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2 things that come into contact and do not skid on each another, like where the rubber hits the road in normal operation, must have the same velocity, even just instantaneously.
The infinite part- Bob Bridges, you already brought up that any movement can be said in a fashion to be infinitely faster than zero.
Now the zero velocity at the bottom of the tire- it can be seen maybe more palatably throug the analogy of a walking human being: the sole of the foot is momentarily stationary on the ground while the rest of the body moves on.
Yep, that was my guess too. But I'm just a little uncomfortable with the term "infinite" being used as a comparison, division by zero notwithstanding. Why not phrase the riddle as: What part of a moving vehicle is always at rest? Granted, the "part" (bottom of tire) in contact with the road is changing continuously and being replaced by another "part" of the tire, but can still be logically described as a "part" of the vehicle, i.e., the part of the tire in contact with the road.
Weirder yet, on a train, where the point of contact with the rail is the center of the wheel, the flanges on either side of that contact point (that hold the train on the rail) are actually moving backward relative to the rail/earth when they are at the bottom of a rotation cycle.
For an excellent video of what I just described, see the physics classic Frames of Reference:
If you don't want the whole physics lesson, just jump to the 8:53 mark. This video was produced in 1960, but I used it every year in my physics class. Even with the grainy B/W, it's still one of the best collection of demos out there.
RobertB said:
Risking the patience of an English forum here, but thanks for a wonderful video.
Might well risk some more: the top of the tire is twice as fast as the car.
Not at all! Mathematics is as much a language as English, imho. It has words, syntax, humor, etc. And this whole thread is about "translating" math into English unambiguously. But the physics might be a bit of a stretch. You are correct that the top of the tire has twice the speed of the car. On a train, >2V depending on the actual diameter of the flange.
Glad you enjoyed that video. I hadn't seen it in close to 10 years, and had to watch it all the way through. Just goes to show you what two creative teachers can do when backed by a university-level machine shop and production crew.
Martha Barnette
Grant Barrett
Grant Barrett
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