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



On 07/26/2013 07:31 PM, Anthony Lapinski wrote:

I can comment on most of these questions. Some might seem underspecified,
but I listed them in general terms without all the specifics. Didn't mean
to confuse anyone. I wonder what questions teachers on this list ask their
students.

#1: Might have been better to ask what is the acceleration at any time

True for one definition of acceleration ... but not the other!

(10 > m/s2 downward as gravity is constant).

The ball decelerates on the way up, and accelerates on the
way down. Maybe this isn't the answer you had in mind, but
how am I supposed to read your mind????

I find arbitrary definitions to be very counterintuitive.

#2: Acceleration on incline depends on angle, so concave hill wins since
ball has highest acceleration at start.

Wrong physics. Initial acceleration does not fully determine the
outcome. A long initial vertical drop gives maximal acceleration,
but is not the minimum-time strategy.

It's a famous problem. It is a calculus of variations problem.
http://en.wikipedia.org/wiki/Brachistochrone_curve
Just saying "highest acceleration at start" is not a valid analysis.
The correct analysis is more complicated than that. Roughly speaking,
there has to be a tradeoff between highest average speed and shortest
path. If you emphasize the initial speed only, you are no more correct
than the student who emphasizes path-length only, and chooses the
straight (i.e. shortest) path.

Just because you know the desired answer and can throw together
some words that make that answer plausible does *not* mean that
your reasoning is correct ... or that students should be able to
follow your so-called reasoning.

Just because your plausible mnemonic verbiage is simple does not
mean that the correct analysis is equally simple.

I find it entirely likely that your students are not as smart as
Jean Bernoulli. However, that does not mean that they are ignorant
of basic physics principles. Stumping them with this problem does
not prove they lack reasonable reasoning skills.

I am quite aware that when the path-curvature is *near* zero, it
is possible to do a qualitative analysis that is less involved
than the full calculus-of-variations formalism ... but it is still
quite a bit trickier than merely saying "highest acceleration at
start".

#3: Car acceleration is constant. Balloon moves forward as it has less
inertia (like bubble in level with cart/pulley). These are similar.

The original problem statement forgot to specify that the
acceleration was constant. In the form presented to this
list, which is all I have to go on, the original problem
statement was ill-posed.

#5: Both teams pull with same force (related to #4 -- like a two-person
game). Winner determined by friction on ground. Can show this with
two-person game with different shoes, or one person on skateboard, etc.

You've obviously never been in a real tug-of-war.

In reality, the winner /might/ be determined by ground
friction ... or it might not. Consider the two-person
game with their feet braced against immovable logs.
Victory might go to the person with more initial arm
strength, or to the person with the greatest finger
strength, or to the person with the most endurance.

No wonder students find physics counterintuitive. I find
wrong answers to be very counterintuitive.

#13 "Three bulbs in series" is not vague, even without a diagram. Not sure
what the confusion is here. Kids just think electrons leave the negative
battery terminal, so the bulb closest to this should receive current
first. They forget what is happening at the other terminal and where the
electrons are in the circuit.

Wrong physics. Electrons have got essentially nothing to
do with which bulb lights receives current first. Any of
the three bulbs could be the first to receive the signal,
depending on the /geometry/ of the circuit, which is quite
impossible to determine from the original word-picture
alone, without a diagram or a verrrrry much more detailed
word-picture.

If it's not obvious what I'm talking about, re-implement
the circuit using coaxial cables instead of bare wires.
The speed of propagation in the coax is well characterized.
The bare wires are not much different in principle, just
less-well characterized. There are logarithmic factors
that change the details but not the basic principles.
The sign of the charge carriers does not enter into the
propagation equations, not at any frequency relevant to
the introductory course.

Whichever bulb is closest to the switch will receive the
signal first. Determining which bulb this is, based on
the original statement, is purely an ESP exercise.

I find wrong answers to be very counterintuitive.

#14: This is asked after I teach about reflection, refraction,
diffraction, and interference.

OK, I'll bite. Is reflection the right answer to the
original question? Is refraction the right answer?
Is diffraction the right answer? Is interference the
right answer? I hope none of the above.

#15: Mirror is fixed (can't be wiggled).

The original problem statement didn't specify that, and
didn't specify a whole lot of other crucial details.
The original problem was very ill-posed.

#16: Kids don't draw ray diagram.

Well, you should teach them to draw the diagram, before
asking them questions about what happens.

Ditto for circuits. Teach them to draw the diagram.

To say the same thing the other way: I see no point
in throwing non-swimmers into the deep end of the
pool. I suggest the the goal is to make the right
answer become intuitive. Asking questions when they
don't have the tools to find the answer is not the
right way to go about it. You can quote the PER
literature about peer instruction; I'll quote some
other literature about the blind leading the blind.