Should Uncertainty Dictate Significant Figures in Physics Calculations?

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Discussion Overview

The discussion revolves around the application of significant figures in physics calculations, particularly in relation to how uncertainty should influence the number of significant figures in reported results. Participants explore the implications of their teacher's guidance on significant figures and uncertainties in their calculations.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions their teacher's instruction that the resultant answer should have the same number of significant figures as the uncertainty, noting discrepancies in their calculations.
  • Another participant suggests that the teacher might have meant to refer to the same number of decimal places instead of significant figures.
  • A participant expresses confusion about the relationship between significant figures and uncertainty, specifically how to round results based on their teacher's guidance.
  • One post provides a general rule of thumb for handling uncertainties in calculations, distinguishing between addition/subtraction and multiplication/division.
  • A later reply agrees with the initial participant's concern, emphasizing that results should reflect the scale of uncertainty in practical terms.

Areas of Agreement / Disagreement

Participants express differing interpretations of how significant figures should relate to uncertainty, with no consensus reached on the correct approach. Some participants agree on the confusion surrounding the teacher's instructions, while others propose alternative interpretations.

Contextual Notes

Participants highlight potential limitations in understanding the application of significant figures and uncertainties, particularly in the context of rounding rules and the definitions of significant figures.

jgens
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My physics teacher said that our resultant answer should have the same number of sig. figs. as our uncertainty (he allows 2 sig. figs. at most); however, having done some of the problems, I get answers - when not rounded - such as 24.0 +- 0.1m, which when rounded to 1 sig. fig. results in 20 +- 0.1m which is not even near the minimum possible value. Did he perhaps mean to say the same number of decimal places?

Thanks!
 
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Did he perhaps mean to say the same number of decimal places?
Check out this http://en.wikipedia.org/wiki/Significant_figures"
 
Last edited by a moderator:
Having read the wikipedia page, I still don't think I have an answer. 24.0 has three sig. figs. while 0.1 has only one; therefore, if I round to 1 sig. fig. like my teacher said - result should have as many sig. figs. as uncertainty - I would get 20 +- 0.1. Is this correct?
 
The simplified rule of thumb for uncertainties is:

When you add or subtract, round the final result to the least number of decimal places.

When you multiply or divide, round the final result to the least number of significant figures.
 
jgens said:
My physics teacher said that our resultant answer should have the same number of sig. figs. as our uncertainty (he allows 2 sig. figs. at most); however, having done some of the problems, I get answers - when not rounded - such as 24.0 +- 0.1m, which when rounded to 1 sig. fig. results in 20 +- 0.1m which is not even near the minimum possible value. Did he perhaps mean to say the same number of decimal places?

You're correct. If your experiment has uncertainty of tens of centimetres, then report your result to tens of centimetres.
 

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