Below I am reposting a rather long piece taken verbatim from the website of Steve Detweiler who just says that it’s an “amusing anecdote from a friend of mine.” So, I’m not sure of the veracity of the story, and some claim that it’s an urban legend. It may well be. But it opened up some deep discussion on the PHYSLRNR email list, which I attempt to summarize below.
About 6-7 years ago, I was in a philosophy class at the University of Wisconsin, Madison (good science/engineering school) and the teaching assistant was explaining Descartes.
He was trying to show how things don’t always happen the way we think they will and explained that, while a pen always falls when you drop it on Earth, it would just float away if you let go of it on the Moon. My jaw dropped a little. I blurted “What?!” Looking around the room, I saw that only my friend Mark and one other student looked confused by the TA’s statement. The other 17 people just looked at me like “What’s your problem?” “But a pen would fall if you dropped it on the Moon, just more slowly.” I protested.
“No it wouldn’t.” the TA explained calmly, “because you’re too far away from the Earth’s gravity.” Think. Think. Aha! “You saw the APOLLO astronauts walking around on the Moon, didn’t you?”
I countered, “why didn’t they float away?”
“Because they were wearing heavy boots.” he responded, as if this made perfect sense (remember, this is a Philosophy TA who’s had plenty of logic classes). By then I realized that we were each living in totally different worlds, and did not speak each others language, so I gave up.
As we left the room, my friend Mark was raging. “My God! How can all those people be so stupid?” I tried to be understanding. “Mark, they knew this stuff at one time, but it’s not part of their basic view of the world, so they’ve forgotten it. Most people could probably make the same mistake.”
To prove my point, we went back to our dorm room and began randomly selecting names from the campus phone book. We called about 30 people and asked each this question:
1. If you’re standing on the Moon holding a pen, and you let go, will it
a) float away,
b) float where it is,
or c) fall to the ground?
About 47 percent got this question correct. Of the ones who got it wrong, we asked the obvious follow-up question:
2. You’ve seen films of the APOLLO astronauts walking around on the Moon, why didn’t they fall off?
About 20 percent of the people changed their answer to the first question when they heard this one! But the most amazing part was that about half of them confidently answered, “Because they were wearing heavy boots.”
MORE ON THE BURNING QUESTION OF HEAVY BOOTS
I decided to settle this question once and for all. Therefore, I put two multiple choice questions on my Physics 111 test, after the study of elementary mechanics and gravity.
13. If you are standing on the Moon, and holding a rock, and you let it go, it will:
(a) float away
(b) float where it is
(c) move sideways
(d) fall to the ground
(e) none of the above
25. When the Apollo astronauts were on the Moon, they did not fall off because:
(a) the Earth’s gravity extends to the Moon
(b) the Moon has gravity
(c) they wore heavy boots
(d) they had safety ropes
(e) they had spiked shoes
The response showed some interesting patterns! The first question was generally of average difficulty, compared with the rest of the test: 57% got it right. The second question was easier: 73% got it right. So, we need more research to explain the people who got #25 right but did not get #13 right!
The second interesting point is that these questions proved to be excellent discriminators: that is, success on these two questions proved to be an extremely good predictor of overall success on the test. On the first question, 92% of those in the upper quarter of the test score got it right; only 20% of those in the bottom quarter did. They generally chose answers (a) or (b). On the second question, 97% in the upper quarter got it right and 33% in the lower quarter did. The big popular choice of this group was (c)…33% chose heavy boots, followed closely by safety ropes at 27%.
A telling comment on the issue of fairness in teaching elementary physics: Two students asked if I was going to continue asking them about things they had never studied in the class.
First off, here’s the physics. Earth is not the only thing with gravity. The moon exerts a gravitational force on things, but it just exerts less force, mostly because it’s just got less stuff. Stuff attracts stuff, and so less stuff will attract other stuff less strongly. If you drop a pen, it will fall slowly, because the acceleration due to gravity is weaker. Earth is far away, but that doesn’t really matter — when you are on the surface of the moon, the gravitational attraction of the moon is stronger than that of the earth. That’s why the astronauts could jump very high on the moon. You would weigh less on the moon than you do on the earth. If the astronauts jumped really really really hard, they could float away from the moon. The same is true on the earth (but to jump that hard, you need rockets, and that’s what the space shuttle does).
