So actually I think there is a way push vs. pull is relevant, and physicists may not be seeing the forest for the atoms that make them up. Seeing it the way I’m about to describe helps me answer the OP question simply: No.
A push involves exerting a force such that the object upon which it’s exerted travels farther away from the object exerting the force. This can happen when the distance between the two is effectively zero–a ball hits a bat–or when the distance is greater than zero–two repulsive magnets (like super-ugly magnets). In the former case, the exerter may follow the exerted: a hamster in a ball that’s rolling across a floor pushes on the ball, pushing that surface away, but then follows along with the ball. A weightlifter pushes the weight up, but gravity pushes back, keeping the weights in the weightlifter’s hands.
A pull brings the object of the force closer to that which exerts the force. In general, that can only happen when the object and exerter are some distance greater than zero apart. Gravity and magnets are the main examples of this type of force. The only other example I can think of involves adhesives, and in this case, it’s the opposite of the hamster on a wheel: I attach a double-sided piece of tape to a paper and touch it and pull my hand back, and the paper comes with me. And I don’t know of any sports with adhesives or magnets.
This may not be useful in terms of understanding the four fundamental forces, but for understanding that an object may be affected by multiple forces simultaneously (the pull of gravity, the push of a child’s hand), I think it’s a useful model. I’ll always explain that things get more complicated once you learn more (I had that conversation with students recently, explaining that third grade math didn’t really touch on integers, just whole numbers), but when kids need to learn about speed and how gravity affects everyday objects in visible ways, the model suffices, I believe.
Oh–and model rockets, built by students out of card stock and powered by an air compressor, are our first activity in the unit :). We’ll finish the unit by building remote-controlled vehicles powered by wifi-enabled, programmable motors. I’m all about the hands-on.
You know, what you have almost, but not quite got to is Newton 2. Indeed it is almost as if you have overshot it without noticing on the way past.
You can do almost all your examples perfectly with Newtons three laws. And it isn’t exactly going to be hard for kids to get.
A weightlifter:
Bar on ground. It isn’t moving. Newton 1.
Force - weightlifter must overcome gravity, residual force left and the bar moves up.
More force, faster it moves up. Newton 2.
Holds bar over head. Force up balances force down.
(And there are more bits in here. Newton 1 is how you get the bar over your head.)
Your model rockets are identical. But you get reactive mass to do action and reaction.
Clearly at this age you don’t get to the mathematics. But kids will likely understand the basic idea of linear relationships.
Seriously, it would be fun to ask the kids what the reaction to a weightlifter lifting the weight is. The idea that the Earth moves an amount that is so tiny nothing can measure it would be usefully mindblowing.
Heck, a really mindblowing question. How come a tiny magnet can hold up something against the entire pull of the Earth? Neat answer for the kids. Nobody knows, but there is a free trip to Stockholm* and nice medal for the first person to find out.
I always wondered if you had to pay for your ticket.
This is a really helpful post, and I might rework how I teach it to include simplified Newton’s laws. Thanks!
Also, the magnet bit cracks me up. Years ago when I was student teaching I had to teach electomagnets, and I figured I ought to understand how magnets work, so I figured I’d pull up a Wikipedia article, spend five minutes iwth it, and I’d be good.
Like three weeks later I finally gave up. It’s nice to know that it’s not just foolishness on my part.
I read the article to which you linked, which was perhaps foolish. I’m not claiming to be a candidate for an Ed.D.; I’m just saying I read the cite you provided and I have some doubts about the concept as presented there. Those doubts were echoed in the article, which described the thoughts of Kim Kastens, a geophysicist at Carleton College. Clearly, I’m not the only STEM-oriented person with reservations.
[QUOTE=Wikipedia]
To me, this all goes back to the credited-to-Einstein quote I mentioned: everything should be as simple as possible, but no simpler. This was meant to apply to hypotheses, but it works pretty well for science education, IMHO. In most cases, it’s possible to tell a simplified, accessible story without telling an untrue story.
