That would be shear forces!!! Don’t you guys look at the stress tensor?
Pulling generally means using some form of tension <––> instead of compression --><-- on the elements. Sorry for the ascii diagram. And the difference in direction has real effects as materials behave differently under different forces. Brittle materials, for example, don’t deal with tension as well as compression. Liquids tend to deform readily under shear forces but might be perfectly incompressible in pure compression. Etc.
Heh–bump, because I was thinking of starting this thread last week, but it occurred to me to search to see if I’d already done so. Almost a year ago, imagine that.
So, folks with physics backgrounds: how would YOU best dramatize forces to kids? Keep in mind we’re gonna start simple: kids I’m teaching don’t understand that speed=distance over time, not even intuitively. I’d love to get them to the point where they can use something like vectors to explain why it’s easier to run downhill than uphill, but they aren’t going to be drawing three-dimensional vector diagrams most likely.
Hm. Tug of war for Newton’s third law/friction/inertia. Have everyone pull on one side of the rope. Easy. Now have half on each end and pull. Not so easy. Tie something heavy (a block) to one end - try to pull on dry grass, then wet it and try to pull the block.
Maybe the Galileo experiment? Make a ramp from wood or such. Put a wheel at the top, let it roll down the ramp and then the length of a hallway. Repeat with ramp at different heights. Ideally the ramp slows things enough you can count time out loud without friction skewing the numbers too much.
For differences between speed, momentum, acceleration and kinetic energy, some things that are the same size & shape but different density (tennis ball, tennis ball opened up and filled with sand). Not sure what to do with them after that.
Something involving a dog. Any demonstrations with a doggy assistant should be memorable.
I agree with Chronos and others that the push/pull distinction is both arbitrary and not especially useful. But I’d go beyond that and say it’s a false dichotomy.
As a kid, I’d always heard that Bernoulli’s principle is what allows planes to fly. The curved upper surface of the wing, I’d been told, forces the air on that surface to flow faster than the air below and therefore its pressure is lower, so the plane is sucked upwards.
Well, not quite, or inverted flight would be impossible. In reality, both the airfoil and its angle of attack matter tremendously. I used to hear debates about whether the plane was sucked up by Bernoulli’s principle or pushed up from the higher pressure below the wing.
In reality, all that matters is that there’s a prayer differential between the bottom of the wing and the top of the wing. That differential is created by a number of factors, including Bernoulli.
The proble, is that the the push-pull (or, here, suck-blow) model makes people think that there’s only one explanation, when in reality, things are more complex than that.
Man, some people here really have a problem with lies-to-children, don’t they?
I don’t. So, shot put is a push. Archery is a pull for people but a push for the bow. Rowing and swimming are pushes, IMO.
To me, if I was explaining it to a third grader (so, 8 y.o.), I’d say a push is where stuff moves away from you (the shot, the arrow), or you move away from stuff (the water), and a pull is where stuff moves towards you (the bowstring), or you move towards stuff (tug-of-war).
Compression vs extension can come then (possibly illustrated with real springs of both types, and magnets with clearly-marked poles)
I tend to agree with the false dichotomy side.
From a pedagogical point of view - what is the goal of even mentioning push versus pull? It doesn’t seem to have a purpose except to perhaps confuse the teaching of Newton 3 later.
Indeed, if you have time to worry about trying to talk about push and pull, maybe it is time to talk Newton 3.
I think Newton’s Laws may be a leeetle advanced for the average 8 year old.
How about a lever? You can introduce the concept of forces and distances. It’s abstract enough to challenge them but also they can see it with their eyes.
In sports the player is generally pulling and pushing at the same time. Even in events like weightlifting and powerlifting, the athlete has to push against the ground to generate a pull. Chin-ups and pull-ups are the only “event” I can think of where there is no push.
If “for every action there is an equal and opposite reaction” I can’t think of any pure pull, or pure push for that matter.
In a word: gravity. Getting students to understand that sometimes the instigator of a force moves something closer (gravity), and other times the instigator of a force moves something further away (a baseball bat), but in both cases you’re dealing with a force, is a difficult enough concept for kids.
If you’re talking about Bernoulli’s principle, you may know some brilliant third graders, but I need to design lessons that help kids who don’t really understand gravity. Thursday I blew their minds by explaining that everything with mass had gravity; Friday I showed them a video of a feather and a bowling ball falling in a vacuum chamber. This is about where the kids are.
I like the idea of tug-of-war as a way of explaining force–thanks!
I found air hockey a great way to talk to my 9yo about friction, BTW…
You’d be surprised; eight-year-olds are subject to them just like everybody else.
If you go down the tug-of-war route, the underlying concept that seems most valuable to me is that things move (properly they accelerate) NOT due to force. But due to *net *force.
Have 3 kids on each end of the rope pull so the rope is stationary. Not moving = no net force. Kids agree they’re exerting “effort” which we’re not going to define further.
Tie one end of rope to solid object. Have 3 kids pull. Same effort, no motion, no net force. So therefore the solid object was pulling back just as much as the other three kids were in the previous experiment.
Have kids drop the rope. Still tied to solid object. Rope still not moving = no net force. So somehow the solid object stopped pulling just when the kids did.
Clean up mess from many small minds asploding.
Which leads naturally to ideas like "the reason stuff doesn’t fall through a table or through the floor is the table or floor are pushing up just as hard as gravity is pulling down. The experiment is kids jumping off a stool or something. Gravity pulls them down, then they stop descending when their feet touch the floor. Why the stop? Floor pushes up.
