground trying to apply a torque to the wheel, backwards at the bottom, but resisted by the wheel, hence the tyre will bunch up behind the axle position just as your eraser does on the desk. How could it jump - or the road suck it to a position - further forward of where the axle has got to?
The force is from the rear, becuase the wheel drives the car by thrust. The rest is flummery to avoid that fact, which is a simple and patently obvious mechanical denial of the treadmill. That is (perhaps) why 3bodyproblem remarks that "frames of reference" is a marketed idea.
