Finally, An Answer

Several years ago, I posted a problem describing a plane on a massive conveyor belt:

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

Similar, but not exact, question was busted on Mythbusters:

An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.

Every since that episode aired, I had serious doubts about the validity of the test; although I could never fully articulate those doubts, even to myself. The biggest issues I had was the speed at which the conveyor belt (or treadmill) was moving.

And that’s the rub. The first question posits that the “belt is designed to exactly match the speed of the wheels, moving in the opposite direction,” while the second questions says “a speed equal to its normal ground speed“. These, my friends, are two entirely different questions.

I thought all hope had been lost, until Randall “xkcd” Munroe became my hero. He asks the version of question I originally posted a couple years ago. It’s actually almost the exact same wording, only he adds in a bit about it being a 747 and then provides an answer:


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?

You think you have it all wrapped up in a nice little package, and then you get to the “But there’s a problem.” And you utter to yourself, “Crap.”

I’m not going to spoil Randal’s rather excellent explanation of the problem. However, he does do a pretty good job explaining it, and you really should read it.

I will, however, add a couple of footnotes:

  • A PID controller is a Proportional, Integral, Derivative controller, which is a type of feedback controller. For example, let’s say you’re running on a treadmill and you start running faster, a PID controller can measure the speed you’re running at and automatically increase the speed of the treadmill so that you don’t run off the end.
  • I’m not sure, but I believe the “‘JetBlue’ scenario” that Randall mentions may refer to the JetBlue Flight 292 incident of a few years ago.

If you also remember the discussion from last time, I think Chris Barnhart is the winner.

P.S. Mr. Munroe claims that xkcd doesn’t stand for anything. However, if you assign each letter a number (A=1, B=2, C=3, etc), X+K+C+D => 24+11+3+4 = 42. Check that out, you can’t make up that shit. And no, I didn’t figure that out all by myself.

Image Credit: Randall Munroe


Boston: Day Zero

Fall break has come (and now gone) and I elected a rather long time ago to spend my four wonderful days of in Boston (or as I like to call it, Bah-ston).

The previous week had been a shit week, which I think is the typical feeling on Friday for many Miners. I hadn’t been feeling well as of late Thursday night and I now had a paint in my lower left ribcage that was coming and going throughout the day. To compound things, several exams had reared their ugly heads that I would need to spend a portion of my vacation studying for.

Feeling rather anxious (in a bad way), I packed my carry on with all the reference material I might need to study and Mom dropped me off at the airport just a little past 10:30pm.

Day 0: 11:35pm to 6:30am
jetBlue has a wonderful red eye flight at that takes off from Denver at 11:35pm local and lands in Boston at 5:15am local. Just enough time to get about three hours of sleep.

I was only able to get about an hour of sleep before arriving. I gathered my things, let Jeff know that I had landed, and starting walking toward the arrival area.

Perhaps the fun part about being up so early is that you can see all shops opening. I don’t why I like the feeling, but it feels good and exciting to see places opening for business…especially early in the morning. I feel part of the city at that point I guess.

There was also a man sleeping in a wheelchair. I couldn’t figure out if he was homeless or not…I should have taken a picture, but I didn’t.

Jeff and Tim picked me up (using James’ van). We made a stop at Dunkin’ Donuts for some bagels, cream cheese, and a bag of coffee to be made later, then proceeded to the dorm unit to catch some more shuteye.

Photo copyright © by legge_e_mare, used under Creative Commons Attribution-Noncommercial 2.0 Generic