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Myth: The astronauts didn’t float away because they had heavy boots (sciencegeekgirl.com)
169 points by andrelaszlo on Nov 28, 2013 | hide | past | web | favorite | 165 comments

I think the people who answered wrongly aren't thinking in a first principles, physics-based mindset, but just instead just recalling previous memories and reasoning in an intuitive, fuzzy kind of pattern-matching.

We've all seen images of pens floating weightless in space on films and TV, and we probably haven't ever seen pens falling on the moon. But the moon is in space right? so... yeah, why not, it floats.

And for the guys on the moon, well I haven't got any memories of astronauts without boots on, and those boots sure look big and heavy, they're special gravity boots, I'm sure I heard that on TV once, yeah, that's probably it.

The first principles approach of applying the concept of universal gravitational attraction probably isn't even entering their heads, even if they've been taught it.

This used to get on my nerves, how can people be so stupid? Why won't they just think for a moment!? But recently I've been focussing some effort on learning a language (Spanish) and realise that in that this case the first-principles style of learning that works so well for physics and maths is largely useless, and it's better to turn off the analytical side of your brain and not think too much, just keep exposing yourself, trying to communicate, and let your subconscious do it's stupid fuzzy pattern-matching, and it does a remarkably good job!

On earth it's entirely true that heavy things fall and light things float (like dust, feathers, etc). So you can't blame someone for applying this experience to the moon if they've never thought it through. Also, I wonder if the "heavy boots" part comes from the fact that the Apollo astronauts on video look like they're underwater. In water, you'd need heavy boots to stop yourself from floating. I wouldn't call these misconceptions. More like non-conceptions: there was no preexisting idea before the question was asked that could have been incorrect.

There's also the issue of quality of effort. If you apply effort to reason something through, you can weed out obvious misconceptions on your own and give a better answer. Part of the reason why the people that gave ridiculous answers got lower test scores could be that they are not in the habit of applying much effort when asked science-type questions, so they blurt out the first thing that comes to mind and move on. I suspect that on a more-favored subject most of those people would apply effort and give higher-quality answers. Otherwise, they'd be crippled in all aspects of life. This explains why my mom can't use a computer and I can't use her ultra-sophisticated microwave. She doesn't want to expend effort on the computer and I don't want to expend effort in preparing food.

I think this is an accurate guess as to what is going on.

I wonder if there's a reliable way to get people to start using critical analysis on a subject. Such as when you're training someone how to send an email, this would be helpful information!

They need some internal motivation and the belief that their effort will pay off, i.e. the subject is within their reach to comprehend, and the effort invested (and risk) will yield an acceptable return. If they don't like a subject, there's going to be a debt of remedial knowledge that's missing. Like fully understanding that gravity affects dust and feathers, even though they float in the air.

> This used to get on my nerves, how can people be so stupid? Why won't they just think for a moment!? But recently I've been focussing some effort on learning a language (Spanish) and realise that in that this case the first-principles style of learning that works so well for physics and maths is largely useless

I had the exact same experience. I'm an Engineer, and my entire life I've only been able to learn (and self-teach) by reasoning through first principles. When I can't reason it through, I don't want to learn it "just because"

I never put any effort into learning a language, and always thought it was "dumb". In high school I could say hello and count to 10 in a couple of languages, but that was it.

When I was 27 I started traveling through Central and South America, and wanted to learn Spanish. I honestly didn't know if I was capable, or if my brain and learning style would allow for it.

For the first couple of months I was frustrated and always saying things like "but that doesn't make any sense", "It should be like this" etc.

After a few months, I just stopped trying to reason it through and started going with it, like you said. After about a year and a half I stopped translating into English in my head, I started dreaming in Spanish, and I can now guess words and verb conjugations without consciously realizing I even know that given word. It first started happening when I was a little drunk, which is a good time for me to actually get stuff done in my head, because I stop trying to reason everything through and just go with it.

Stick with it, you'll get it. I can't more highly recommend immersing yourself for a prolonged period.

Thanks! Yeah, getting a little drunk may help, although there's definitely an optimal point after which it has the opposite effect!

Actually I'm living in Madrid now so kind of immersed, the problem is that my girlfriend and I tend to always speak English with each other and I don't need to use Spanish to work either. It's what led me to create my current project: http://readlang.com

That's a nice site. Having seen what you can do with live webcams, e.g. livejasmin (NSFW), I was thinking a similar model could work for language learning. The teachers could be paid. They wouldn't be contracted and they'd be minimally screened, but they'd accumulate popularity. Maybe this could be combined with the readlang model so that you could learn by reading a text together with a teacher. I think it's evident that learning from an actual person, especially one you can hear and practice with, is the most productive way of acquiring a language.

I haven't tried it but I hear that http://www.italki.com/ is pretty good for online Skype lessons where the teachers are paid and have reputations as you describe. Completely agree that interaction with a real person is very important.

I consider it a sign of a good idea when someone else has beaten you to it. Some of the XXX cam sites draw people in by allowing non-paying users to watch and chat, and once you get involved, you pay only for what you use. The home screen shows captures of current action, so it always looks like there's a lot going on. I think there's a formula in there for getting a self-sustaining system going.

I grew up as a space junky, with NASA still running manned missions when I was a kid. It didn't take me very long to understand the concepts of gravity on the moon vs. earth. I trust that if I asked grade-school me, I'd get the right answer.

While it didn't take long to get the physics of the problem, it took a lot longer for me to grasp the social side of things. Specifically, if your grade six teacher says something stupid like this, there is absolutely nothing to be gained from correcting her. I wish I could go back in time and clue me in.

OTOH, if it's a university lab at a science and engineering school, then give 'em hell.

You may not gain anything correcting your sixth grade teacher, but by not correcting her, something can be lost. A world where teachers can spout bullshit without any opposition is not one I want to live in. And given we only have one world...

Perhaps it was irrelevant in your case. Or perhaps your skepticism inspired a fellow student to become an engineer. Who knows?

Don't worry, I get a feeling you wouldn't of listened to yourself anyway, just ask your grade six teacher! (I mean no offence by this)

> I think the people who answered wrongly aren't thinking in a first principles, physics-based mindset, but just instead just recalling previous memories and reasoning in an intuitive, fuzzy kind of pattern-matching.

