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Re: [Phys-L] The Make-Believe World of Real-World Physics



On 07/17/2013 07:25 PM, Anthony Lapinski wrote:

For the helium balloon problem, I first do a related demo in class. It's a
pulley, cart, and hanging mass, and I attach a horizontal level to the
cart. When the mass is released, the cart accelerates to the right. Which
way does the bubble move? This is about inertia.

OK, that's one experiment. Here's another: Just now I grabbed
a brownie pan, approximately 40 x 30 x 5 cm. The details don't
matter, over a wide range. I put about 2 cm of water in the bottom.
I grabbed a slab of hardwood, approximately 30 x 24 x 2 cm, and
floated it in the water, initially centered over the middle of
the pan. Then I jerked the pan hard to the right. Now, relative
to the pan, which way did the wood move?

So the helium balloon question is similar.

Similar to what?
-- Similar to the floating-bubble demo?
-- Similar to the floating-wood demo?

I'm a little frustrated here, because warned everybody in a previous
message that the physics permits two different answers. Indeed the
physics /demands/ two different answers.

The claim that the balloon-in-car experiment "must" be like the
floating-bubble experiment is not merely counterintuitive, it is
counterfactual. It is wrong physics. Much depends on timescales
and other details of the experimental conditions.

The balloon-in-car question is what I call a telepathy exam,
because it requires students to read your mind, to figure out
what experimental conditions you had in mind. The first law
of motion demands one answer, while the equivalence principle
demands another, and there is no way of knowing which law you
will have in mind on any given day.
For the ball thrown straight upward, I want the students to know that the
Earth's gravity is a constant 10 m/s2 downward.

Again, that's in the category of telepathy exam. You know what
you want, but the students have no way of knowing what you want.
All they have to go on is the wording of the question, and the
question is ambiguous, because the English language is ambiguous.
There are two definitions of "acceleration". Sooner or later
they will learn that you are fishing for one definition to the
exclusion of the other ... but this is a telepathy issue and a
vocabulary issue, not a physics issue.

For the ISS question, the spacecraft is accelerating so a force (due to
gravity) must be acting.

Sorry, that's just wrong physics. According to common sense, and
according to any modern (post-1907) understanding of physics, and
according to the astronauts themselves, objects aboard the spacecraft
are weightless relative to the comoving frame. Given
W = 0
and
W = m g
and since the mass is nonzero, we can conclude with absolute confidence
that g = 0. That is to say, there is negligible gravity in this frame.
The assertion that gravity "must" be acting is not merely counterintuitive
... it is just plain wrong.

You are free to choose a frame in which g is nonzero, but that is
a choice, not a law of physics, and other folks are free to choose
differently.

Again I am a little frustrated, because I warned everybody in a
previous message that Newton's law of universal gravitation gives
us a value for δg but not for g itself.

I don't deceive/trap my students

Maybe the three questions discussed above are not /intended/ to
be deceptive, but they are deceptive nevertheless.

the questions I ask relate to what I am teaching.

But you are teaching them stuff that conflicts with their prior
conceptions ... in these three cases, not prior misconceptions
but correct, well-attested prior conceptions.

If you teach them that the laws of physics contradict each other
-- for instance that the equivalence principle contradicts the
first law of motion -- then it's no wonder they find the subject
to be counterintuitive.

Fortunately, this is a fixable problem. The laws of physics
_when correctly stated_ do not contradict each other.

I don't want physics to be scary and confusing

Good!

most textbooks already do this.

Agreed, indeed they do ... some of them much worse than others.

================================

I think we have here the beginnings of what may be a very
fruitful line of inquiry.

I will check out your "deceptive" questions

Exactly. I reckon that whenever students predictably find
a particular question to be "counterintuitive" or otherwise
troublesome, we should enquire very closely to see where
the students are coming from. Sometimes the students have
a good, solid, non-crazy reason for interpreting the question
differently from what was intended.

Sometimes they can readily articulate their reasoning, but
sometimes not, in which case it may take a tremendous amount
of fishing to figure out where they're coming from.

I get a lot of mileage out of assuming the students are smart.
If they're stuck on something, there's probably a good reason
why. There's probably some evidence out there that is propping
up the "wrong" conception ... or possibly the "wrong" conception
isn't as wrong as I thought it was. The trick is to find the
conflicting evidence and either disprove it or encapsulate it.
Encapsulation is useful when the conflicting evidence is valid
in one situation, but we are interested in another situation.
Encapsulation requires figuring out how to distinguish one situation
from the other. For example, consider scalar acceleration (as
opposed to vector acceleration) ... that's something that took
me maybe 100 hours to encapsulate the first time. That's 100
hours of seriously hard work. First I had to notice that there
was an inconsistency, and then I had to find a way to resolve
the inconsistency, to reconcile the way people /claim/ to define
the word with how they /actually use/ the word.

===================

These three examples provide excellent illustrations of the point
that Mazur was making, as discussed way back at the beginning of
this thread. Student have a lot of good, solid reasons for thinking
that the physics they are being taught is wrong or irrelevant or
both.

Fortunately, this is a fixable problem.