Teach Fzx Tuesday- THREE Significant Digits

The AP Physics test evaluators care not a whit about significant digits.  If the answer is 3.62 m/s and you show some basic work and write "4 m/s" - full credit.

Our physics and chemistry textbooks are inconsistent with their own rules in problems and answers.  They promptly drop significant digits after introducing the topic.  From chapter 20 in the physics textbook shown below we can see that they truncate or round depending upon their mood.

     

A civil engineer builds a bridge that is capable of supporting 90 tons.  Since it has to survive hot and cold, rust and salt, the rated limit is 10 tons so after 30 years it'll still be standing.

Sure, he'll have to put together some solid blueprints with good digits and details, but he's an engineer and he has to overbuild it.  I'm not an engineer and almost none of my students will be so why should I put yet another obstacle in the way of my students learning or liking physics?

Many modern devices have digital readouts and only require simple calibration.  They give us the proper values and we don't even have to think about recording a measurement to the limit of the instrument.

     

Our calculators, on the other hand, lead students to believe that every answer has 11 digits.  Some students copy them all.  Silly.

From the 17th century til the last decades of the 20th century, the slide rule was king - it ruled the calculation world with only three digits.  Someone should use this man-sized one to teach me how to use it.  Then I could strap it to my back and calculate my way through an EMP and the ensuing apocalypse.

    

Digits are important for chemists or pharmacists or particle physicists.  They become important when I build a deck or put an addition on my house.  But high school students have more important things to worry about.  They'll learn the digits and the algebra and the calculus when they need to.

Let's minimize those sig figs and focus on the big picture.  I have my students report their answers to 3 digits unless it's a lab and the results are questionable - then we use 2 or even 1.  It's messy but it's realistic.

"We made it to the moon with a slide rule and 3 digits" so why waste precious time on significant digits now?  (Dennis Dudley)