## Ireland

I will be in Ireland this Saturday, at SkyCon at the University of Limerick. I’m guessing they found me through my LimerickDB project, which may not be what they think it is. Since they rejected my original idea for a talk, which was based on John Cage’s 4’33” I will be talking about doing internet comics for a living, and related subjects such as frozen pizzas and MarioKart.

My apologies to everyone who’s extended a speaking/convention invitation who I didn’t get back to — the answer is usually ‘no, thank you’, but I’m too meek to say so (which is probably part of why my email piles up so badly). I love traveling and talking to people all day, but it’s so exhausting that I only do a couple of them a year.

## A Math Problem

Courtesy LispClub.com:

Sue and Bob take turns rolling a 6-sided die. Once either person rolls a 6, the game is over. Sue rolls first. If she doesn’t roll a 6, Bob rolls the die; if he doesn’t roll a 6, Sue rolls again. They continue taking turns until one of them rolls a 6.

Bob rolls a 6 before Sue.

What is the probability Bob rolled the 6 on his second turn?

I love puzzles which are simple to state but have a fiendishly tricky or counterintuitive answer. I just threw up a page on the xkcd IRC wiki to hold some of the better ones I’ve found over the years. I’ll be adding more over the next few weeks as I remember or find good ones. Feel free to add some of your own!

Edit: Buttons and then Daniel Barkalow got the correct answer first.  Here it is, rot13‘d.  Check your answer against this before posting smugly or people (I) will tease you:  gjb friragl svir bire gjryir avargl fvk, be nobhg gjragl-bar cbvag bar creprag.

## A Laptop Problem

I need help finding a new laptop.

For the last couple years, I’ve have a Fujitsu P1610 (which replaced my stolen P1510). It should last me for another year or two, but I’m pretty hard on my computers. This one has started developing problems typical of an overused, aging machine — your standard hardware failures, screen damage, random crashes, unexpected hangings, paradoxical package syndrome, retrograde memory leakage, Hamitic keyboard, fan tearing, Indian screen burn-in, dysfunctional Haskell, LCD torsion, tab key thrombosis, trackpad sclerosis, dry RAM syndrome, scroll-wheel arousal, systemic rainbow failure, peristeronic disk fragmentation, hyperactive tilde key, heat-sink apricity, kernel duplicity, space bar plasticity, and cross-site chlamydia.

So I know that in the next year or two, I’ll be in the market for a new laptop. Here’s my problem: as far as I can tell, nobody makes the laptop I want. I really only have two criteria:

1. Weight under 2.5 lbs
2. Screen at least 1280 pixels wide

This isn’t an impossible combination. My current laptop fits it. The Fujitsu Q2010 fit it. The Fujitsu U820 fits it but is actually too small and light (the size of a CD case, with a 270 dpi screen!). The problem doesn’t seem to be technology. And, given the recent popularity of netbooks, it’s probably not lack of demand for small laptops, either. Everyone just seems to assume that if you want small, you also want a cheap and crappy display. Is there nothing between the netbooks and the Lenovo X series (which are a bit bulkier than I want)? Other than buying a laptop that is several years old (hunting down a Q2010) or has a touch screen that I don’t really want (the newer model of my current one, the P1630), do I have any options?

(Side note: I find it odd that there are so few laptops with high-DPI screens (>175)? Everything above 150 on that Wikipedia list is either a Fujitsu or a smartphone. Aren’t we supposed to be moving out of the era of pixels by now?)

## Sledding

I went sledding in Danehy Park in Cambridge recently, with my brother and some friends (including Mike, one of the other Boston-area people with a ball pit).  The snow was packed, icy, and awfully slick, and we were wondering just how fast the sleds were going at the bottom.

When you slide down something with no friction, your speed doesn’t depend on the path you take — just on how far you fall.  There are a number of simple equations derived from F=ma that are handy to memorize.  One of them gives the speed of an object after it’s fallen height “h” in Earth’s gravity:

In this equation, “h” is in meters and the answer is in m/s.  It’s actually 2*a*h, where a is the acceleration of gravity.  We round 9.8 m/s^2 up to 10.  (Other handy ones are that the time to fall that distance is sqrt(h/5) and the maximum range of a thrown projectile (45-degree angle) is v^2/10.)

This formula tells you that if your car nosedives off a 50-meter bridge (about double eastern US treetop height) you’ll be going about 30 m/s (interstate speed) when you hit the ground, making the crash the equivalent of hitting a concrete wall at highway speed.  It also tells you that if a (purely gravity-based) roller coaster’s highest cumulative drop, top to bottom, is 35 m (a typical large coaster), it can’t go faster than 26 m/s (which is roughly the old speed limit of US interstates).