Well, that’s not quite cricket, is it? When you quoted me, you omitted the next sentence, which pointed out that the OP was asking people with a strong physics background to weigh in on whether he was describing the physics accurately.
More importantly, we’re teetering on the brink of another false dichotomy: one between pedagogy and subject matter. Yes, effective teaching is of critical importance, but teaching the wrong thing effectively is a failure. And teaching the right thing ineffectively is no better. We obviously need both. Agreed?
I like this compromise very much. My concern with the OP’s case was that he was considering teaching “that all forces either exert a push or a pull on the object of the force.” Well, some do, and some don’t, and the OP himself is already running into examples (like swimming) where it’s a little of each. Plenty of everyday things (like magnetism) fall into neither category.
As Frances Vaughan points out, breaking phenomena down into these particular rigid categories will be counterproductive because the categories are not, as LSLGuy put it, “upward compatible.” As long as upward compatibility is generally there, I don’t see a problem at all.
In fact, the very project of science is to describe things in an upward-compatible way. Once a theory shows itself not to be upward compatible (i.e., it’s shown to be incompatible with new information) that theory is tossed and replaced with one that is upward-compatible. Science!
Kudos, OP, for thinking this approach through and wanting to get the physics right-or at least, upward compatible!
N.B.: My previous post included some markup that I decided not to use but failed to delete. I missed the edit window. D’oh!
Insane Clown Posse has a nails-on-chalkboard song (I know, they’re all like that) called “Miracles,” in which they famously asked, “F**king magnets: how do they work?” My engineer friends and I snickered at that for a long time until we realized that (a) it was hard to explain and (b) we couldn’t even provide a clean, accurate description to ourselves.
In short, don’t feel bad. To paraphrase both Barbie and James Clerk Maxwell, magnetism is hard.
My problem with lies-to-children is not moral or philosophical, but practical. I’ve had a lot of experience teaching introductory-college physics, and I have to spend at least twice as much time on un-teaching as on teaching. Students come to college with heads full of lies-to-children, and in order to make room for the truth, I must first chisel away all of those calcified lies. I’d have a much easier time if they had never learned anything on the subject at all, to say nothing of if they had actually been taught truths to begin with. What’s more, often the truth is actually simpler to understand than the lies.
My favorite example here is mirrors. When you stand, say, two feet in front of an ordinary mirror and look at your image, where is the image? Ask any four-year-old, and they’ll tell you that the image is standing two feet behind the mirror, looking through it like a window. To such a child, it’s easy to teach the truth, because the truth is that the image is behind the mirror, just like it looks like (and in fact, that’s exactly why it looks like it, because our brains have evolved to be very good at dealing with the real world). But put that same child through our education system, and by the time they get to college, they’ll “know” that a mirror is like a TV screen, with the image right at the surface of the glass, and lots of luck convincing them to believe their own eyes over the accumulated weight of authority of all of their teachers.
Oh, and as an addendum: One might object that most students don’t ever go on to take college physics courses, or indeed to pursue physics in any other way, and so do not need to have their lies dislodged. This is perhaps true… but in that case, the lies still aren’t doing any good. If they don’t prepare the child for anything further (because they work contrary to that goal), and they don’t have some abstract value in knowing the truth for truth’s sake (because they’re not), then why tell the lies to begin with?
I should clarify here that my objection is not moral either. It’s both philosophical and practical.
That’s both fascinating and unsurprising (at least to me). I’m very curious about one of those lies in particular. I was a TA for engineering classes in grad school and students would routinely tell me that “fictitious forces” didn’t exist. These are things like centrifugal and Coriolis forces. They’re real (i.e., useful) and very important if you’re in a rotating or otherwise non-inertial reference frame. Like, you know, on Earth.
(For the non-physicists: Einstein argued that gravity is a fictitious force. No one in my classroom would have claimed that gravity didn’t exist, but that’s the logical conclusion one must reach if one is A-OK with general relativity but also arguing that fictitious forces don’t exist).
