Spinning Wheels, NASA, and Learning
September 9, 2013
In high school I had a great physics teacher, Doc Collins. I remember one day he had a strange contraption in his lab. It consisted of a bicycle wheel (with axle) and a spinnable stool. To operate the contraption, you held the wheel vertically in front of you by the axle, and sat on the spinnable stool. Then you spun the wheel. If the wheel was vertical, nothing happened. But if you tilted the wheel left or right, your stool would begin to spin, as if by magic. This really blew my mind at the time. A good science teacher does things like that. The question in every student’s mind, upon sitting in the stool and feeling the spin, was, “How?”
I didn’t really understand it until years later. Doc was a great physics teacher, but I hadn’t yet achieved the mental maturity for understanding Newtonian mechanics. One of my highschool friends, who always had an intuitive grasp of such things, went on to become a mechanical engineer. A few years ago I asked him if he remembered Doc’s contraption, and if he could explain it to me.
As a little quiz, you might want to try and figure it out for yourself. I’ll give you that opportunity, by briefly digressing. NASA’s recent Lunar Atmosphere and Dust Environment Explorer (LADEE) launch was successful, but for one tiny glitch. The glitch concerned the reaction wheels. I’ve never heard of reaction wheels in a spacecraft before. I knew the Lunar Rovers had wheels for driving around on the moon, but LADEE won’t be driving around on the moon. It will end its mission in a suicide dive. So what does it need wheels for?
According to NASA, the reaction wheels help control the spacecraft’s position and orientation while flying. This allows LADEE to conserve thruster fuel, which would otherwise be spent on these tasks. I was like, “Wheels for flying? How does that work?” After thinking about it for a while, I realized the answer. I learned about it first by feeling it happen in 9th grade, then by having it explained by my engineer friend, years later when I was intellectually ready.
OMG nerd alert!!! NASA engineers build LADEE.
Have you figured it out yet? How can a wheel, in mid air, turn a stool, or a spacecraft? The answer has to do with torque and Newton’s Third Law, that every action has an equal and opposite reaction. When you spin a wheel, in Russia (and everywhere else) the wheel spins you. Usually, the things we attach to wheels are very heavy, or anchored against the earth, so we don’t really notice the wheel spinning back. If you generate a lot of torque by flooring your motorcycle, the wheel might spin you back by popping a wheelie. The spinning wheel twists against the motorcycle’s body, causing the motorcycle to rotate in the opposite direction.
The same thing happens in Doc Collins’ contraption. The spinning wheel twists against your body, and since your body is free to rotate, due to the spinnable stool, the wheel succeeds in twisting you in the opposite direction.
And of course the same thing happens with LADEE. If LADEE wants to twist around on one of its axes, it need only spin the wheel corresponding to that axis in the opposite direction. In the olden days, they used thruster bursts to control spacecrafts’ stability and orientation, which used a lot of fuel. So, by taking advantage of the torque of wheels, LADEE is able to achieve the same effects with much less fuel, which in turn saves it more fuel for performing its exosphere experiments.
Pretty cool story, but there’s a deeper point to it that I want to bring out. This cool story ultimately grows out of a seed that my high school physics teacher planted in my head. I failed to fully grasp the lesson at the time, but that’s okay, because a teacher’s job is not to cram as much information into the student as possible, contrary to the assumptions of standardized tests.
Instead, the teacher’s job is to motivate the student’s curiosity and offer intellectual guidance, giving the student exactly what they need at the time to make some progress. Doc’s lesson began in 2003 with a spinning wheel, and though I didn’t fully grasp it at the time, I held onto its sheer coolness. Its coolness kept me curious enough to keep thinking about it, until I finally asked my friend about it, who picked up Doc’s lesson and helped me further along in it. Teaching is a long-term team effort like that, and each teacher’s task is to help nudge the students along from wherever they are in the learning process, and then hand them off to the next teacher to do the same. With the lesson completed, I was able to recognize its application in a real-world NASA situation and figure out how something worked. That feels really good, and I’m thankful to Doc Collins and my friend for helping me out along the way.
About the Author: Seth KurtenbachSeth Kurtenbach is pursuing his PhD in computer science at the University of Missouri. His current research focuses on the application of formal logic to questions about knowledge and rationality. He has his Master's degree in philosophy from the University of Missouri, and is growing an epic beard in order to maintain his philosophical powers. You can email Seth at Seth.Kurtenbach@gmail.com or follow him on Twitter: @SJKur.
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