Re: [Phys-l] [PTSOS] Significant Figures

["LaMontagne, Bob" <RLAMONT@providence.edu>]

Reason number  73 why students hate physics and physics teachers:

         Spend days covering significant figures instead of doing some physics.

Over the years I have learned to leave the rigid and often incorrect rules of significant figures to the chemistry teachers and spend my class time discussing physics. Worrying about whether a student has 2 pencils or 2.00 pencils is not a good way to engage students.

Bob at PC


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From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet [bernardcleyet@redshift.com]
Sent: Sunday, March 20, 2011 2:19 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] [PTSOS] Significant Figures

On 2011, Mar 19, , at 10:22, jonathanhanna wrote:

> Hi Everyone,
>
> I teach physics primarily to 9th graders, so I'm not very picky when it comes to significant figures. As long as a student does not write every single digit on the calculator when solving a problem I'm happy!
>
> However, there is an issue during lab that really bugs me. The other day we were using digital multi-meters to measure the voltage drops in a circuit. If the students get a measurement of "1.56" they will write down "1.56". However, if the students get a measurement of "2.00" they will write down "2"! I have the same issue with analog measurements when using metric rulers. I've tried every trick in the book I can think of to convince them that "2" and "2.00" do not mean the same thing, but it's not sticking. One student commented that "2" and "2.00" is the same thing since their math teacher says so!
>
> Sure, I could just tell them to write down their measurements to the hundredths place each time, but I want there to be meaning behind why we write down "2.00" as oppose to "2" alone. Any suggestions?

I have written to PTSOS (and the NCNAAPT list) at least twice to forget sig. figs. and use +/- estimated error.  and refer to JD's sig. fig. exposition web page(s).

Here are replies:

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I explained significant figures to students in terms of birthdays.


If I say "I am 41 years and 0 days old" - that is very different
from saying "I am 41 years old."  I asked them to discuss the
difference between these two statements.  Then I'd ask them
if there was a difference between someone giving them directions
if they said "From this spot, it's a mile down the road." compared
to "From this spot, it is 1.000 miles down the road."
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What you need to ponder, I suppose, is how important is it that students know the deep meaning behind the difference between 2 and 2.00. You get a fixed number of hours to teach them, so if you want to pursue the rules and philosophy of significant figures, you can create a whole unit around it. But you'll have to leave other things out. Physics things.

Speaking only for myself, I don't see that topic worthy of pushing much physics curriculum out of my school year.

I teach some bare bones minima on the topic and revisit the conventions from time to time when discussing calculated results. I doc points from lab reports that show too many or too few. Not many, but they get tense at the difference between a 79 and an 80.

Remember, these are 9th-graders. Far from fully formed*. They will have repeated experiences in other science classes in HS and college before sig figs have sig meaning to them. And if you let it go completely, nearly all the sky will remain aloft. (I stopped teaching the ins and outs of vectors years ago, and I was sure the sky would fall. It did not.)
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bc about to give up.

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