Four? For me that would be a rounding error. 
Username checks out.
Seems like a good idea to me. It would only be suitable for fairly smooth surfaces, but be as good as a skateboard. It’ll have less mass than those giant monowheel boards. Of course it needs a self-balancing mechanism to be useful.
Even better, instead of a normal wheel, have one of those onmidirectional ones. That way you don’t need any left-right balance.
No Penny Farthing. What a disappointment.
But that would just lead to disagreement on wheel diameter. Which is the determining size for the sake of the graph - the large wheel? The small wheel? An average of both? Note that there was no old-style tractor there, either, despite the fact that tractors have very large rear wheels.
And he has an 18 wheeler at the 16 wheel mark.
Ridiculous. I’m taking my mono caster and going home. See you at the hospital!
Just a board with a hemispherical dent and a rigid sphere. Like the old fashioned computer mouses that had an internal ball rolling over the mousepad surface.
Which raises a very interesting question with probably a very unhappy answer:
- What does happen when a comet collides with the Moon?
I think that might make a pretty good xkcd What-if question. Or a decent SF novel.
Probably not. Comets are too light and lacking in mass to do much against a moon-sized object.
Now a wandering asteroid of decent size…
There are a couple of different hazards. One is the idea that maybe it smacks hard enough to knock a big hunk off the Moon. So mechanical damage. I agree with you that doesn’t seem too likely.
A different hazard is that 100% of the relative kinetic energy of that collision is going somewhere. That might be enough radiant energy to do some damage here on Earth.
I just looked at wiki and Halley’s comet masses something around 2E14 kg. WIki also says (edited)
Unlike the overwhelming majority of objects in the Solar System, Halley’s orbit is retrograde. … That retrograde orbit grants Halley one of the highest velocities relative to the Earth of any object in the Solar System. The 1910 passage was at a relative velocity of 70.56 km/s
Wolfram Alpha sez that’s ~5E23 Joules of kinetic energy. I would not want to be standing near that energy release.
I’d thought of that but wasn’t sure if it qualified as a “wheel”. The tech is easy; this is from 14 years ago, and advances in batteries, motors, computing, and inertial measurement would make it trivial at a smaller scale these days:
Continuing this musing …
The “yield” of the Chicxulub impactor is estimated (wiki again) at ~3E23 J. So Halley hitting the Moon would be an energy release akin to one or maybe two Chicxulubs.
The “kiloton of TNT equivalent” used as a convenient unit of measure for nukes is ~4E9 J. So we’re looking at ~1E14 KT of yield. Or ~1E6 Tsar Bombas detonating at their full design power of 100MT. Or ~2E6 Tsar Bombas as actually tested at ~58MT yield.
Not an Earth-Shattering Kablooie, but still quite a sight to behold.
The Moon masses ~1% of Earth. The more I think about it, 1 or 2 Chicxulubs hitting something 1% as massive as Earth might well spall off some pretty big pieces on the far side from the impact. Plus dig a very impressive crater at the point of impact. And at an impact speed of 7E4 m/s, even a loosy-goosy slushball is going to behave pretty much like a rigid body.
I’d imagine the amount of moondust kicked up would not be trivial and might impact the earth’s insolation for a year or 6 … that seems to be the most immediate effect
Astronomical collisions are typically at speeds that make the material strength of both the impactor and the target irrelevant. A startling fact is that the kinetic energy of an object is equal to its weight in TNT at a relative velocity of only three kilometers per second.
Or possibly make some beautiful rings.
We’d only get a tiny fraction of that dust, though.
Say goodbye to the moon dinosaurs
While not explicit about what does it to the moon, check out Neal Stephenson’s Seveneves. The answer is, it’s not good - in a very XKCD way.