I’d try to explain that these are “real” forces but, unlike non-pseudo-forces, depend on one’s frame of reference. They’re not “fake” or mistaken ideas. But these students would routinely push back and tell me that there was no such thing as centrifugal force. I’d invoke Newton’s third law (each force has an equal and opposite reaction) to try to connect centripetal to centrifugal, but it rarely worked. I didn’t teach dynamics, so it was never directly relevant to the coursework. If they insisted, I’d just let it go.
Do/did you ever get that one? In my experience, it was so common that I was convinced that legions of high-school physics teachers must be teaching their students that fictitious forces are the unicorns of physics: something everyone has heard of but which has never existed. Is this something you have to deal with, or are physics students better prepared than engineering students?
On that particular topic, I always make sure to get in the opening salvo, and state the “reality” of pseudo-forces as soon as they come up, including a statement that many misguided high school teachers say otherwise. I’ve never gotten much pushback on it, though I’m not sure if that’s because my opening salvo is effective enough, or because the high school teachers are getting better.
I will say, though, that trying to invoke Newton’s 3rd is the wrong tactic. Pseudo-forces are not subject to Newton’s 3rd, and it’s in fact precisely that lack that makes them pseudo. And even setting aside the issue that centrifugal and centripetal don’t both exist in the same reference frame, they’re certainly not a 3rd-law pair (though they can (with appropriate caveats) be regarded as a 2nd-law pair).
The implied assertion that people who are learned in pedagogy are all (or even generally) on board with the idea of “lies” to children is wrong.
And, as has been established by many in the thread, there is no need to distinguish between the words “push” and “pull” in trying to describe the meaning of “force” to 3rd graders. While “push” and “pull” can certainly be discussed, since they are common terms we use, it’s not too hard to quickly show that they are at best ambiguous when used to describe some easy to demonstrate situations. That can easily be used to segue into a discussion of a better way to discuss the idea of force, one that shows among other things the idea of equal and opposite reactions.
When you don’t “lie” to younger students, you have the benefit of not forcing a later instructor to clean up your misguided explanations. As a simple example, I offer one I always had to deal with as a high school Geometry teacher: a square is a rectangle. Because of the fact that, when they were young and learning their shapes, their elementary teachers insisted that the shape we call a “square” was NOT properly called a “rectangle” (a name the teachers associated with a non-square rectangle exclusively), when they arrived in my classroom, they had a hard job over-coming this incorrect classification. And it’s so totally not necessary to do that to them. The usual excuse is that at their young age, they cannot easily attach two words/concepts to the same form. But bi-lingual children do that all the time and suffer no difficulty, so that’s a poor justification for this “lie” being taught.
This is the crux of my practical objection. My moral objection is twofold:
First of all, the “lie-to-children” concept seems to be, at its core, a warmed-over version of Plato’s allegory of the cave–albeit one that advocates remaining in the cave through 12th grade. LTC is sort of an interesting idea, but was also stated more generally and effectively (IMHO) 2500 years ago.
Secondly, I’m not sure it’s the children who are being lied to; the teachers may be lying to themselves. There’s nothing wrong with presenting complex ideas in a simplified and accessible way, but, per my example of fictitious forces upthread, I can’t help wondering whether the teachers telling these noble lies understand the truth themselves. Some must, but I worry that many don’t.
My second objection is mostly resolved by LSLGuy’s idea of making it clear that this is a simplified version of the “real idea.”
Per Mangetout’s comment, the push-pull dichotomy could be a useful tool to prod students to explore the limits of the idea and find examples where it’s inadequate.
Ideally, they’d then be prompted (by the teacher or, better, necessity) to come up with a broader description that incorporates pushing, pulling and whatever they’ve discovered that doesn’t fit into the push/pull model. Not to repeat myself, but if a teacher could achieve that, they’d go beyond teaching scientific facts; they’d be teaching the scientific method. If that’s how it shakes out, that’s amazing pedagogy. I’m skeptical that this happens very often, though I’d love to be wrong about that.
Thanks for answering my question. And I fully approve of putting “reality” in quotes in this context, for what it’s worth.