In the Niven/Pournelle book The Magic Goes Away, one of the wizards takes his apprentices up to a high peak and casts a spell to enable his apprentices to walk on clouds. There is a hidden purpose: In every group class, some of his apprentices goes up the peak and tries to jump on a passing cloud without knowing the spell. This had the effect of raising the intelligence level of his students.

Heinlein proposed that colonists living on the moon would be more intelligent than the general earth population -- simply through natural selection.

Check put the PhD thesis and TED talk of the guy running veritasium. It is all about sciencrofic misconceptions and education. Veritasium is an extension of that work.

This mention of "gravity boots" blew my mind! You're right, for someone regularly exposed to American TV, this phrase will sound very familiar [1]. I am seriously wondering if that could have measurable impact.

Also, your way of learning Spanish is impressive.

[1] http://en.wikipedia.org/wiki/Gravity_boots

That advice only works for endeavours that are similar to things we've been selected for. Otherwise, the human brain will fall remarkably short with plenty of built-in biases. The fuzzy matching heuristics are broken in well-determined ways, and even being consciously aware of them isn't enough to overcome the problems.

One guess is that you can directly see the blackness of space from the moon (pictures of moon landing) so it doesn't have an encapsulated feel like the earth's atmosphere so one less layer of separation. It feels like (visual perception from photos) it is easier to float off the moon.

My intuitive, fuzzy pattern-matching immediately matched the concept of dropping a pen on the moon to the actual videos of astronauts dropping similar things on the moon, which makes it fairly easy for even the most science-ignorant person to figure out.

It would have been really interesting to ask people to draw a free-body diagram listing out each of force acting on the rock, prior to asking the question of what would happen when dropped. It would be hard to make up a force besides lunar gravity and any Physics 101 student should now answer correctly by analytic reasoning.

They would get it wrong, though. Even the physics 101 students probably wouldn't draw the gravity of the sun, the gravity of the earth and the Milky Way.

If we're discussing reasoning from first principles, is it really so intuitive to disregard the gravity of the Earth in this experiment? Why can you disregard it and still get the correct result?

Exactly right: you have to recognize that the moon is in orbit/freefall with respect to Earth, which means there is no gravitation near its center of mass and only tidal forces away from the center of mass. Then you have to estimate that the tidal forces on the astronaut will be small compared to the moon's gravitational forces on him. None of this even counts centrifugal forces from the moon's rotation or effects of the sun. Nested non-intertial reference frames play merry hell with the simplicity of a FBD sans prior knowledge (or solid logical reasoning) about what can be neglected.

Even with a physics degree, my kneejerk reaction didn't come from first principles. I, too, was doing "stupid pattern matching" -- the only initial difference between me and the floating-pen-people was that I happened to know that gravitational acceleration could be reasonably approximated as constant on large astronomical bodies (moon is g/6). It took a few seconds for the "otherwise the planetary body would disintegrate" argument to occur to me and even longer for the formal mathematical picture.

I'm still astonished at this idea though. I don't understand how a student of Philosophy could live life without regularly thinking about the behaviour of the universe outside our home. Thinking about it often enough that gravity on the moon is a concept close to the surface of awareness.

At my daughter's school, they had an assembly for parents to attend that celebrated, amongst other things, the first landing on the moon.

The children boldy proclaimed that the reason the hammer and the feather hit the ground at the same time in the famous experiment was because there was less gravity on the moon.

It still rankles with me a year later.

It's amusing that that kind of is the reason, indirectly. Less gravity means it couldn't retain an atmosphere, and the lack of an atmosphere is why they fall together.

That's a shame. Was there any opportunity to assess the material before it was presented? Was there any method to correct mistakes after?

Your daughter presumably understands that it's not about gravity but air resistance. Has she told all her pals? Because that kind of subversive teaching could be really powerful. TEACHERS HATE HER; THE GRAVITY SECRET THEY DON'T WANT YOU TO KNOW!!! etc.

May I ask, hold old was your daughter? What kind of science teaching should she be getting? (Obviously, not anything that wrong).

She was 6 at the time. I told her afterwards it was wrong but I regret not telling the teachers.

I think the key is this sentence: "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!"

People just DO NOT think.

Everybody has SEEN (on TV, where they spend more time in front of, than in school or reading books) astronauts in orbit demonstrate they're weighless, by showing a pen float in front of them.

So space = floating pens, in their visual memory.

Moon is in space, therefore pens float.

What about a wrench? Oops, no visual memory to answer, bummer, they'll have to think! And of course, some of the little teaching they had pegged, and they can answer correctly.

That's all. People DO NO think.

Why do you think, IBM had to put up "THINK!" panels in its offices?

That's one possible reason. The other might be "pen == light, wrench == heavy".

I've found that the main problem is that people do think. About things that they've seen on tv, or read, that are just plain fictional.

Humans have a huge advantage in being able to communicate knowledge across generations, and a huge disadvantage in being able to communicate bullshit across generations.

> People just DO NOT think.

People think. They just generally do not think about Physics.

I think he's referring to cached thoughts. Thinking critically takes more effort than retrieving cached thoughts. So why think critically in lieu of a ready made meme?

Site seems to be down. Cached version is here: http://webcache.googleusercontent.com/search?q=cache:uWHPLrG...

A significant part of the article is a reposting of http://www.phys.ufl.edu/~det/phy2060/heavyboots.html

Edit: Changed wording re how much of article is repost

>Article is mainly just a reposting of http://www.phys.ufl.edu/~det/phy2060/heavyboots.html though...

That is an unfair characterisation, I think. The article does indeed reproduce heavyboots in its 725 words entirety (according to pages's word count for what that one's worth), but tacks a further 930 words afterwards. And these 930 are interesting indeed as they're further explorations of the phenomenon and discussion on its underlying reasons.

Fair enough - I've changed the wording a bit. Personally, though, I thought that the follow-on discussion didn't add much beyond what was mentioned in the original heavyboots article.

This was a pretty entertaining read. First I thought I would sense a smug sense of superiority of natural science over the humanities, but then I realized he performed the same test on his physics students.

I always sucked in tests. In the first question, I would have hesitated between b) and d).

The reason for choosing b) would have been that the moon's gravity being weak, the earth's gravity nullified the moon's thus having a floating pen.

The reason for choosing d) would be because Moon's gravity is stronger than Earth's.

