Strange that a hand crank would have the power to amputate someone's limb. Especially since the gear reduction doesn't seem to be all that high, there's just not that much friction in the system to overcome.

Looks very cool and I want one because of that, but it seems like a massively inefficient design for something where efficiency is pretty important. Why the steel frame? Why not aluminum or one of those fancy bike alloys if aluminum isn't strong enough? Why the big wheels and fat tires with all their extra weight and increased rolling friction just for the sake of aesthetics? The minibike layout does seem pretty good for an electronic bike though.

I lived in Turkey for four years, and after a while I noticed that Turks didn't respond well to my smiling. They didn't understand it as a friendly signal, and it actually caused friction. I never asked about it, but I somehow caught on that they thought I was stupid. I thought about it, and it made perfect sense to me that it was stupid to smile at things that shouldn't make you happy.

So I stopped smiling in stores and restaurants, with coworkers, and even with Turkish friends. My interactions with people improved noticeably.

After four years, I moved back to Canada, where I continued not smiling for no reason. I've never been able to get back into the habit. I just feel stupid and unnatural smiling for no reason. People smile at me just because they see me, and they smile politely. I can't smile back. I just raise my eyebrows.

People now tell me constantly that I'm too sad, that I should smile more that I'm not happy. Now, there's some truth to that -- I do suffer from depression -- but that predates living in Turkey and it's only since then that anyone's accused me of being sad, or even noticed that I don't smile as much as I should. I've had to train my friends out of referring to me as grumpy.

My job is teaching English as a Second Language to students from all over the world. My Western students -- particularly the Latinos -- tell me daily (literally) that I don't smile enough. My East Asian and Eastern European students have never said a word in that direction. I just realized the divide now after watching this video.

I guess the hint for these is the rotational test that they show at the first.

1) A sticky object that would let go like a wall crawler that climbs down a wall would create this effect. (see below)
2) You can't. As you approach infinite speed it would get very close. (see below)
3) The bike will move forward. (see below)
4) The outside parts of the wheels that overlap the rail. Also if the train has a flywheel that is larger than the wheel size the bottom of the flywheel would also always move backwards faster than the train was moving.

1) He says "what object is inside?" so I'm not sure a liquid would count. Also a viscous liquid would flow a slow rate and would probably not stop and start. You might be able to get a viscous liquid to stop and start if you had fins, but that still might just move slowly or gain enough momentum to roll fast without any flow.

2) A little excel calculation shows that the average velocity approaches twice the initial but will never hit it.

attempted m/s - total time - average m/s
1 100 1
2 50 1.333333333
3 33.33333333 1.5
...
200 0.5 1.990049751
201 0.497512438 1.99009901

3) I'm not sure if the parameters of this experiment are explained sufficiently.

If it is allowed to slip then no matter the mechanical advantage a hard pull should always be able to get the bike to skid back and defeat friction.

If the bike is not allowed to slip on the ground then I don't understand how it could ever move backwards, the only options would be that it doesn't move at all or it moves forward.

If it can't slip then the ratio of the pedal to the wheel is what is in question. Bikes only have gear ratios higher than 1 and the crank is smaller than the tire so the tire will always rotate more than the crank thus the bike should move forward.

For 4, could it be the coupling rod? I guess alternating sides.

And for the bike one, you've got to remember the wheels are able to spin freely in that position, so he could drag it backwards as far as he wants. It's only the force of your body pressing down on the bike that creates enough friction that the gears drive it forward.

Where's the solutions goddammit.

My guesses:

1) a pendulum.
2) 3x faster (this one seems too simple)
3) in this case forwards, but depends on the weight of the bike, and friction between the ground and the tyres?
4) Q2 and 3 of every wheel.

In a world where the world ends at the end of your block
And them uh, little whirlwinds spin friction round the clock
I'll be savage, hunt and gather the average rather cadaver
Cock back hammer, splatter matter all over cell phone and calendar
No peace, yeah, in the middle of a war zone, riddle when the norm thinks slow
No sleep, yeah, with the shades down low, on the down low fighting with a dream
I creep, yeah, all secrets please, I don't need to speak
I'm paranoid like a man in the land of the free
To set up and let us burn and turn cheek, damn
Stop with the octagon, top your block, I'm gone
Off my rock, no songs, no more locked, yeah
Just a little bit a prison for everyone of us
We won't listen till there isn't any more of us
These days we quick to part ways with rights like "okay"
Here, let's be clear, for the record I did not sign up for lockdown
Or any kind of shock and I'm so bored

You must have forgot just who you were dealing with
Nothing less than aggression so naked, so crystal clear with a
Trust in absolutely fucking nothing but Doomtree
Step up your thought game lame, we're all thirsty!