You could extend the experiment by having them jump to land on something smooshable, say lumps of clay or grapes. Which demonstrates that forces can be bigger or smaller than the strength of something. Kids will love jumping off a stool to crush grapes.
It’s not just people here. You know who really can’t stand the lies-to-children concept? Children.
Seriously. This is why authors like Roald Dahl and Lemony Snicket are so popular with curious children: they’re dark enough that children know they’re not being lied to about the world somehow having a benevolent nature.
I’m not suggesting that third graders need to use integral and derivatives to study physics; virtually no adult physicists were doing integrals at the age of eight, so that’s a bit of a strawman, no?
If you want to examine Newtonian mechanics for third graders, why not launch a few model rockets? Have them speculate about what make rockets go and then test their hypotheses. Many people will guess that rockets go by pushing their exhaust against the surrounding air. On an intuitive level, that’s not unreasonable. But then you can ask how rockets can work in space where there is no air.
In third grade, I wondered whether it was the existence of hot exhaust that created thrust from jet engines. So, with a oarent’s supervision, I made a paper airplane and taped a lit cigarette to the back of the plane. When it didn’t fly any farther than a non-cigarette airplane (actually, it was worse to to the shift in the center of gravity) I learned two things:
- Heat alone doesn’t produce thrust, and
- I can guess at what works and then test to see if I guessed right.
Lesson (2) is more formally known as “the scientific method,” and that was how I learned something incredibly useful by taping a cigarette to a paper airplane.
I’m being a bit flippant here, but I have several serious objections to the “lies-to-children” concept. First, it involves lying. It’s entirely possible to simplify without falsifying. To paraphrase some smart guy, explanations should be as simple as possible, but not simpler.
Secondly, these aren’t just lies to children. The adults believe these oversimplified explanations as well, and are therefore unprepared to delve more deeply into these subjects. When adults believe the oversimplifications they give to children, they’re transmitting not knowledge but ignorance.
Finally, children are exquisitely sensitive detectors of adult lies. It’s not that they can’t be lied to, but if they have enough knowledge and context to examine an adult’s answer critically, they’re pretty great at detecting bullshit. And adults who model making up answers (especially when they don’t know the real answer themselves) are implying that it’s somehow shameful to not know the answer to a question. Knowing the right answer is a whole lot less interesting than asking good questions and following them where they lead.
An adult who responds, “I’m not sure why; let’s find out!” is not only encouraging the kid to be inquisitive and to find answers rather than making them up, but also being fundamentally honest with the kid. That’s worth a lot.
I must point out that we’ve now got two at least physicists and one mechanical engineer (me) saying that this push/pull distinction is arbitrary and, worse, doesn’t tell the kids anything useful. This isn’t an appeal to authority; the OP asked how people with physics backgrounds would approach this, and most or all of us are saying the push/pull distinction doesn’t really go anywhere.
The push vs pull thing is actually a good way to get people (esp children) thinking about forces. It’s fertile ground for debate and discussion, which is great for learning.
For example, a person rowing a boat is pulling on the oars
… except he’s not - he’s pushing on the side of the oars that are furthest from him
… except his muscles are organs that only really have a ‘pull’ action
If I operate the ball launching plunger of a pinball machine, I’m pulling it
except I’m not - I’m pushing on the rear surface of the knob
my fingers are pushing against the handle to provide grip
but this pushing force is achieved by muscles that pull tendons in my arm that close my fingers
When I move the handle of the plunger, the rest of the rod moves with it because it is being pulled.
The ball, resting on top of the plunger, rolls down to follow it because gravity is acting on it - is that a push or a pull?
These nitpicks and statements of exception are not a distraction, they are a vital thinking process that should be embraced and encouraged.
You know who I don’t always trust to know what’s best for them? Children.
I suggest you read the actual books about the concept, then, if that’s your take-away…they are lies, but not lying. There’s a difference, IMO.
And I must point out that neither of those professions is qualified in pedagogy.
I see the moral point in objecting to lies-to-children. So instead, start every explanation with “This is a simplified description of what’s really going on. Some day you’ll get into the grittier details. But not today.” If you admit that, then it’s no longer a lie and the moral objection falls away.
Then you’re just left with the practical pedagogical question of which simplifications are A) best understandable from where the student is now, and B) are most “upward compatible” in the sense that we won’t have to tear out too much structure to add in more concepts and details next time around.
In fact IMO the world would be a better place if everybody had totally internalized the idea from school age on up to now that every explanation of everything everywhere is simplified to some degree. And what matters is the quality = honesty of that simplification.
I think what worries a lot of the posters here is the image of what might come next. It brings back nightmare images of a time gone past - where children get little tests with things like:
“name one push force and one pull force”
“which of these is a push force and which is a pull”
Now this builds a false dichotomy. The idea that the kids need to get the idea that the force can come in either form is great, but under no circumstances should it develop into a taxonomy. The core trick seems to be the need to bridge the idea that all these forces are unified in some manner. And in particular forces where one can see the mechanical componentry (ie arms and legs and ropes) is just the same as invisible agents - gravity pulling things down, and magnets doing their thing - is really good.
One feels that the kids are so close to being shown a significant number of really core ideas that it just hurts to think about.
One of the really odd things is that the kids are taught the equivalence principle very early - these kids have, we see, already been taught it. Yet it is one of the deepest questions in physics. Almost none of them will ever see it again. But it is taught religiously, by rote, and I doubt most of the teachers have much clue what it is either. Yet they worry they can’t talk about much simpler concepts like Newtons laws.