
Evidently teaching physics is hard - SlyShy
http://www.falstad.com/gravity.html
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Confusion
I don't find these answers surprising and I don't think it signifies anything
about 'teaching physics'. I'm a physics graduate and, well, it's all just not
very intuitive. Here's a more interesting question to ponder: when on the
moon, can you throw a pen hard enough towards earth for it to pass the
Lagrange point (where Earth's and the moon's gravity are equal) and,
eventually, fall towards earth? This sounds like a plausible option, even to
me. However, when you do the math, you will find it is nowhere near possible.
I should immediately consider the idea ludicrous, but I don't. When things
differ, we tend to exagerate the difference. These students are mistaking
'much less gravity then earth' for 'almost no gravity', while not realizing
the importance of 'no atmosphere at all', while thinking of feathers, while
doubting themselves and expecting a trick question.

~~~
dazzawazza
I'm confused by it not being intuitive. I imagine a chid would say the pen
would drop as they probably know nothing of 'low gravity' and all that math
stuff so they will imagine what would happen on the earth and extrapolate that
to the moon. They may even recall seeing people WALK on the moon and thus
assume it's just like earth and say the pen drops.

This seems to me that years of physics/science teaching has ruined the ability
of these students to actually think.

~~~
Confusion
_I imagine a child would say the pen would drop_

A child that has seen a movie or documentary about astronauts floating around
in a spacecraft, may very well guess that the pen would float. "Space" seems
to be the common denominator, not "on a large rock". From most perspectives,
the earth looks nothing like the moon.

 _This seems to me that years of physics/science teaching has ruined the
ability of these students to actually think._

The more you know, the more options you have to consider before coming to a
conclusion. Considering all the, mutually influential, effects that could play
a role, you may simply get overloaded. The problem isn't the capacity to
think, it's the ability to easily and swiftly dismiss all irrelevant factors.
I believe I could have made this mistake in my first year as a physics
student.

~~~
mechanical_fish
_The more you know, the more options you have to consider before coming to a
conclusion._

This was a big problem when I played rapid-fire, _Jeopardy_ -style trivia
games in school. One learned to dread questions that were in a field you
actually knew. The knowledge slows you down!

Trivia questions are generally written by knowledgeable non-experts. But a
question which is clearly phrased from the perspective of a knowledgeable non-
expert can be imprecise enough to cause an expert to pause, and pauses are
deadly in rapid-fire trivia.

It's hard to construct a perfect example, but suppose you got a question that
boils down to "who first demonstrated radio"? A lot of people will blurt out
"Marconi" -- he did, after all, apparently score the first _patent_ for a
radio _set_ , and he was a good self-promoter. But the physicist or historian,
who might know about Tesla and Oliver Lodge, will object, or at least hesitate
for a fatal moment.

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Sam_Odio
It's interesting that many of the respondents were so concerned about the
Earth's gravity. It's inconsequential given that the moon is in orbit around
the Earth (and therefore so is the pen).

~~~
Qz
You're right about it being inconsequential, but the fact that the moon is in
orbit around the earth has nothing to do with it.

It's an inverse square law based on the distance between the objects (and also
the mass, but the distance is the key factor in this scenario). The earth is
200,000+ miles away, which is why the moon's gravity will win out over the
earth's.

It's really sad that college physics students failed so badly at the question.

~~~
Sam_Odio
No. The fact that the Moon is in orbit around the Earth means that the pen is
in perpetual free-fall. As long as the Moon is in a stable orbit, the distance
to the Earth doesn't matter.

Even if, at 200,000 miles, the Earth's gravity was _stronger_ than that of the
Moon's, the pen would still fall to the Moon.

Think about it.

~~~
ced
Hey, but couldn't the same argument be used to claim that tides shouldn't
exist? The Earth orbits the Moon as well.

That bothered me. So I checked Wikipedia:

 _Thus, the tidal force depends not on the strength of the lunar gravitational
field, but on its gradient (which falls off approximately as the inverse cube
of the distance to the originating gravitational body).[4] [25] The solar
gravitational force on the Earth is on average 179 times stronger than the
lunar, but because the Sun is on average 389 times farther from the Earth, its
field gradient is weaker_

So the distance does matter, it seems. Can anyone figure out under which
conditions a pen doesn't fall to a moon?

