Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] thoughts for future physics regents.



First, a parable: Once upon a time there was a student driver who was
erratic, wandering all over the lane, with occasional excursions outside
the lane toward the right.

The bad driving instructor pounces on the excursion and says "See that?
You need to drive farther over to the left so that doesn't happen."

Of course the driver is still erratic, and the next excursion will go
across the lane boundary to the left.

The second-best moral of this story is this: Don't fixate on the symptom
du jour. Address the underlying problem, not the superficial symptom. In
this scenario, the erratic steering is a deeper issue than any particular
excursion.

An even better moral is this: Solve the whole problem. Smooth, non-erratic
steering is only one of dozens of skills the driver needs to have. The
excursion is two steps removed from the big picture, and the erratic steering
is one step removed from the big picture. The objective is to get the
student to /understand/ the task, to the point where he not only knows the
techniques but knows the standards and the reasons behind the standards,
and can critique his own performance ... et cetera ..........


On 06/16/2011 08:20 PM, Michael Barr wrote:
in the past i have always told students that if they have any doubts about a
units that should use dimensional analysis and leave the units as it. For
example kgm2/s2 instead of Joule. After grading the regents today I am
re-thinking this strategy. I find that the type of student that might not
be sure about what the unit should be is also less likely to properly do
dimensional analysis. Next year, I am just going to make sure that students
look at what is being asked for and use the corresponding unit. I had a few
one or two kids leave kgm/s2 leaving out the square after the meter which
would make it a Joule. The regents takes off a point out of two points for
this type of unit mistake. any thoughts on ways to make sure kids do not
mess up on units.

The suggestion of writing J instead of kg m^2 / s^2 is at least two steps
removed from the big picture. I don't know exactly what the exam question
asked, but based on a plausible guess I would say:

-- The student should have known before starting the problem that the
answer would have dimensions of energy.

-- Even if the student didn't know that, carrying out the calculation
carefully would have resulted in an answer with the correct dimensions
and indeed the correct units.

-- The student should have known by heart that J = kg m^2 /s^2

-- Even if the student didn't know that, he should have been able to
re-derive that fact in N different ways.

-- The student should have been told every year for the last 11 years:
"check your work". Along the same lines: "Any problem worth doing
is worth doing two different ways" so as to improve the reliability
of the answer.

I reckon that the student had many bites at this apple. By the time
we see the symptom -- wrong units in the answer -- we are many steps
removed from the underlying big-picture issue ... namely /understanding/
the physics.

There's time pressure on the Regents exam, which may cause some students
to make dumb mistakes. The obvious suggestion therefore is to make
sure the students can work quickly without getting sloppy. Checking the
units at every step of the calculation is a good practice, and a skilled
practitioner can do this very quickly. It is a skill that can be learned.

A major part of effective learning is this: The learner should turn over
each new idea in his mind, to see how it is connected to other things he
knows. You can encourage this, and check for it.

Specific suggestion:

Question #1: Suppose you forgot how to express the joule in terms of the
SI base units (kilogram, meter, second) and you wanted to re-derive the
answer based on other things you know. How would you do it?

Question #2: Same as question #1, but re-derive it in a /different/ way.
How would you do it?

My answer #1: Energy = force times distance. F=ma. Therefore energy
= kg * m/s/s * m

My answer #2: KE = 1/2 m v^2.

Other equally-good answers abound. If the students understand the physics,
they should be able to come up with answers like this, very quickly.
-- If they can do this, you don't need to tell them to write the answer
in terms of J instead of kg m^2 / s^2.
-- Conversely, if they can't do this, telling them to write the answer in
J isn't going to help.

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

Another suggestion: Change the rules of the Regents' to lessen or eliminate
the time pressure. Some states have exams where students can take as long
as they want.

I'm serious about this. There is no advantage in getting the wrong answer
quickly. Also, in the real world, there is little advantage to answering
trivial questions quickly. When I'm hiring somebody, I do *not* give
precedence to the guy who comes up with the first answer. I'm much more
interested in the guy who comes up with the best answer, even if it takes
longer.

To be sure, fast *and* clever is best, but I will settle for clever without
fast. I will not settle for fast without clever.