This is why I think tests are stupids. If I had chosen b) instead of d) I would have gotten a wrong answer. Even though my thought process would be correct. Just missing the right information (which could have been easily found outside a test environment).

The relative strength of gravity mostly does not matter. If the Earth pulled on you harder than the moon, you'd still fall towards the moon, because the moon is just as strongly affected by that gravity as you are (momentarily ignoring tides).

So no, I'm pretty sure your thought process wasn't remotely correct.

His thought process is correct, but his "pen on the Moon" model is missing vital element (that Moon is in free fall/orbiting Earth).

If Moon was hanging on a stick connected to Earth (instead of orbiting Earth) and Earth gravity forces on a pen were stronger than Moon's gravity forces, then pen would fall in Earth.

Of course you and I know that there are no such strong materials for that Earth-Moon stick, but that's yet another level of complexity that most people don't have to know about.

>If Moon was hanging on a stick connected to Earth (instead of orbiting Earth) and Earth gravity forces on a pen were stronger than Moon's gravity forces, then pen would fall in Earth.

Technically that's true, but it would require a completely different scenario. Earth's gravity is miniscule at that altitude. For the moon to have a weaker force it would have to be so small that it would likely not even qualify as a moon any more.

Well, you are ignoring tides, I'm not. I say that theoretically, you could be at the Roche Limit, and then the pen would stay still. Hypothetical, yet real. So thank you very much but yeah, the thought process stand correct.


You can disagree all you want, it's not because things would just fall towards the moon in practice that it's impossible that the pen stand still.

It's funny how quickly that simple "Pen on the Moon" question can be turned from novice level into physics expert level.

"Please select all possible conditions under which pen on the Moon would not fall on the ground".

Tides are purely about relative differences in gravity between positions, though, not about differences in pull between two bodies. You can still be at a point where the Earth's gravity is greater than the Moon's while on the surface of the Moon without anything floating away and while comfortably outside the Roche limit. In fact, such a situation must necessarily exist outside the Roche limit for any body.

I'd be comfortable marking you wrong, unless you had a reason to think the two forces would be almost exactly the same on the moon's surface. It's not a matter of being confused as to which is stronger when you say 'neither'.

Thanks for posting that. Not one to complain about blogspam, but that's one of the all-time great "internet motherfucker" posts. Neither Sciencegeekgirl.com (lol, brand yourself harder) nor anyone else can do much to add to it.

Actually, Sciencegeekgirl.com adds VERY MUCH to the article, namely a highly interesting discussion of WHY students give these wrong answers. No, it's not because they're stupid or ignorant.

> it's not because they're stupid or ignorant

They may not be stupid, but that's pretty much the definition of ignorance.

Obviously, anyone who answers a question incorrectly is ignorant of the correct answer, but the point is that in many cases the wrong answer seems to be logically based on a wrong mental model of how gravity works. Just like Aristotelian physics. And Newtonian physics as well. Would you call Aristotle and Newton ignorant?

The interesting part is the discussion of what these wrong mental models look like and how you can teach a correct one.

> Would you call Aristotle and Newton ignorant?

Absolutely. There was much they did not know, as is true of anyone. "Ignorant" is not an insult, it simply means you do not know something. Would you say Aristotle and Newton knew everything about everything? If not, then they would be, by definition, ignorant of the things they did not know.

> Would you call Aristotle and Newton ignorant?

By modern standards, absolutely.

> Would you call Aristotle and Newton ignorant?

By your definition they're ignorant, but not stupid?

There's absolutely no way to justify a student attending a physics class and not knowing that the Moon does have gravity as well.

The problem is that they may know that the Moon has gravity, but they don't understand what that concretely means when it comes to questions like this.

I would dispute the validity of calling that state of mind "knowing that the moon has gravity". I would instead call it "memorized a couple of correlated keywords".

No. Where writing is concerned, 2+2 is greater than 4.

'Heavy boots' is actually a reasonable answer in that it doesn't preclude the moon having gravity.

On its own yes, but if combined with the previous statement that a pen would float away, it just exhibits a piecemeal and inconsistent understanding of physics.

Why? On earth, heavy boots hold you down, but dust and feathers float away.

The moon has less gravity than the Earth, so a pen is like a feather.

Blah blah atmosphere electro-magnetism blah, the universe is big ad complicated.

Oh god. Feathers and dust don't float away because they're weightless.

They float away because air currents exert forces on them that are greater than their weight. Eventually, in a still-air environment, the feather and the dust will settle to the ground. You can verify this by going to your nearest attic.

To be fair, your nearest attic might have dust on the walls and ceiling as well, though maybe not as much...

See the famous video of one of the lunar astronauts dropping a hammer and a feather at the same time. Both of them drop fairly rapidly to the surface.

From the comments:

> I answered gravity on a physics test in high school for the question “What force works at a distance” and got it wrong because the professor wanted electromagnetism. When I took the paper up to him to contest it, he told me that gravity doesn’t count because it only exists between planets. I got the physics book and ..."

I was sure at that point he was going to say "dropped it on the floor" but no such luck.

> "...read to him out of it but he cut me off and claimed that what I had read proved his point."

See also this website. They ask the question to students, and we know the courses the students are taking.


> Physics 324 - Modern Physics for Engineers

> "A body is at rest tends to stay at rest, plus there's no gravity"

> "The gravity of the moon can be said to be negligible, and also the moon's a vacuum, there is no external force on the pen. Therefore it will float where it is."

> "The pen will float away because the gravitational pull of the moon, being approximately 1/6 that of the earth, will not be enough to cause the pen to fall nor remain stationary where it is. The gravatational pull of other objects would influence the pen"

It's a bit scary seeing what the answers are.

I guess we need to remember just how long ago it was that anyone was on the moon, or any distance into space.

An ASCII diagram.

       e   g                          m

       earth geosynchronous           moon
Since 1980 we've sent people maybe a pixel from e.

>> "The gravity of the moon can be said to be negligible, and also the moon's a vacuum, there is no external force on the pen. Therefore it will float where it is."

That one's almost correct; it's just an issue of scale; they don't realize how big/heavy the moon actually is.

The issue is that they presumably understand that the moons gravity isn't negligible, since damn near everybody on earth has seen the Apollo 11 moon landing videos. They are not merely missing information needed to answer the question and making the wrong assumption. They have all of the necessary information, but get it wrong anyway because for whatever reason, they aren't using all of the information they have.