The rotational speed matters as well. If a planet were to turn fast enough
that the centrifugal force is stronger than the gravitional force, it would
shed matter, and a pen would be first to go. (Do such bodies exist? Anyone
knows? Presumably, it would have to be solid to stand a chance)

[This is all quite off-topic, of course. Yay for random thoughts on physics]

~~~
pmjordan
_So the distance does matter, it seems. Can anyone figure out under which
conditions a pen doesn't fall to a moon?_

When there is a force holding the moon in place that doesn't act on the pen as
well, or the pen is really far from the moon.

Orbit is a special case, so assume earth, moon and pen start at rest. They
will all start accelerating towards their combined centre of mass. The pen
essentially doesn't contribute at all to the location of the centre of mass;
If moon and earth had quasi-zero radius, the pen would hit the object first on
whose side of the centre of mass (CM) it started on. (the objects'
acceleration will be such that their individual CMs would hit the CM at
exactly the same time, so the objects nearest the CM will accelerate slowest)

    
    
      earth - pen - CM - moon -> pen hits earth first.
      earth - CM - pen - moon -> pen hits moon first.
    

Earth is bigger than the moon, which is much bigger than the pen, so this
won't quite be true, but the pen will still have to be quite far from the moon
to begin with for it to make a difference. (distance pen-moon vs. radius of
moon vs. distance moon-earth)

The original question is clearly the latter case. In fact the pen and the moon
are so close compared to any third objects, and the pen's mass so irrelevant,
that you can treat them as being in the moon's frame of reference. So unless
there's a force acting on either the moon or the pen which _isn't_ acting on
the other (this can never be true for gravity), the pen will always drop to
the moon.

Because this is true in general, it is also true in orbit.

Aside: Note that there cannot be any tides on the moon because the moon itself
rotates around its own axis at the same rate as it rotates around earth. Also,
the water involved in tides doesn't start floating off - it is still very much
attracted to the earth, and the deformation of such a gigantic body of water
is extremely slight - metres of deformation of a shell with a radius of
~6300km. You won't notice the pen's reaction to that sort of force.

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roundsquare
To some degree, I think this type of thing happens when you ask a simple
question in a class where its not expected. People start over thinking and
confusing themselves.

I just asked my friend the question (since I wanted to send him the link) and
he started asking all sorts of follow up questions because he couldn't believe
I'd be asking him such an easy question.

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tjmc
Evidently reading the question is hard too as everyone here and on the site
has missed it: "If a pen is dropped on _a_ moon" Not _the_ _M_ oon.

The moon in question could be Phobos, Titan, Europa etc

~~~
char
Yes, but then you'd have to consider the gravitational effect of
Mars/Saturn/Jupiter. What happens to the pen then?!?

~~~
JeanPierre
It will still be attracted, because all other gravitational forces apart from
the gravitational force on that moon will be negligible.

~~~
char
Yes, I know. I was trying to be sarcastic. :)

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rajat
Well, see, it depends on how the question was asked. Was it part of a test
that the student thought would be graded?

When I was in college, every once in a great while, I'd be stopped on the
street and asked some question, usually by a psychology student. I'm not
necessarily happy today with the smart-aleck I was then, but my answers were
always completely off the wall. Q: Who's the Vice President? A: Paul Newman.
Q: What's to the north of the U.S.? A: Africa. (I'm making up the questions.
They were about general knowledge, but I no longer remember what they were).

I mean, there wasn't anything at stake. Why the hell wouldn't I give
completely off the wall answers? So, somewhere, there are a bunch of people
laughing at stupid American college students at least partially because of my
answers.

Ok, so if someone asked me a question like that in the street today, I would
probably answer exactly the same way (haven't matured all that much
apparently). Wouldn't you?

~~~
lotharbot
If you've watched Jay Leno's "Jaywalking" segment, you've seen this
phenomenon. He asks people on the street a bunch of questions and looks for
hilariously dumb answers. I think some of his responders really _are_ that
dumb, but some of them are just going for lulz (Example: "What is the opening
line of the Bible?" "A long time ago, in a galaxy far far away.")

That said, the page said pretty explicitly that the question was asked on
actual quizzes, which presumably means the students expected to be graded.

On the positive side: they presented 40 of the 48 wrong answers they got. 34
of the 40 came from first-semester physics or astronomy. So at least it
appears the majority of second semester and higher level students got it
right.

~~~
klipt
_"What is the opening line of the Bible?" "A long time ago, in a galaxy far
far away."_

Only in my dreams ... only in my dreams.

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nzmsv
The "heavy boots" link at the bottom of the page is even more... interesting.

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spamizbad
I cannot understand how you could get this question wrong. The Apollo 15
Astronauts dropped a feather and a hammer on the moon and showed they both hit
the moon's surface at the same time.
<http://www.youtube.com/watch?v=5C5_dOEyAfk>

You don't even need to take the class to know the answer is C.

~~~
mechanical_fish
People who were born in the year of Apollo 15 are now 39 years old. I should
know.

So the only people who have ever seen this video you refer to are people well
over forty, people who have been shown a videotape in (e.g.) a physics class,
and people who have caught it on YouTube, a medium which is only a handful of
years old and where it is hard to find moonwalk videos among all the piano-
playing cats.

It's probably hard for young people today to appreciate what life was like
before home video recording. You had to hope for the TV networks to rerun
something in order to see it a second time! There were also only half a dozen
channels on TV. And we had to walk to school uphill both ways in the snow...

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ced
_If a pen is dropped on _a_ moon, will it_

Hey, the pen really could float away, no? The centrifugal force on a
rotatating body is mv^2/2. If this is exactly equal to the gravitational
force, the pen would "stay where it is". And if it rotates faster than that,
the pen would "float away".

