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15 August 2014

Mail Time: How much detailed attention to significant figures in AP Physics 1? (Answer: NONE.)

Mo here, a retired engineer teaching AP Cal AB & BC (for 13 yrs.), and now taking over the AP Physics program. 

I can’t seem to find clear information anywhere with regard to significant figures (SF) and how they’re handled by AP Physics readers. Being an engineer, you can imagine I tend to be very strict about SF, but I don’t want to overwhelm my students unnecessarily if the College Board does not stress it.

I would greatly appreciate any advice.

Hey, Mo... this is certainly a frequently asked question.  On one hand, don't harp on students about sig figs. If they put four rather than two sig figs, that's usually not in any way an issue.  However, if they're writing down every digit on their calculator, they're missing something, and they're possibly losing credit.  

The new exam does discuss experimental uncertainty.  Rather than stress rules of significant digits, instead you might talk in context about the uncertainty in a measurement.  Two measurements of 0.19 kg and 0.20 kg are equal, because by definition "0.19" means somewhere in bewteen 0.18 and 0.20.  However, two measurements of 0.191 g and 0.202 kg are NOT equal.  

But most importantly, your question about significant figures is nearly irrelevant for the new exams.  

Take a look at the released practice exam, and see how many questions require a calculated numerical answer.  Only one part of one free response question does:  that's 1(b), which asks for a calculation of external force on a cart based on a graph of experimental data.  Now look at the rubric for that part: of the four points awarded for this calculation, not a single one is awarded for the numerical answer.  

As with everything on the new exams, rote rules about arithmetic are far subordinate to the physical meaning of predictions and experimental results.  And calculation -- whatever the number of significant figures on the answer -- is far subordinate to explanation and description.  

Hope this helps...


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