> since damn near everybody on earth has seen the Apollo 11 moon landing videos.

Is that true? I suspect it's true for people over 30 or who have special interest in STEM, but not so true for anyone else.

I would say that the footage has been used enough in various forms of media to have just about everyone covered. Stuff like this at least: https://www.youtube.com/watch?v=182oUgBfoLE

One way or another, I think everyone but very small children or reclusive hermits have seen at least some footage of an Apollo astronaut bouncing around on the moon.

I would add another answer choice to the question 25:

25. When the Apollo astronauts were on the Moon, they did not fall off because: (f) They were on Earth

I think a lot of people who are skeptical of this story just don't realize that they appended to be surrounded by people who have developed unusually accurate physical models.

I recommend browsing Amazon reviews of (non heat pump) electric heaters for statements about which ones are more efficient. (The less efficient one produce more waste heat?)

Because there are few consumer products that exercise people's understanding of gravity, basic thermodynamics might be a good proxy. However the real point of the article isn't all the misconceptions that must be "plucked out" one by one, but the struggle of trying to figure out how to encourage students to form more comprehensive models.

I'd also add that an important goal in teaching is students' facility in applying principles, and confidence to question intuition and consensus.

Well, in case of electric heaters used to heat a home the efficiency should really be measured in terms of "how much electricity is consumed if the heated room is kept at constant 20C measured at e.g. center of room". This is somewhat dependent on the quality of the thermostat controlling the heater.

Then, I think that there are some real differences found in the efficiency of the heaters.

If you're still talking about electrical heaters here, there's literally no where for the energy used to go other than into heat for the room.

If you have two 1000w heaters, one made from a potato and ten clothes hangers and the other made from the highest quality materials known to man, they will still both heat a room the same amount.

Granted, one might have a better fan or more airflow and might therefore provide a more even heat distribution in the room, but they will both be providing the same amount of heat to the interior of the room. That's just how energy balances work. If 1000w is flowing from the plug into the heater and it's not being stored anywhere in the heater itself there's nowhere for it to go but into heat for the room.

I don't think energy efficiency is really so critical as the control system: how much steady-state error is there, and how quickly can it initially heat the room? Overheating the room is obviously a bit of a waste of power, but it's also uncomfortable.

>don't realize that they appended to be surrounded by

autocorrected, should have been:

>don't realize that they happen to be surrounded by

This must be nonsense. Or an April Fools joke. Or something. I don't want it to be true. It can't be.

sobs quietly

Veritasium [1] often interviews people as part of his videos, and the results are consistent with what's being described in the post: somewhat distressing [2]. To the point where you might think he was purposefully making fun of them [3].

1: http://www.youtube.com/user/1veritasium/videos

2: http://www.youtube.com/watch?v=TQxeutcYP6I

3: http://www.youtube.com/watch?v=cY_o4A1wzsg

Go forth and ask random people whether (a) the earth revolves around the sun, or (b) the sun revolves around the earth.

The proportion giving answer (b) will cause you to sob even more.

(Yes, I have done this. Afterwards, I hid under my bed bewailing the ignorance of my fellow hominids.)

Both a and b are true (after you fill in the missing information). With two objects it's only a matter of chosen frame (or point) of reference. I'd argue that since you're on Earth, b makes more sense.

The importance of the heliocentric model comes from the fact that there are more objects in the solar system. With multiple objects you could also hold any one of them still but it makes the model much more complicated and the attraction between the objects less intuitive.

b is ruled out to a "reasonable" person because the Earth is not an inertial reference frame, but is clearly the 'rotating' frame, while the Sun is (approximately) stationary.

This is confirmed through the phenomena of stellar parallax and stellar aberration, which are only visible from Earth because of its motion. Even back in the 1573, Thomas Digges and many successors realized they could prove the Earth was in motion using parallax (though had trouble detecting it), and in 1729 James Bradley gave the first proof of heliocentrism using aberration. Both of these observations were taken as proof by the scientific world that the Earth orbits the Sun, and not the other way around.

The difference is that when we say that the Earth revolves around the Sun, it's implied that we are using an inertial reference frame. Yes, you can take whichever reference point you want, but it's not the default assumption.

A tiny number of people saying "The Sun revolves around the Earth" have any idea of the subtleties of different frames of reference.

The Earth and the Sun both revolve around a separate point that is roughly in between them (ignoring pesky other planets). However this additional point in space happens to be located inside of the sun, so unless you are going to throw in non-inertial reference frames for some unstated reason, "the earth revolves around the sun" is rather unambiguously the 'correct' answer.

Well, they orbit each other, don't they? [Edit, well, to be precise, they are both orbiting the barycenter, which, given the great mass of the sun, is much closer to the center of the sun than the center of the earth]

For the Earth-Sun system, the center of mass is only shifted a few 100 km towards the earth, but the radius of the sun is 695500 km. So the offset is less than 1/1000 of the radius of the sun.

But we don't live in an Earth-Sun system. The point around which planets revolve in our solar system is often outside of the sun as can be seen here: http://en.wikipedia.org/wiki/File:Solar_system_barycenter.sv...

That wibble-wobbles around much faster than I would have expected.

It looks like Jupiter is way faster than I thought it was. My guestimate for gas giant orbits was skewed towards Uranus/Neptune (84/165 years), but Jupiter orbits in only 12!. It's crazy how close Jupiter is to us.

Thanks. That is a really cool diagram! I had no idea that it varied so much over time.

Stuff and nonsense! The only reason the lamestream media promulgates such poppycock, ist das it makes the maths easier!

Honestly the thing that caused me the most concern was that the "which way does the rock on the moon drop" question was considered of average difficulty, in a University course. And only ~half of the students got it right.

That's incredibly damning of your grade school science curriculum :(

Read to the end and the bit about buoyancy, you'll be less angry.

That merely demonstrates people aren't thinking, they're just applying patterns.

You say tomato, I say tomato, let's call the whole thing off.

Why does people's ignorance in Physics bother you?

Specialization allows people to stay ignorant about many things.

I was sold a space faring civilisation. I was promised flying cars for 2000.

I blame my uprooting to this retarded planet on people's ignorance in Physics.

That's why.