~~~
lolcraft
The centrifugal (pseudo-)force is not mv^2/2 (kinetic energy), but mv^2/r
(much less). And the centrifugal force must be orders of magnitude less than
the gravitational pull: if not, the moon wouldn't be there in solid state, to
start with, as all its particles would separate from its center of mass. Not
very plausible.

------
tokenadult
"We read an article claiming that the average American does not know the
correct answer to the following question:

"If a pen is dropped on a moon, will it: A) Float away B) Float where it is C)
Fall to the surface of the moon

"So a bunch of us TA's got together and gave our physics classes quizzes
asking this question. Out of 168 people taking the quiz, 48 missed the
question. The responses are below. Some people didn't write comments. The
spelling and grammar were not changed, however, clarifying comments are
enclosed in []'s."

All I can say after reading the comments is "Wow!" (I had to laugh to keep
from crying.) The student responses from Physics 324 - Modern Physics for
Engineers have to be seen to be believed.

~~~
greenlblue
I blame the teachers, not the students. It's not like Newtonian physics is
intuitive, in fact it's the exact opposite so if the professors don't hammer
the concepts the students will just retain their old intuitive views of how
objects interact.

~~~
Qz
You mean the _high school_ teachers right?

Also, I don't really understand how it would be intuitive for the Earth to
attract the pen and not the Moon.

~~~
greenlblue
It's not like it is complete nonsense. The earth is bigger and more massive
and has a stronger gravitational field so in principle it is possible for the
pen to float towards the earth, the pen would have to be a lot higher than a
person's height but it could work. Anyway, I'm getting off track here. A while
ago I read about David Hestenes and Eric Mazur and their experiments about
students' intuitive understanding of physics before taking a physics class and
after taking a physics class. These results are perfectly in line what
Hestenes and Mazur discovered and both of them are excellent teachers. So it's
not just the teachers but the way the classes are structured as well.

~~~
tjmc
A hell of a lot higher than a person's height. Gravitational force is in
inverse proportion to the square of the distance from a body.

The classic gravity analogy is billiard balls sitting on a stretched sheet of
plastic. The force of gravity is like the slope of the dents on the plastic -
very steep around individual bodies and dropping off quickly after that. So
drop something close to any given body and it's going to have a much greater
influence than a larger body further away.

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jaytee_clone
I used to teach Physics. This is actually a misleading problem for people
over-think the problem (but do not have solid understanding of Physics
fundamentals).

The problem can be easily answered by using a thought experiment.
Unfortunately many of such techniques are usually not taught in classes unless
you are a in a Physics class for Physics majors.

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petercooper
You don't need to know anything about physics to get this right. You just had
to have seen a _video_ of people walking on the Moon to realize that when
they're bounding along, they don't shoot off into space or float in mid-air.

~~~
whatusername
"but people are heavier than a pen - and they almost float away - look how
they bounce. A pen would obviously float."

~~~
petercooper
And to think that would require that you've done the "dropping something heavy
vs something light and noticing they take the same time to fall" test. I
admit, I don't consider that to be _really_ "knowing" physics, it's just a day
to day observation.

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MaysonL
See "Confessions of a Converted Lecturer" by Eric Mazur:
<http://www.youtube.com/watch?v=WwslBPj8GgI>

(on HN a few months ago).

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char
As someone with a degree in astrophysics, that was extremely, extremely
painful to read. My favorite was, "It shouldn't float away just because I've
never seen it happen".

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hristov
This is bad, but when you think about it, the answer is not as obvious as it
initially seems.

I believe the earth's gravity is stronger than the moon's even on the surface
of the moon. In order to get the correct answer a student must think about
orbital velocities and the way velocity helps objects stay in orbit and not
fall to the ground, even if there is a gravity force exerted on them. And then
the student would have to imagine the pen not as something stationary on the
moon, but as something that is travelling around the earth (with the moon) at
high velocity, and then the student would have to realise that the pen has
sufficient orbital velocity not to fall to the earth, but in relation to the
moon it has very little orbital velocity, so it will fall to the moon and not
to the earth.

So my point is this is not that simple and it requires some imagination and
complicated reasoning to get the right answer.

Of course most people should get it by remembering the moon landing videos and
remembering how the astronauts were bound by the moon's gravity in those
videos, but maybe kids these days have not even seen the moon landing videos.

