Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Significant figures and uncertainty

When asked to estimate the number of steps required to walk the length of the Appalachian Trail,

Which student's answer is more sensible? Why?

Scientists pay careful attention to the way they write numerical values. They use only as many digits as they can justify, a practice we describe as providing only the significant digits. There is a connection between the number of significant digits in a written quantity and the uncertainty in that quantity.

Is there a connection between +/- uncertainties and significant figures? Yes, indeed. For example, how many students at RIT are on campus today?



      N  =  12,439  +/-   338

    

This number could be as big as 12,777, or as small as 12,101.



          12,101   <   N    <    12,777

    

The first digit -- 1 -- doesn't change. The second digit -- 2 -- doesn't change. But the third digit could be 1, or 2, or 3, or 4, or 5, or 6, or 7. So, if we write the number N using significant digits, we should write



      N  =  12,000             

    

since these are the only digits we are SURE are correct.


Significant figures are like viruses: they infect any numbers they touch. If you combine two quantities, then the number of significant digits in the result is equal to the SMALLER of the number of significant figures in the input values.

For example, if you add two values, one with 5 significant figures and another with 1, then the result will have only 1 significant figure.

Can you write the answers to the following combinations?



         83 meters/sec  *   1.459 seconds        =      



         0.04 kg        /   250 cubic meters     =



         $12 / hour     *   2.000 x 10^3 hours   =


Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.