The heavy-boots answer seems pretty justifiable in the case where you have a correct intuition about how gravity itself works but simply underestimate how strong the Moon's gravity happens to be. Set the parameters just-so and there would be a threshold effect, as on an asteroid massive enough to walk on but not-massive enough to jump off.

Exactly! This reminded me of the Little Planet what if http://what-if.xkcd.com/68/ Munroe claims on a small, super massive planet, you could overcome escape velocity with a sprint. It doesn't work the same on the moon obviously, but only because the moon has significant gravity.

>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.

Why is this such a stupid assumption? That's how it works on Earth.

I don't believe that these kids are failing to think; they're using reasonable analogies, but lacking specific knowledge about the environment on the moon.

>"All science is either physics or stamp collecting." - Ernest Rutherford

Basically the authors are complaining that the students haven't collected a specific stamp.

No, on the contrary!

It's a packers vs. mappers question.

The packers collect stamps, and when asked whether the pen falls on the Moon, get out the stamp that says that pens don't fall in space.

The mappers will apply the physics model, the formula, and come out with the right answer (or approximation) corresponding to the right situation.

That's a big problem with packers: you teach them stuff, but they retain only half of it, forgetting the applicability conditions. "Nothing can go faster than light!" Only material stuff, and only in the vacuum. But there are mediums where light will go slower than a walking human. "There's no perpetual motion machine!" Right, only when you consider a CLOSED system. There's no physical entity smaller than the Universe that's a closed system. Even black holes have inputs AND outputs! And so on.

Well, the lack of an atmosphere on the Moon should rule out buoyancy. Myself I don't doubt that many of the wrong answers really were the result of faulty intuitions or dodgy reasoning.

Of course, but that's another environmental variable.

When I took physics in high school, Gravity was introduced to me with an equation. After 5 minutes discussion, everyone in the room would have known how to answer the question about "Dropping a rock on the moon."

I find it hard to believe that a high school physics student, just fresh from studying gravity module, wouldn't realize that a rock dropped on the moon would fall down. Doesn't pass the smell test.

Being introduced to gravity with an equation is not nearly enough to figure out how things will behave. It takes calculus to describe the motion of falling things even without air resistance. Even if someone straight-up tells the students that gravity accelerates things at the same rate regardless of mass, they're probably not going to believe it without some experiments.

Edit: a quiz to maybe prove my point :) On the moon, if you throw a rock, what shape would its arc describe?

Edit2: not a straight line when you drop it, silly!

It's not even guaranteed to be a straight line if you drop it, due to variations in the density of different parts of the moon, radiation pressure, particles in the way (yes, the vacuum isn't absolute), any magnetic effects, gravitational gradient caused by other celestial bodies, etc.

The arc will approximate a conic section of some kind, depending on how hard you throw it. Within the usual level of ability of a space-suited human throw, it will approximate to an ellipse so close to parabolic as to be impossible to tell the difference before it hits the moon. Throw it harder and it'll approximate to an ellipse. You are only going to be able to approximate a circle if you stand on the highest point on the Moon and throw at exactly the circular orbital speed horizontally, although gravitational perturbations will usually break the orbit. It'll approximate a parabola if you throw at exactly the escape velocity, and a hyperbola if you throw even harder than that.

> tells the students that gravity accelerates things at the same rate regardless of mass

Please get your wording right, there is a big difference between force and acceleration. It is trivial to prove, that things are not accelerated by gravity at equal rate.

PS: I am not usually wording-nazi but since you started with calculus...

You have it backwards, qwerta. If the feather and the hammer fall to the ground in the same time they must have accelerated at the same rate. Of course the hammer has more force applied by gravity and the feather has less; that is why the hammer is heavier!

Yes, but in atmosphere it would be accelerated at a different rate. Stationary objects are not accelerated at all, because gravitational force is compensated.

As I was saying, there is difference between force and acceleration.


I read sp332's original comment as having an implicit "in the absence of other external forces" (gravity accelerates things at the same rate regardless of mass) as that's often implied when generally discussing forces and acceleration.

After all, it becomes hard to talk about the relationship between force and acceleration, if you can't even say that the acceleration of an object is proportional to a force acting upon it - because there might be other forces you've not taken into consideration! Yes, I won't accelerate an object if I try to push it into a barrier, because the barrier will exert an opposing force that resists my effort. Yes, an object might fall up if I "drop" it, because it's in an updraft that combined with it's drag coefficient causes more force up than gravity exerts.

But springing those sorts of situations on people as "gotchas" isn't useful in a general discussion about the essence of a concept. Unless otherwise specified, assume ideal point masses in a vacuum (or whatever). If you want to add opposing forces of some kind to complicate^W make the situation more realistic later on as a more advanced topic, that's fine, so long as that's specified up-front.

If complicating factors are not specified up-front, it's assumed that they're absent, because there are an infinte possible set of complicating factors which could be present but have not been mentioned yet.

After all, in an atmosphere it could be accelerated at the same rate, as there's a (previously unmentioned) anemometer and computer-controlled rocket attached to the object which are set up to produce a thrust which exactly compensates for the wind resistence! So ner! :-)

(And, in fact, sp332 specifically stated "even without air resistance" to explicitly constrain the conditions under which their example was valid)

This is not about edge conditions, but about two different concepts. Saying "gravity accelerates" does not make sense by definition, at least we should say "gravitational force accelerates".


> gravity accelerates things

The confusion is between the transitive and intransitive forms of the word "acceleration". If I say "the pen accelerates", this implies net acceleration of the object. If I say "gravity accelerates the pen", this implies gravity's contribution towards net acceleration. Air resistance diminishes net acceleration, but not the acceleration as contributed by gravity.

sp332 was fine. It's common for astrophysicists to talk about gravitational fields in terms of "acceleration fields" rather than "force fields". This is because the acceleration contributed by gravity only depends on a single argument (distance from the celestial body's center) as opposed to a force-field's two arguments (distance, the pen's mass). And next time you hear "g = 9.81 m/s^2 for all objects on earth's surface", the physicist is referring to gravitational acceleration, regardless of whether or not the object is moving.

Depends how hard you throw it. ;)

Just like on Earth. That's the point.

That wasn't my point really. Oh well! I though most people would pick a parabola, which is a decent approximation for short throws but not the real shape.

