HN Theater @HNTheaterMonth

That doesn’t seem to be a charitable reading and in any case it has to do with wheel taper, more like cone than cylinder. More specifically the nature of the solution was getting two parts of the org, track and train, to work together. While that may seem elementary stuff, anyone at a big org can attest to how often it is tremendously difficult to do sometimes.

I write this not as an apologist for commuter rail, which I find excessively expensive and inflexible outside of very circumstances, but as an excuse to link to the lovely Feynman description of why train wheels are tapered:

https://m.youtube.com/watch?v=y7h4OtFDnYE

⬐ gooseyman

I used to joke when the Subway wheels squealed “yeah that’s how the engineers designed that”

Turns out, it was.

Mandatory Feynman explanation:

https://m.youtube.com/watch?v=y7h4OtFDnYE

Indeed. Feynman talked in ‘surely you’re joking..’ about his affection for MIT students in his fraternity setting one another puzzles, and he talked about the question of how trains go round corners being one he found particularly interesting. In fact he repeats it in this interview: https://www.youtube.com/watch?v=y7h4OtFDnYE

He doesn’t answer the question ‘how does a train stay on the track’ with ‘some trains don’t’; he doesn’t switch to talking about rollercoaster trains and insist that the way they stay on the track is with horizontal wheels. He finds the logical, geometrically satisfying answer quite delightful.

And in fact, in many cases the flanges which he says are just for safety and aren’t supposed to impact the track because otherwise they make a terrible sound… are expected to hit the track to help the train make it round particularly tight turns. Coming from New York and having ridden the subways you’d think Feynman would know there are some corners subway trains take where the flanges rub all the way round, and the lovely conical wheels aren’t what keeps the train on the track at all.

So I definitely find this a weird characterization of a Feynman-ish approach to this kind of problem. I think he’d probably have delighted in the geometric neatness of why circular or triangular manhole covers can’t fall down their holes, and been happy to ignore the practical fact of square ones for the sake of a good puzzle story.

> doesn't a speed change imply a force on the bearing .... imply friction

Not necessarily. A speed change could also be because the track is longer. That's how trains stay on the tracks, the wheels are slanted, and if they turn off the track the length of the wheel changes, which changes the speed (relative to the wheel on the other side) and steers it back onto the track.

Feynman explains it: https://www.youtube.com/watch?v=y7h4OtFDnYE

⬐ function_seven

> and if they turn off the track the length of the wheel changes

But is doesn't actually change. A different part of the wheel comes into contact with the rail, but that different part always had a higher linear speed. It just wasn't in contact with the rail until the turn.

But with these bearings, it appears that the balls themselves actually do speed up or slow down to maintain separation from one another. In that case, there must be a force coming from somewhere to effect those changes.

additionally, here is Richard Feynman explaining it: https://www.youtube.com/watch?v=y7h4OtFDnYE&index=7&list=PL0...

⬐ awalGarg

Love to hear Richard Feynman explain anything.

⬐ JohannesH

Right? I wish he was alive today so he could do a ton of YouTube videos explaining simple counterintuitive concepts like this.

⬐ RoboTeddy

Here's him explaining how computers work (first 35 minutes): https://www.youtube.com/watch?v=EKWGGDXe5MA

⬐ fmela

That whole interview is so great.

⬐ cmsmith

A great video, and I noticed the actual linked article here suspiciously follows the exact same structure and examples that Feynman does in his explanation. It would have been nice to acknowledge the fact that the author was probably watching Feynman explain it while writing. But it seems to be a student fun journal, and has made a few hundred people think, so why get upset?

⬐ None

None

⬐ hackuser

Maybe both Feynman and the author copied the explanation from a third source.

Feynman explains it well [1].

[1]: https://www.youtube.com/watch?v=y7h4OtFDnYE

If Feynman were still around, he'd set things straight. I don't think a bicycle staying upright is much different from a train keeping course [1], but I can't describe why. Instead of the rail veering off and the wheel adapting, it's like the bike does what it wants and the ground shifts beneath it.

If you've read about Einstein's pail-of-milk-on-a-lazy-susan, it's a similarly unintuitive frame of reference. Also, if the experts haven't figured it out: I don't know what I'm talking about.

I find it fascinating that we as a species can observe, define, and exploit Maxwell's law, neutrino physics, etc, but we can't clearly explain bicycles. Or why wings provide lift.

EDIT: forgot the Feynman video

[1] http://www.youtube.com/watch?v=y7h4OtFDnYE

Reminds me of 'How does a train stay on the tracks?' http://www.youtube.com/watch?v=y7h4OtFDnYE

⬐ pitiburi

beautiful.

⬐ arketyp

Wow, neat. I'm not sure I should attribute all my amazement to Feynman, but it sure is lovely hearing that man explain things.

⬐ Luc

So now all I need to do is figure out what determines the angle of the taper on the wheels...

⬐ pbhj

The difference between the circumferences would have to be such that the train could round the sharpest bend on the track.

That should approximate to a maximum angular change (in direction of travel) per metre:

angle per metre = tan( 2 PI (R-r) / inner track gauge ) x 2 PI r

where R is the largest contact circumference of the wheel and r the smallest.

?

⬐ TrevorJ

What I don't get is how the train rounds a variety of curve radii. How do they control the part of the wheel that is riding the rail precisely enough to allow the train to turn at differing angles depending on the radius of the curve?

⬐ TrevorJ

What I don't get is how the train rounds a variety of curve radii. How do they control the part of the wheel that is riding the rail precisely enough to allow the train to turn at differing angles depending on the radius of the curve?

⬐ TrevorJ

Hmm, sorry for the double post, not sure what happened there.

⬐ jhayes

This clip (and the others posted by 'ChristopherJSykes') are from the BBC show "Fun to Imagine". Been a few months, but I remember the 'Magnets (and 'Why?' questions...)' segment being particularly worthwhile if you're looking for good bang-for-buck timewise.

Incidentally, the guy releasing this also directed it.

Also in case you prefer text: http://varatek.com/scott/feynman_problems.html