I eyeballed the height of the hill to be about 11 meters, since I was about eye-level with the top windows of nearby three-story houses (Google Earth later verified this). So, the theoretical maximum sledding speed in Danehy park is sqrt(20*11), or about 15 m/s. In practice, because of friction, it will be lower (interesting note: the ratio of the vertical to horizontal distance the sled travels is roughly the coefficient of friction of the sled on the snow.)

Checking with our handy table, we see that 15 m/s is faster than the fastest sprinter, about the speed of a cat or rabbit (but — critically — slightly slower than a raptor), and not near highway speed.  We got the GPS from the car and did a few runs with it, recording the maximum speed each time.  It was a pretty reliable 10 m/s (11 if we pushed), which is a lot faster than running speed for everyone except Usain Bolt.

So, in every state except Wyoming, North Dakota, South Dakota, and Maine, there is a building high enough that Marty McFly could have taken the innards of his DeLorean up a freight elevator and acheived the required 88 mph (40 m/s) by jumping off the roof.  (Because of air resistance, I wouldn’t try it in NH/MT/ID/WV/AK/VT, either.) He just needs to leave a note at the bottom explaining that in 30 years they should set out a trampoline.

Edit: Sorry for the brief downtime today.  Also, to everyone posting that the GPS will only give horizontal speed and will underestimate speed on the slope: the sled reaches top speed near the bottom.  It only starts to decellerate when the grade of the slope is less than the sled’s coefficient of friction (plus a bit for air resistance), which seems to be less than 0.10 (people have trouble eyeballing slope grades, which are almost always shallower than you’d guess).  Since sin(x) ~= x for small x, this correction (1/cos(grade)) comes out to at most a percent or two in reality.  However, if the fastest part of the slope looked like the one in the second drawing, it would indeed be a big correction.

## Some Lists

After a hectic few months, I finally have some projects that are in the early stages.  But since nothing is worth posting about yet, here instead is a list of phrases that (at the time of this posting) turn up no hits on Google:

• “ate a violin”
• “driver-side bidet”
• “unlike normal furries,”
• “Sarah, plain and tall and a cyborg”
• “people are too civil on the internet”
• “his penis shattered my world”
• “more like LAME-arkian theory”
• “my little horse must think it gay”
• “it turned out her bottom half was a robot”
• “Aww, a baby hooker!”

Here are some phrases that I had hoped were original when I typed them in but was disappointed:

• “full-body glissando”
• “passenger-side bidet”
• “underwater Linux”
• “Nu-Polka”
• “erotic colonoscopy”
• “Spocktoberfest”
• “Frankly, my dear, I don’t give a frack.”
• “my bologna has a first name, it’s A-D-O-L-F”

And lastly, here’s a fun bash export I stumbled on:

• `export PS1='C:\${PWD////\}>'`

## Debate Drinking Game, New Yorker, Burlington Vermont

I am currently at the tail end of the Debate Drinking Game, wherein you take a drink every time McCain says “my friends” and Obama starts a response with “Look,”.   Since both candidates had their verbal tics under control, I just took a drink every time someone said a pronoun.  I am not in a good state.  Fortunately Google’s sobriety test has not yet extended to the WordPress default implementation.   Why would you ask for the arctan of pi/2?  Maybe it will make sense when I’m sober.

I did an interview with the New Yorker Cartoon Blog.  It’s available here.

I will be in Burlington, Vermont tomorrow, to try kite photography.  Thanks to the excellent (albeit poorly-organized) CHDK project, I’ve picked up a cheap Canon Powershot A720 and modified it to do time-lapse photography.  This makes kite photography much simpler than it’s been in the past.

If there are any readers in the Burlington area who are interested in kite flying and know of a good location, email me at xkcd@xkcd.com.  I will send out an email containing plans to anyone who contacts me, sometime soon after noon.  If you know someone in Burlington (e.g. at UVm,) let them know!  Thanks.

Wow. It seems someone at YouTUBE took this comic seriously and decided to add an “Audio Preview” feature. Now you can hear your comments read aloud to you.

Of course, it’s an optional button using speech synth rather than a mandatory dramatic reading, so it’ll just be used for entertainment by people who haven’t played with a speech synthesizer before:

But then, after a little more browsing around, I see this:

So maybe there’s hope after all.

## Federal Reserve Skateboard: A Short Story

(Written after sitting in a car for five hours listening to financial news stories.)

——-

Damn these subprime lenders, thought Fed Chairman Ben Bernanke, barely keeping his balance on the wobbling skateboard. We can’t afford more debt. He snapped a grappling-hook-tipped quarrel into his crossbow as the skateboard slowed. When the country owes trillions and is asking for more, its shadowy creditors start calling in favors.