You got me, what's the real shape? I thought it was a parabola if there was no air friction. Or are you taking the curvature of the moon into account?

It's an ellipse, but it is so elongated that it is extremely close to a parabola. If the entire mass of the moon was concentrated in a single point at its centre, and the projectile could pass through the moon, then the projectile would follow a parabola-like path and go down below the moon's surface, until it started curving around the other side of the centre and back up to its starting point. It's only when you look at the uninterrupted path that the difference between the parabola approximation and the ellipse becomes apparent.

The parabola results from assuming that the force due to gravity on the object is constant (which is a decent approximation over short vertical and horizontal distances). In fact it will change magnitude and direction as the projectile moves.

The curvature of the surface of the moon doesn't matter, but the fact that the rock is orbiting the center of the moon just like a satellite would is the insight that Newton needed a smack on the head to get :)

Would I be wrong if I said that the deviation between the parabola and the elliptical orbit trajectories is probably much smaller than the tidal effect of the earth and the sun, solar wind and other external forces on such short trajectories? And as such the elliptical model is "as wrong" as the parabolic one?

After all, a model is only as good as the precision of the results it produces...

It's not numerical approximations I'm worried about. It's the fact that if you don't understand what's going on underneath, you're not going to have good "intuition" when someone asks a question like: what happens when you don't ignore air resistance.

I distinctly remember my physics I teacher in highschool giving the class the classic "You fire a bullet and drop a bullet at the same time" question, at the conclusion of a lecture on gravity. Barely a tenth of the class got it correct, even though they were capable of deducing the correct answer from what they had just learned.

Many people don't think.

On a few sites, a commenter known as "Russ Brown" claims he posted the original on usenet, referring to a class he took in 1981. He claims that the quiz results were added to his version of events.



I believe this is the original: https://groups.google.com/forum/#!search/"heavy$20boots"$20a...

I call it bullshit. I just asked that question to a seminar of paedagogic students, without a single person with physics background. 2 Choose answer the first answer, 3 the sencond one, the other 20 choose the correct answer. I expect much better results for students with physics background.

/edit, well bullshit is a bit strong, but replication failed

Only counts if you forbade them to communicate with each-other.

I'm more stunned by this than when people go around and ask college students who the VP is or where is France. Those are facts and you either studied and memorized them or not.

That many people suck at "reasoning" about problems boggles my mind. Most these students knew the facts of Newtonian mechanics but could not apply them (in my opinion) to a very simple problem.

Not bragging. I must be good enough at reasoning that I am cognitively unable to comprehend that other people have trouble with it. Everyone knows how to breath or think about future. I assumed everyone can take things they know, combine/apply them to new situations. What I would call reason about things.

Maybe they can. And the results are an artifact of college environment (study/memorize/pass test vs actually learn/challenge/grow).

See, I so can't comprehend it that I'm trying to come up with rationalizations.

Does a lack of knowledge of physics really matter in the real world for most people?

I was recently wondering if our terrible road safety problem in South Africa can partly be explained due to a lack of understanding of physics. After all, intuitively Newton's First Law explains why seatbelts are necessary. Concepts like momentum and kinetic energy explain why heavy trucks, speeding down steep inclines are dangerous - authorities don't seem to grasp that very often, and tragedies involving runaway trucks on poorly engineered roads are not infrequent.

But I then realised that two of the most reckless drivers I know are my uncle, an Engineering dropout, and my friend who is a practicing engineer. Being good at physics hasn't saved them from being in multiple car crashes.

Are there any cases where an understanding of physics helps the average person in everyday life?

The thing is, you can go on in life without understanding barely anything. That's one of the wonder of the modern life. You just need to be able to do enough arithmetics to buy stuff at the grocery and pays your bills and taxes.

For your uncle and friend, understanding something doesn't mean you feel it's going to apply to you. Road accident are not only a matter of physics but also a matter of statistics and risk calculation. Some people are reckless because they can't compute the probability of accident and often underestimate it, feeling that since they think they are good drivers it won't happen to them. Humans in general are bad at assessing risk, it's a very difficult matter.

This isn't a lack of physics knowledge, it's a lack of basic reasoning. If they'd said "the astronauts had magnetic boots", for example, that would be wrong, very wrong, but it would have made sense - at least, it's not outright contradicted by their previous answer.

That was my first reaction too. After thinking and discussing it with some non-technical people, though, I realize it's not a contradiction unless you also assume some physics knowledge. I (and probably you) have so deeply digested the fact that a pen and a heavy boot fall at the same rate that it seems like a logical failure to not incorporate it into a conclusion. It's quite possible that a person's understanding is simply inconsistent (at the frontiers of anyone's knowledge i think inconsistency is quite common; "learning" is largely a process of finding more data and working through implications to resolve them), but it not a foregone conclusion based on these answers.

This is the main challenge in teaching people any technical subject - discovering the basic knowledge you have assimilated so deeply that you can't even imagine that others don't share it.

> Does a lack of knowledge of physics really matter in the real world for most people?

That's an interesting argument to make.

In the context of students taking courses with a physics component it's not so relevant.


Physics 221 - First Semester Calculus-based Introductory Physics

> "Because the gravitational pull of the moon is much weaker than that of the earth. And object such as a pen is so lite that it will float"

Some of the answers are borderline incoherent. I'm not sure if English is their first language, or how old people who take Physics 221 are, but it's kind of scary.

This reminds me of my reaction to the evolution/creationism debate- Does it really matter which one people believe? It has pretty much zero practical impact on their lives, and there aren't a lot of decisions they can make for others that depend on the information.

Now, I'm not in favor of teaching creationism in schools. Schools should teach what we as a civilization understand to be true. But as far as what individuals choose to believe? I don't really much care. I think it's an odd thing to get emotional over, but both sides certainly do (this is me preempting emotional replies :-) )

Of course, even when evolution is taught, it's almost always taught wrong. Even a large percentage of graduate biology students have deep misconceptions about what evolution is and how it works.

It just occurred to me that machine learning students would probably have the best understanding of evolution of any group of students. Ha!

If you choose creationism, you also choose the existence of a creator. A great many people who hold this belief seem to base many decisions in their lives on what this creator would want them to do (or at least, say they do). This can be a very, very long way from "zero practical impact on their lives".

Could you give an example of practical impact on life of person who chose to believe in creationism?

