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Re: [Phys-l] Student Misconceptions

Philip,
I think it's more important for high schoolers (and all of us) to honestly look for sources of error before taking measurements and getting to the nitty-gritty of calculating errors. If you don't know what they are, your calculations of error will be meaningless. Say you identified some and came up with +-1% error in your measurement. If you fail to identify an error that is 5% by itself, that 1% is misleading (consider the magnetic monopole fiasco of 20 years ago).

NEVER let them say "human error." If it is something the experimenter does, force them to be specific, e.g., failure to account for parallax in reading the meter stick, using the inches scale but recording cm (arghhh!!!). Force them to identify errors inherent to the method (can't be eliminated but may be systematic) vs errors that are controllable (checking the zero point of the micrometer or ruler). Have them recognize the limitations of their instrumentation.

Sources of error in dropping picket fences:
Air resistance - very small
Uncertainty in spacing of the bands - very small
Tilt of the picket fence - large

One could ask "Does the release mechanism affect the slope?" and have 5 different students drop the fence from a fixed height using their fingers, each 5 times, and compare the standard deviations of the slopes obtained. Then have a mechanical release system and do the same.

Then use the mechanical release to drop (5X) from 5 different heights and see if that has a big effect. (there's nothing magic about 5, but I like to do more than 3. If you have time, do more).

Finally, use the mechanical release and drop the gate tilted slightly (by suspending at near one of the corners).

Bill

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller
Sent: Friday, September 30, 2011 10:50 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Student Misconceptions

BC -- the videographer had already spent his whole demo budget getting
to the moon! I was referring to the video taken during the Apollo 15
mission.

As for how I handle error in my high school physics classes...well I
know the list won't approve, but I am going to give the honest answer
(and one that I believe is actually true for many high school classes):
I don't. There are several reasons for this. One is that is seems
boring and I'd feel bad cutting other material to include this or even
delaying. It's hard enough to climb past kinematics and vectors. I
don't want to get bogged down in error analysis. But another reason
comes from a rule that following this list has led me to. I call it
the "first-do-no-harm" rule: if I don't know how to teach something
right, at least try not to teach it wrong.

You'd be amazed how following this rule can shorten a presentation.
For example, when I teach thermo to my AP classes, the topic of entropy
comes up. What can I say that isn't wrong? "Entropy is a number
that..." Ok, that's as far as I get. (Surprisingly, that's more than
they need to know for their AP exam.)

So what correct things can I say about the error when finding the
acceleration due to gravity from the slope of a velocity graph
collected with a falling picket fence and photogate? _________



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet
Sent: Friday, September 30, 2011 12:57 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Student Misconceptions


On 2011, Sep 29, , at 13:09, Philip Keller wrote:

But we absolutely DO discuss the fact that the heavier ones do fall
a LITTLE
faster...the equipment is good enough to pick that up.. but these are
complete
beginners and I have to choose my conceptual battles one at a time.
So on
this day, I wave my hands at air resistance and show the video of the
hammer*** and feather :)
________________________________________


Aaa a practical instructor. How do you "handle" error?


*** The videographer couldn't afford a guinea?

bc corrected.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l