The crossbow twanged, carrying his climbing rope up the side of the Federal Reserve building. As he began his ascent, he reflected on the years past. I inherited a broken system, he insisted to himself. We’re simply doing what’s required to prevent a catastrophe. It’s not my fault.

He tossed his skateboard over the parapet and hauled himself over. He dropped six feet to the roof, landed heavily on the board, and trundled on into the night.

——-

From her perch in a tree across the street, the blogger watched through her blogoscope as Bernanke disappeared over the wall. She spoke quietly into her radio: “Subject is in the haybarn. The chickens are in danger of roosting.”

“Roger that,” came the reply. “Deploying Agent Harpsichord.”

——-

Inside, Bernanke moved along the wall like a shadow, elongating and contracting as the light sources shifted around him. In the midst of a sea of filing cabinets, he froze. He sniffed the air, then dropped to his knees, licked the floor, and paused. Yes, he thought, Greenspan was definitely here.

——-

The blogger had waited five minutes and was starting to get impatient. She picked up the radio. “Situation imminent. Pass the ducklings through the snorkel. Repeat: Pass the ducklings through the snorkel.”

“We are go for mode Sinatra,” replied the commander. “Reticulate core and set throttle to ‘cryptic’. Prepare to jitterbug.”

——-

Bernanke forced the door on yet another inner office, realizing too late that the light was on inside. The chair in the corner swiveled around, and Bernanke found himself face-to-face with Alan Greenspan. There was silence for a moment.

“You won’t get away with this,” said Greenspan, rising to his feet. “The Fed is subject to general congressional oversight. But you never understood that, did you?”

“Congress sold out the country, not me,” replied Bernanke. “Don’t shoot the messenger.”

“I wasn’t planning to,” said Greenspan. He flicked open a switchblade.

——-

The blogger peered once more into the eyepiece of her blogoscope. She threw the switch labeled “overlay building schematics.” The external view of the building disappeared, but instead of blueprints, she was presented with a green puzzle piece. “This view requires the Adobe Flash Player plug-in. Do you want to search for this plug-in now?”

Shit, she thought.

——-

Bernanke, trying not to slip in the patches of blood on the floor, struggled with Greenspan. The older man moved like a snake that moved like a former Fed Chairman who moved like a ninja. At last, Bernanke got a solid grip on Greenspan’s collar and hurled him through the fourth wall, knocking you to the ground.

Improvising a tourniquet from the remains of the snake left over from the earlier simile, Bernanke moved on through the hallways.

——-

The moonlight-bathed roof of the Federal Reserve building fell suddenly into shadow. A pair of night watchman looked up in alarm to see what had occluded the sky.

“Is that …” one whispered to the other, “… is that a blimp?”

——-

Bernanke reached the central vaults, accessed the Gibson mainframe, and began transmitting the requested files to his distant masters. He didn’t hear the gentle thud on the rooftop, the muffled explosive charges, or the sound of the door opening behind him. But at the last minute some sixth sense kicked in. He spun around just in time to see a golf-ball-sized lump of gold rapidly expanding in his vision. It struck him in the forehead, and he collapsed to the ground like a burlap sack full of scrapple.

Congressman Ron Paul retrieved the gold nugget from the floor and returned it to his satchel. “Try that,” he said, donning his sunglasses, “with a fiat currency.” He spun on his heel, cape swirling behind him, and swept from the room.

Read more of these adventures in the thrilling new novel, Ron Paul and the Chamber of Commerce — in bookstores now!

## The Goddamn Airplane on the Goddamn Treadmill

Sorry for the forum/blog downtime today. Many things went wrong during davean’s heroic upgrade. (I blame the LHC.)

Feynman used to tell a story about a simple lawn-sprinkler physics problem. The nifty thing about the problem was that the answer was immediately obvious, but to some people it was immediately obvious one way and to some it was immediately obvious the other. (For the record, the answer to Feynman problem, which he never tells you in his book, was that the sprinkler doesn’t move at all. Moreover, he only brought it up to start an argument to act as a diversion while he seduced your mother in the other room.)

The airplane/treadmill problem is similar. It contains a basic ambiguity, and people resolve it one of a couple different ways. The tricky thing is, each group thinks the other is making a very simple physics mistake. So you get two groups each condescendingly explaining basic physics and math to the other. This is why, for example, the airplane/treadmill problem is a banned topic on the xkcd forums (along with argument about whether 0.999… = 1).

The problem is as follows:

Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?

The practical answer is “yes”. A 747’s engines produce a quarter of a million pounds of thrust. That is, each engine is powerful enough to launch a brachiosaurus straight up (see diagram). With that kind of force, no matter what’s happening to the treadmill and wheels, the plane is going to move forward and take off.

But there’s a problem. Let’s take a look at the statement “The conveyor belt is designed to exactly match the speed of the wheels”. What does that mean?