A literal belief in fairy tales leading to them making major life decisions based on the collected, millennia-old edited ramblings of a desert tribe. Start with lynching witches and go from there.

You've made several leaps that do not necessarily follow.

Yes I have. Nonetheless, given the number of people who subscribe to creationism and yet do not hold any religious views (vanishingly small in my sampling, but I haven't conducted a large-scale international review), whilst they do not have to follow, they do seem to follow in the vast majority of cases.

Or any programmer who works on legacy code... sure destroys the misconception that natural selection must end up with the best option ;)

We live among the dinosaurs!

I can't find the reference now, but in the 1980's, there was a study done on honors physics students -- undergraduates IIRC -- at Johns Hopkins. They were asked questions that they had the physics training to answer, but that were not problem set or exam questions. For instance: Coil a garden hose in a spiral, then force a ping pong ball out through the hose from the center to the periphery with a blast of air; how does it move once it leaves the hose: (a) continues in spiral (b) straight out - the tangent of the spiral where the hose ends (c) bending outward, away from the spiral.

Some large percentage of the students got it wrong. Explanations: (i) students limited their "physics know-how" to the textbook context, and this was something different; (ii) textbook problem sets are, over time, edited to weed out confusing problems, even if they're valid.

I've encountered something similar in teaching basic financial present value analysis to engineers - even the students who do well in the class go back to "folk" thinking when we discuss their mortgages and credit cards.

To me, it says that the cue of the classroom, or the awareness that "this is part of Physics 101" is critical to getting some people to apply a particular frame, and without that cue, they go back to "common sense". That does explain what happened (in the blog post) in the Philosophy class, and in the phone poll, but not as well what happened when the blog author tested her physics students.

Well shit, now I want to know the answer. It's "straight out" right?

From a simple F=m*a standpoint maybe, but I'm thinking that the hose would probably put a spin on the ball so it would likely curve outwards?

A ball with no spin on it would travel straight outwards because there would be no force acting on it to make it curve in any other direction.

I think a ball with spin on it would curve outwards because the ball would rotate the air around itself, which would provide the force needed to accelerate it outwards.

One explanation for the wrong answers is how people use the phrase 'pen floats', and how people are imprecise about using words.

The gravitational constant is 1/6 of the Earth's. This means that I can jump on the Moon 6 times higher than here and the pen will tale 6 times longer to fall down.

It will take 6 times longer for gravity to 'kick-in' and cause the perceivable movement, as we got used to here on Earth - where on Earth the pen moved already 6 cm on the Moon the movement is still 1 cm.

It looks as if the things are moving slower. I can catch a falling egg before it reaches the ground, my reactions are like of a fly on Earth.

So I can say that the things are floating on Moon, because they are not accelerating and falling fast enough, in common terms. In interstellar space the pen will fall onto the neighbor galaxy, but will do this so slow, that I will perceive as if it is floating. If the pen will move alongside with an object much smaller than the Moon (a rock), the pen will fall onto the rock with much slower acceleration. For time perception of humans it will float in space.

Another explanation for wrong answers is that if you feed people with stupid questions, you will get stupid answers. Have you asked me to answer these questions I will surely answer wrongly 'pen floats' and 'heavy boots' on purpose only to prove the point. These answers seem to be serious enough to mislead you.

No, it does not take the pen 6 times longer to fall down on the Moon compared to Earth. Not even close. That's not how acceleration works. [I can give the equation, but I think it's more instructive if people think about it themselves :-) ]

Also, actually try dropping a pen, like right now. Gravity "kicks in" immediately - there's no perceptible delay. (Unless your perception is way different from mine.)

I suspected that time difference between falls would be not linear, but I was not sure about actual ratio.

Playing with freefall distance equations showed that time to fall difference is square root of 6. Here we are:

  Height = gEarth*timeOnEarth^2/2 = gMoon*timeOnMoon^2/2
  gEarth = gMoon * 6
  6 * gMoon * timeOnEarth^2/2 = gMoon * timeOnMoon^2/2
  6 * timeOnEarth^2 = timeOnMoon^2
  timeOnMoon = sqrt(6 * timeOnEarth^2)
  timeOnMoon = sqrt(6) * timeOnEarth = 2.45 * timeOnEarth

The gravitational constant is constant DUH! It's worth the same 6.67384 × 10-11 m3 kg-1 s-2 on the Moon than on Earth than in the other corner of the Universe! (AFAWCK for the other corner of the Universe, from the kind of stellar light we get from there).

Thanks, you caught me in my incompetency. The standard gravity on Moon it supposed to be.

There's a similarity between teachers and journalists in that they're supposed to double-check their informations before transmitting them. Not doing it plagues society.

Towards the end, there are some guesses as to what theory of gravity the students hold, if it's a buoyancy analogy, or a threshold theory... A lot of people don't really subscribe to any coherent system of the world at all.

When you press someone, it's not uncommon for them to just pile on another fact about the subject matter and reassert the claim.

It's like brute force puzzle solving, kind of like I used to do in adventure games - you know the result you want, juggle all the stuff in your inventory or mind until it works.

And for a lot of subjects, for a lot of education, that approach works really well. You're told three facts about some historical event. You're asked a question on a test about the event. List one of those facts to win.

I'm guessing they jump to "boots" because the bootprint on the moon is a pretty widely seen and memorable image, and boots in our normal life are sometimes heavy. Heavy things make it harder to float, that's what heaviness sort of means. Having arrived at "floating's hard," they then call it a day without really understanding anything about how they got there.

There's no underlying theory, they kick away the scaffolding as soon as they get to any answer.

People don't have a list of facts in their heads that are constantly checked for mutual consistency, and people can stubbornly hang on to any premise they want.

In other words, the students (+ phil TA) aren't just stupid on this point, they are so stupid, the physics teachers don't even understand how stupid they are.

I think the false association in this context is the following:

    space -->  weightlessness
Thinking this makes people look for "alternate" explanations, like the weight of the boots, etc.

People who have not been trained in physics often have a different "intuition" about the laws of physics. An interesting effort to qualify these "physics misconceptions" is the Force Concept Inventory (FCI) questionnaire[1], which which asks students to answer some basic qualitative problems from Newtonian mechanics. You should try it and see how you fare...

