4 ms·

Evidently teaching physics is hard

• tokenadult 17y ago
  "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 17y ago
    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 17y ago
      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 17y ago
        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 17y ago
          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.
        • infinite8s 17y ago
          By that reasoning, we should all fall toward the sun.
        • tokenadult 17y ago
          The other article they linked, which inspired the TA's question, suggests the follow-up question of "Then how is the astronaut standing on the moon without floating off?"
      • patio11 17y ago
        The right answer is so obvious that a student who has been well-prepared by years of instruction to spot tricky questions will immediately assume that it cannot be right. Here's another fun one: A student is standing on a rock orbiting a stellar body at a distance of eight light-minutes. The stellar body is much, much more massive than the rock. The student drops a pen. What happens to the pen?
        • noonespecial 17y ago
          Oh! I know this one! It hangs from the chain that attaches it to the desk to keep it from being stolen even though the bank is "too big to fail".
• rajat 17y ago
  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 17y ago
    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 17y ago
      "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.
• Sam_Odio 17y ago
  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 17y ago
    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 17y ago
      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.
      • CoreDumpling 17y ago
        Despite how readily we poke fun at the answers given on the linked page, I found it interesting that I had to drill down this far to find a satisfactory answer. I liked the explanation given by my high school physics teacher: in the end, it boils down to frame of reference -- since both the moon and the pen are in orbit (aka perpetual free-fall), the earth's gravity basically becomes "background" to the situation and has no effect on the relationship between the pen and the moon. It's not the concept of gravity, the mathematical formulas, or anything else that's really hard to grasp. Looking at things from the right frame of reference is hard, and not just in physics either. I struggle with this in my daily work, even for simple things like getting widgets to align correctly relative to each other.
      • Qz 17y ago
• hristov 17y ago
  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.
• nzmsv 17y ago
  The "heavy boots" link at the bottom of the page is even more... interesting.
• char 17y ago
  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".
• tjmc 17y ago
  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 Moon. The moon in question could be Phobos, Titan, Europa etc
  • char 17y ago
    Yes, but then you'd have to consider the gravitational effect of Mars/Saturn/Jupiter. What happens to the pen then?!?
    • JeanPierre 17y ago
      It will still be attracted, because all other gravitational forces apart from the gravitational force on that moon will be negligible.
      • char 17y ago
        Yes, I know. I was trying to be sarcastic. :)
    • tjmc 17y ago
      According to Wikipedia, the terms "moon" and "satellite" are basically interchangeable. There's some use of the term "moonlet" for very small bodies but there isn't a defined size you have to reach to be officially a moon (unlike a planet to the disappointment of Pluto fans). So drop your pen in a dusty part of Saturn's rings for example and it might be the biggest "moon" around and start attracting smaller bodies to itself.
  • petercooper 17y ago
    Or even http://3.bp.blogspot.com/_g7KcbMxmLEU/R4-JxU_YZgI/AAAAAAAADCY/0qIPTnxKZLI/s400/mooning.jpg http://3.bp.blogspot.com/_g7KcbMxmLEU/R4-JxU_YZgI/AAAAAAAADC... I've seen people drop pens on to moons like that, and can confirm that they fall to the "surface" pretty quickly. (Sorry, I'll be back off to Reddit now to say 20 Hail Marys.. :-))
    • 17y ago
• jaytee_clone 17y ago
  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.
• petercooper 17y ago
  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 17y ago
    "but people are heavier than a pen - and they almost float away - look how they bounce. A pen would obviously float."
    • petercooper 17y ago
      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.
• spamizbad 17y ago
  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 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 17y ago
    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...
• roundsquare 17y ago
  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.
• Confusion 17y ago
  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 17y ago
    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 17y ago
      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.
• ced 17y ago
  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 17y ago
    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.
• MaysonL 17y ago
  See "Confessions of a Converted Lecturer" by Eric Mazur: http://www.youtube.com/watch?v=WwslBPj8GgI http://www.youtube.com/watch?v=WwslBPj8GgI (on HN a few months ago).