How many sig figs do i use when making conversions

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Discussion Overview

The discussion revolves around the application of significant figures in conversion problems, specifically when converting measurements such as miles to kilometers. Participants explore how to determine the appropriate number of significant figures based on the precision of the initial measurements and the nature of conversion factors.

Discussion Character

  • Homework-related
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant asks how to calculate significant figures when converting measurements, using the example of converting 12 miles to kilometers.
  • Another participant suggests that the final result should retain the same number of significant figures as the least precise measurement, which in this case is "12" with two significant figures, leading to "19 km".
  • A later reply clarifies that conversion factors do apply and that the number of significant figures in the result is limited by the least precise measurement, which is the "12 miles".
  • Some participants note that certain conversion factors are exact and can be considered to have an infinite number of significant figures, providing examples such as "1 cm = 0.01 m".
  • There is a discussion about the ambiguity of interpreting "12 miles" as either a measured value with two significant figures or an exact integer with infinite significant figures.
  • One participant expresses that they would interpret "12 miles" as having two significant figures, while another agrees, stating that it is unlikely to know a distance exactly as 12 miles.
  • Another participant points out that the interpretation of the question could lead to different answers, highlighting the importance of context in determining significant figures.
  • One participant introduces a different example involving fathoms, suggesting that the number of significant figures can vary based on the context of the problem.

Areas of Agreement / Disagreement

Participants generally agree that "12 miles" has two significant figures, but there is disagreement regarding the interpretation of whether it is a measured value or an exact number. The discussion remains unresolved on how to handle ambiguous cases.

Contextual Notes

The discussion highlights the limitations of significant figure rules based on the context of measurements and conversions, as well as the potential ambiguity in interpreting numerical values.

kevinlikesphysics
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Homework Statement



I have to do a bunch of problems such as converting meter to km and cups to mililiters etc. How do i calculate sig figures for these problems

for example

i did 12 miles to kilometers

12 miles x 1.609 km/1mile = 19.308 km = 19 km ? do i leave the sig figs at 19.308 or do i change the answer to 19 to make the answer only 2 signigicant figures? because 12 miles is only 12 sig figs i made the answer two is that correct...thanks



Homework Equations





The Attempt at a Solution

 
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How many significant figures in your final result depends on the the starting accuracy of your information. In your case, you began with two figures, "12". You should keep TWO significant figures for your result, in your case, "19 km".
 
awesome thanks ...so the conversion ratio doesn't apply at all in these cases then right?
 
kevinlikesphysics said:
awesome thanks ...so the conversion ratio doesn't apply at all in these cases then right?

The conversion factor definitely applies. The limitation on significant figures depends on which number has the smallest amount of significant figures in the conversion expression. In your case, your 12 miles is what restricts you to two signif. figure. Your conversion factor has plenty of figures.
...in fact your conversion factor would limit you to FIVE significant figures; but since your number to convert only has TWO sigfigs, you are limited to two significant figures.
 
Note that some conversion factors are exact by definition, and can be considered to have an infinite number of significant figures.

Examples:
1 cm = 0.01 m
1 foot = 12 inches
1 inch = 2.54 cm
 
kevinlikesphysics said:
12 miles to kilometers

Could be my English fails me, but this sounds ambiguous to me. Is it 12 miles as measured (in which case it has 2 significant digits) or is it exactly 12 miles (12 is an integer, in which case it has infinite number of SD)?
 
I would interpret that as having two significant figures.
 
I agree, "12 miles" has 2 sig figs. I can't imagine a situation, in real life, where you would know something is exactly 12 miles.
 
Redbelly98 said:
I agree, "12 miles" has 2 sig figs. I can't imagine a situation, in real life, where you would know something is exactly 12 miles.

12 miles is exactly 12 miles :smile:

Note, that HallsofIvy to some extent confirmed ambiguity writing "I would interpret". I would interpret the question the same way most likely, assuming it is 2 SD. But it is just an interpretation.

Question doesn't state whether it asks about the real world measurements, or about abstract conversion factors. I can easily imagine student that interprets 12 as an exact number and gives 19.312128 km as an answer. Not accepting this answer when the exact meaning of the question is open to interpretation will be IMHO wrong.

If the question would say something like "I have measured distance between my home and school to be 12 miles, how far it is in km", that'll be completely different story.
 
  • #10
One might have been given the problem of determining how many miles were in 10,560 fathoms...

In that case the number of sig figs is not 2.
 

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