Interestingly, researchers have found that even after taking a Newtonian mechanics class, many students still hold on to their misconceptions---their score on the FCI is only slightly improved...

[1] http://prst-per.aps.org/multimedia/PRSTPER/v3/i1/e010102/e01...

And that's my basic problem with talking with other people: It's hard for me to understand how one can not know or comprehend what I know and classify as "basic knowledge"...

Of course I understand that and can for instance explain it to them, but it often still remains surprising for me.

My wife posited that perhaps their gravity suits kept them on the ground (which i found charming, at least she was thinking of gravity, and/or science fiction). My 8 year old daughter thought for a second and said the rock would fall, then on her way out of the room said it would float (no doubt taking some time to catalogue everything she's ever seen or read about space). I'm fairly sure if the question were posed as multiple choice, they both would have answered correctly.

No indictment of either (or anyone else who would answer incorrectly). After all, not many people think of physics very often on a day to day basis, particularly physics of extraterrestrial bodies.

I could have benefited from an unhurried, contemplative learning about science in elementary school. The primary concern approach with example and counter example even seems fun. The "heavy boots" problem of focusing on what is not the primary concern is a problem in understanding other issues like in politics. In politics though people have a self interested agenda and will warp reality to make it fit like having the earth the center of the galaxy. People should strive in political learning to use the first principle approach of the article, like eliminate oppression, and unbiasedly apply it universally.

A long while back, I read a scifi book (really a kids scifi book) about a kid that finds a red pebble which cancels gravity for anything it's in contact with (except kids, obviously), causing those objects to be pulled strongly up into the sky. Why? Because they were attracted by the moon's gravity. They even got it to spin round and suggested using that for free energy. Can't for the life of me remember the title or author, but I remember it really grated against my sensibilities, and I never read anything by the same author again.

I couldn't resist crunching the numbers: the acceleration due to the moon's gravity as felt from the Earth is roughly 3.319×10^-5 m/s². By contrast, the centripetal acceleration due to Earth's rotation is 0.0337 m/s², several orders of magnitude larger (and just about perceptible as a gentle tendency to float up).

(A similar example to your red pebble would be "Cavorite" from the H. G. Wells story "The first men in the moon", which also negated gravity causing objects to shoot violently upwards. The explanation given however was buoyancy, which makes more sense I suppose.)

If you had a way to cancel gravity, you could use that to get free energy because the no-curl condition for energy conserving fields would no longer hold. You could create a closed path in space where gravity is always pulling you along the path.

Pretty sure it was Antigrav by Nicholas Fisk. It was a cool story - if you were a kid. I never did find any red pebbles though.

That sounds about right.

When I read the title I thought this would be about space walks. Having heavy boots would help create a greater gravitational attraction to the space station, making it harder to float away.

It's interesting to consider the forces involved. At 10m, with a 100kg astronaut and a 15 ton space station the gravitational force is about 1e-7 Newtons. At that acceleration, if the astronaut kicked off at the speed of 1cm/s--or 36 meters per hour--and assuming the force doesn't change, it would take about 24 days to get back.

Here is a related question that has bothered me for some time: in Jules Verne's "From the Earth to the Moon" (1865), he writes the travelers as only achieving weightlessness for a small moment, at just the point where the moon's gravity exactly cancels out the earths gravity.

This makes an intuitive kind of sense: you would gradually lose weight as you get farther from earth, then after a moment of weightlessness gradually 'gain' weight as you approach the moon.

Obviously this is wrong, but why?

(I'm not a physicist, so take the following with a grain of salt!)

It's not wrong, but it would never happen.

Let's say we've built a moon elevator. Like a space elevator, but tethered to the moon. You travel from the earth; there is a brief period of acceleration, then you settle down to a steady speed for your journey.

After you reach the steady state, you are going to experience normal Earth gravity. As you move further from the earth, however, you will experience less of it. As you approach the moon, its gravity will eventually be stronger - with this transition occurring at the Lagrange point L1 between the earth and the moon, where the gravitational pull from each is equal.

In practice, until we build a moon elevator, this isn't going to happen. Instead, we have to throw you at the moon using a rocket. Most of the energy we use for this is actually spent making you go sideways, instead of directly up. It's kind of cool actually - if you are orbiting the earth in, say, the ISS, you are actually experiencing nearly the same gravitational pull as if you were on the surface. You are just moving so ridiculously quickly in a circle around the earth that the centripetal force cancels out gravity - like someone is swinging you around on an invisible rope. Since this is happening to both you and the spaceship you are in, there is no relative acceleration - so you appear to be weightless.

If we fire some more rockets to accelerate you towards a lunar orbit, nothing else changes - you are still not accelerating relative to your spacecraft (assuming you are strapped in!) and so you continue to appear weightless. You transfer into orbit around the moon - and you are in the same situation you were in when orbiting earth. But if you drop a lunar lander, which slows itself down - it'll fall!

Orbital mechanics and gravity can certainly be counterintuitive, but it's thoroughly fascinating.

If 47% of students fail that "pen on the Moon" question -- that means that about half of professionals are clueless about physics.

On the other hand, most professionals are quite functional in their role (if it wasn't the case, why would employers pay them?).

These two observations mean that understanding of physics is not mandatory for being functional professional.

No, it means they're not functional, and that employers pay them to heat their chairs. Why else would employers insist on having people come to the office, instead of working at home, as we almost all could, in this day and age?

Further: http://www.technologyreview.com/view/519241/report-suggests-...

That entire article made me angry like when I hear about creationism and evolution being taught as equal theoretical concepts.

Great video of Apollo 15 astronaut dropping feather and hammer on the moon:


Too bad this isn't part of current events any more.

Fascinating, I was so confused at first because I first misunderstood the fact who was right and who was wrong in this philosophy class.

I will ask this question on parties, this will be funny :D

I wonder what the poll percentages would be like in other countries? I suspect that fewer people would be this ignorant in industrialized countries other than the US.

This is why Super Mario Galaxy is a great educational tool!

it blows my mind that half got the first question wrong... :(

Myth: This story

Truth: There are dumbasses in this world... those who believe this story, because it gives them a superiority over (mythical) others.

Lol, those stupid uni students who don't even know there's gravity on the moon. Lucky I didn't get a degree and become dumber/Lucky my Uni was better.

It's even a friend of a friend of a friend.

Seriously, why is this on the front page?

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