Well, as I see it, there are three possible interpretations.  Let’s consider each one based on this diagram:

1. vB=vC: The belt always moves at the same speed as the bottom of the wheel. This is always true if the wheels aren’t sliding, and could simply describe a treadmill with no motor. I haven’t seen many people subscribe to this interpretation.

2. vC=vW: That is, if the axle is moving forward (relative to the ground, not the treadmill) at 5 m/s, the treadmill moves backward at 5 m/s. This is physically plausible. All it means is that the wheels will spin twice as fast as normal, but that won’t stop the plane from taking off. People who subscribe to this interpretation tend to assume the people who disagree with them think airplanes are powered by their wheels.

3. vC=vW+vB: What if we hook up a speedometer to the wheel, and make the treadmill spin backward as fast as the speedometer says the plane is going forward? Then the “speedometer speed” would be vW+vB — the relative speed of the wheel over the treadmill. This is, for example, how a car-on–a-treadmill setup would work. This is the assumption that most of the ‘stationary plane’ people subscribe to. The problem with this is that it’s an ill-defined system. For non-slip tires, vB=vC. So vC=vW+vC. If we make vW positive, there is no value vC can take to make the equation true. (For those stubbornly clinging to vestiges of reality, in a system where the treadmill responds via a PID controller, the result would be the treadmill quickly spinning up to infinity.) So, in this system, the plane cannot have a nonzero speed. (We’ll call this the “JetBlue” scenario.)

But if we push with the engines, what happens? The terms of the problem tell us that the plane cannot have a nonzero speed, but there’s no physical mechanism that would plausibly make this happen. The treadmill could spin the wheels, but the acceleration would destroy them before it stopped the plane. The problem is basically asking “what happens if you take a plane that can’t move and move it?” It might intrigue literary critics, but it’s a poor physics question.

So, people who go with interpretation #3 notice immediately that the plane cannot move and keep trying to condescendingly explain to the #2 crowd that nothing they say changes the basic facts of the problem. The #2 crowd is busy explaining to the #3 crowd that planes aren’t driven by their wheels. Of course, this being the internet, there’s also a #4 crowd loudly arguing that even if the plane was able to move, it couldn’t have been what hit the Pentagon.

All in all, it’s a lovely recipe for an internet argument, and it’s been had too many times. So let’s see if we can avoid that. I suggest posting stories about something that happened to you recently, and post nice things about other peoples’ stories. If you’re desperate to tell me that I’m wrong on the internet, don’t bother. I’ve snuck onto the plane into first class with the #5 crowd and we’re busy finding out how many cocktails they’ll serve while we’re waiting for the treadmill to start. God help us if, after the fourth round of drinks, someone brings up the two envelopes paradox.

## Pi-Con, Math, Gender, Glaubama

Pi-Con: This weekend I’ll be at Pi-Con, a convention in West Springfield, MA.  It’s run by some old friends of mine, and will feature Cory Doctorow.  I’ve never actually met Mr. Doctorow in person, so it should be fun.  In my mind’s eye, he still wears a red cape and goggles everywhere he goes.  I hope that’s okay with him.  Anyway, feel free to stop by!  You don’t need to pre-register or anything.

I’ll be spending most of my time there on panels.  It still baffles me that sometimes I’m put on panels to talk about serious business.  A convention once saw, for example, that I had worked at NASA, and put me on a panel about the future of space exploration.  I felt a little out-of-place, given that my main NASA achievement was that I once lassoed a robot with cat-6 cable and had it pull me around the hallways charioteer-style.  So I sit, surrounded by Ph. Ds, and look thoughtful while drawing stick figures on convention stationery.

Project Euler: As is my habit lately when I travel, I’ll probably be working on Project Euler.  For those unfamiliar, Project Euler is a site with a lot of math-oriented programming problems that you can solve in any language.  They start out easy (First problem: Find the sum of all the multiples of 3 or 5 below 1000.)  Then they get harder rather quickly.  I think it’s a great way to learn a language; I started picking up Python last year by doing the first couple dozen Project Euler problems in it.  Many of the problems lend themselves to clever recursive solutions, and as such I imagine it’d be a great way to learn Lisp.

Book recommendation: Self-Made Man, by Norah Vincent.  I inhaled this book in practically one sitting, as did my cousin and several other friends.  In it, the author passes as a man for a year to try to understand the male world, taking notes the whole way.  It’s full of fascinating little stories and avoids getting too preachy — just shows over and over again the surprising ways in which gender influences basic social stuff.  Thanks to creature of #xkcd for first sending me a copy.

Notice: Whoever’s been sending me periodic text messages, formatted like official VP announcements, telling me that Obama has selected Summer Glau as his running mate, please stop.  You get my hopes up every time.