It's like they leave us no option
Walking these streets, heat is watching
These preachers speak from their pockets
These teachers...bring it back c'mon
These teachers reach but can't stop it
Seedlings so poisoned, so lost and
Follow these prophets to nonsense
Tossing what's right to the dust
And I ain't no casualty
Got no surface with spotless morality
My dirt might have to cover up my grave
But I keep my fear of faith
And filth clutter up my cave
Got me looking for disinfectant
(I don't know how to behave)
God, I'm bored!

You must have forgot just who you were dealing with
Nothing less than aggression so naked, so crystal clear with a
Trust in absolutely fucking nothing but Doomtree
Step up your thought game lames, we're all thirsty!

Mooooooooore!...
So thirsty for mooooooooore!...

Since I quite enjoyed the talk I'm willing to overlook that fact He did also have some good examples of actual cold welding.

NASA has an interesting lessons learned article about the Galileo high gain antenna failure, which also seems to be more nuanced than "it was cold welding" - http://llis.nasa.gov/lesson/492

p.s. I got curious about the reference to Gemini, and I'm not 100% sure but I think it might come from a 1991 paper "On-Orbit Coldwelding Fact or Friction?" by Dursch, H. & Spear, S. (Bibliographic Code: 1991NASCP3134.1565D) or else it's from the paper it references as ref 5 (I. Stambler "Surface Effects in Space", Space/Aeronautics, Vol 45 No. 2, 1966 pp. 63-67).

That paper gives the opposite impression to the start of Derek's talk, rather than cold welding being discovered around the time of Gemini, it was often thought to be a problem around that time but as he says later was subsequently found to be quite rare (Dursch and Spear found no actual cases of cold welding causing spacecraft issues, they were usually friction issues due to fretting or galling caused by loss of lubricants, but still recommended taking precautions to avoid coldwelding).

Love me some sand dunes. I've heard low droning sounds while out in Guadalupe sand dunes in central california and in Death Valley. I remember researching why and there were some theories - something about grain size (changes pitch), friction & amplification from a layer of moisture below the surface and sand collision upon the surface creating vibrations that in turn create a feedback loop of low frequency. Stony stuff, lol!

Does anyone know what the mechanism is behind that high-pitched, start-up whine? I love that sound but realized I don't actually know what causes it. It didn't look like it had enough RPM to be friction.

I wonder if you can use the weight at the back of the car to make an aerofoil shape and reduce downward force -> tyre friction.

Good question. I think that the entire device is unstable no matter what, its impossible to keep it straight no matter where the point of gravity as long as it needs to balance on a single tip. So zero speed would mean tip over in all cases unless you make a more stable tip (square) which would mean it cant spin very well which means you havent made a spinning top.

You can find more about the physics of the spinning top here:
http://hyperphysics.phy-astr.gsu.edu/hbase/top.html
and here:
http://physics.stackexchange.com/questions/271/why-dont-spinning-tops-fall-over

So what is the optimal distance of the center of gravity to the tip?

There are several things working at the same time. Most importantly is the energy. You need to store as much energy as possible so that the top can spin for as long as possible. When the top slows spinning the friction at the tip becomes larger (the precession becomes bigger) so it starts to lose more energy and slows the spinning even more. You store energy by adding weight with a center of mass that is further away from the tip. When the top then "falls" the center of gravity moves down and reduces potential energy. Due to energy conservation kinetic energy goes up meaning speed of precession or of spinning goes up and creates a force pushing the top back up.

Off course, more mass means more friction at the tip, so there is for sure an optimal here, most likely depending on mass, size and shape of spinning top, etc.

Last but not least, more rotation speed I assume also means more friction, so its a trade-off.

If you move the center of mass down below the tip, well, if you move it as far off as you would above, the energy you can save is about the same, but the entire thing would be harder to build and you would need to make sure the sides fit around the ground plateau. Also, when the precessions become bigger the sides will hit the plateau, meaning game over.

In the end you are better of keeping the center of gravity above the tip point.

I was incorrect, apparently when you group them together, they become far more sensitive to ignition.

"Sparkler bombs are constructed by binding together as many as 300 sparklers with tape, leaving one extended to use as a fuse. In 2008 three deaths were attributed to the devices, which can be ignited accidentally by heat or friction. Because they usually contain more than 50 milligrams of the same explosive powder found in firecrackers, they are illegal under U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) regulations."

I'm curious how, in an automated system like this, they deal with clogs or residue. I presume there's very little, or such a system wouldn't work, but I'm curious how a system relies on friction for heating while not causing buildup.

I absolutely love this man. His videos have become some of my most looked-forward-to sources of general information.

Did I sense some... friction between those two?