So, here’s the discussion. One instructor said that she had used similar questions in her class, and gotten similar results. Many students thought the pen would float away. One year, she asked them instead about a crescent wrench instead of the “apocryphal pen.” They all answered that question correctly! Another instructor, however, gave a similar set of questions to his class, and most of them answered correctly. What’s he doing differently?
What’s the problem? This question forces students to challenge a preconception that they had walking in the door — perhaps that “things float in space” or “heavy things get weighed down.” Apparently the misconception that the moon has no gravitational attraction persists through most physics courses. Even though they might be able to state that the moon has gravity (as evidenced by correct answering of the second question, as to why the astronauts stayed on the moon), they have trouble transferring that understanding to the “what happens when you drop a pen on the moon” question. They are thinking, argued one instructor, in terms of the surface features of the problem (we’re on the moon!) rather than the underlying features (all chunks of matter have gravity). Students transfer more when they’re interactively engaged in the material, says the research (e.g., Cognitive Development, 6, 449-468 (1991), Learning and Transfer: Instructional Conditions and Conceptual Change, Michelle Perry).
John Clement gave a few ideas for ways to address this misconception in class:
Given enough time you could propose a number of what-if questions which might help the TA understand what is going on. Why did the rocket have to fire its engines to prevent a crash? Why don’t rocks fly away from the moon? What force pulled Apollo 13 around the Moon? Whey when the astronaut dropped a feather and hammer did they both fall to the surface of the Moon? This last one has no heavy boots!!!
Another really important question is to ask why they think there is no gravitational attraction on the Moon. A number of students will reply “because there is no air”. The common misconception is think that “gravity” is due to the air pressing you down. Or they may say because the Moon does not rotate, as this is another common misconception. These are explained in the teachers manual for Minds on Physics, and students are asked questions
to bring out these misconceptions while building a coherent model of gravitational attraction.
So rather than attacking the “heavy boots” conception, the student has to internalize the model that there is (at least in classical mechanics) no threshold to the action of forces, and that unbalanced forces cause acceleration. Then they have to apply it to a variety of cases, of course, along the way. It helps to have them apply these conceptions to objects on other planets. So blocks of wood in a water filled bowl all float at the same level on the Moon and the Earth, but springs supporting masses are stretched less on the Moon.
So the “heavy boots” is not the primary concern. The concept of forces and acceleration are the primary concern. Once the students have a firm model of forces, and of NTNs general gravitational law, the idea that things can float on the Moon will go away.
A few more comments that I liked:
Can the boots be heavy if the astronaut is not? Are the boots heavier than the astronaut? If not, do the boots weigh down the astronaut or does the astronaut way down the boots? I think a few questions like this can make the logical inconsistency evident. (Jerry Touger)
But, countered Dave Van Domelen:
Actually, it goes along with ideas like blankets being intrinsically warm. Qualities as properties of things, rather than the result of interactions.
And from John Clement, an idea I’d never heard before:
This comes from the concept that “gravity” exhibits a threshold effect. You have to have enough of it to be pulled down, otherwise you float.
Which, pointed out a discussant, suggests that students are using buoyancy as an analogy — if you’re heavy enough you sink, if you’re light enough you float. Or, perhaps, friction is the correct model — there is a threshold at which the force becomes effective.
Of course, trying to address these misconceptions as “problems” to be plucked out of the students minds won’t work. They’re using these ideas because they fit with their experience of the world. Trying to understand their underlying conceptions (without perjoratively labeling them as misconceptions) and working from there, will be most productive. Dewey Dykstra has written quite a bit about this, and you can see my previous post on that.
- Minds on Physics (vol 4) has a good section on moon/earth comparisons
- An entire thesis was written on the “heavy boots” problem
- Chapter 4 (p 44-46) of another thesis also deals with this problem
- David Hammer, More Than Misconceptions” published in AJP in 1996.