You’re right; that was the wrong way to approach the subject. My hope was to try to cross reference frames (for the student, not formally), but all I did was convey that they might be a 3rd-law pair, an idea which garbles the “real” idea–precisely the behavior I find so irritating in teachers who are not me.
I was a TA and only there for the tuition remission and stipend so that I could finish grad school without taking on (additional) debt. I’ve taught enough to know that effective teaching is as much a skill to master as anything else that’s worth doing. I’m not great at it (witness the garbled explanation) and I admire those who are.
This is a great example. You don’t even need to go to multiple languages, though. Most young school-age children would agree that all robins are birds, but not all birds are robins. Even younger children would agree that their mothers are people, but certainly not all people are their mothers.
Asserting that young, school-age children can’t distinguish between the general case (rectangles) and the special case (squares) seems patronizing to me.
As an aside, teh google tells me that this relationship is described by linguists as the hyponym/hypernym relationship; this is essentially the same idea of a subset and a superset, of course.
A wing’s job is to push the aircraft up by pushing air down - simple Newtonian action-reaction. You can see this at work by watching a helicopter (whose rotor blades are nothing but wings moving in a circular path) hovering over water: vast quantities of air are being accelerated downward to keep the helicopter aloft. Though it’s less easy to see, exactly the same thing is happening when a fixed-wing airplane flies.
Yes, of course, there’s a pressure (prayer?) difference between the top and bottom of the wing. But it’s not particularly helpful to focus solely on that without noting what causes it: the large mass of air that’s being continuously deflected downward.
This is a good point. In explanations to grade-school students, it’s often clear that a plane in level flight generates enough lift to overcome its own mass times the force of gravity. But it’s rarely mentioned that a lot of that lift (and the a lot of downwash) comes from the angle of attack in addition all that Bernoulli goodness.
Once that’s established, we can talk about how that force gets generated. F=ma, so a helicopter’s rotors “blow” air downwards (lots of air mass, a little acceleration), a Harrier landing vertically uses direct thrust from rotary fans (relatively little mass but lots of acceleration) and an airplane wing can be thought of as a linear fan that achieves the same effect.
Naturally, the reality is more complicated (some acceleration of the air happens normal to gravity) but “downward deflection of air” is critical to how airplanes fly and a lot more intuitive to understand than the Bernoulli effect. But only the latter seems to figure in most explanations.
I’m also glad you mentioned the typo, because I missed the edit window. Some mis-typing of “pressure” got corrected to “prayer” on my phone. I’m going to avoid posting from my phone in the future.
Agree with this and the whole second page convo since.
The challenge goes far beyond physics. Which, as much as it pains us STEM folks to admit, many people don’t actually need much after high school.
The same nature of simplifications / lies get told in history, civics, economics, math, language studies, etc. And what is *not *taught at every step of the way is that these are profound simplifications, the barest glance at the real deep truths.
Which is how we get a populace that believes deeply in the profound accuracy of their embedded third grade = 8 year-old’s view of how human society operates.
With rather predictable consequences for our society.
What about words like “attract” and “repel”? Again, I think there’s a useful role for these words, inasmuch as they help students understand some very basic vectors–which is where we’re going at this young age. It’s not lying to describe a force as moving something either toward, or away from, the thing exerting the force, is it?
Two things:
I spend a lot of time overcoming this idea with students, in third grade. When I taught second grade, I spent a lot of time overcoming this idea. Your kids may have been taught correctly at elementary level, but the idea of a “rectangle” as a “non-square rectangle” is so thoroughly embedded in culture that it’s a hard idea to shake.
The shape we conventionally call a rectangle, i.e., a rectangle with nonequal adjacent sides, has no name as far as I know, but it’s a useful shape to be able to refer to. I suspect that people call it a rectangle for the same reason that people incorrectly call a drawing on a piece of paper two-dimensional: mathematically it’s untrue, but it’s really useful to have a term to refer to that state. The mathematical truth, that a square is a rectangle (and a parallelogram and a rhombus and a quadrilateral and even according to certain definitions a trapezoid), is less useful in everyday life than the conventional truth that “square” and “rectangle” refer to different things.