All measurements are not certain, for this we have Significant Figures, or Sig Figs. Many things are to blame for measurements not being exact, but the main two are the person measuring and the device used to measure. Here are some rules to Sig Figs.

1.    Non zero Numbers- all digits that are non zero are significant.

o    The number 75 has two Sig Figs; the number 89.73 has four Sig Figs.

2.    Zeros Held Captive- zeros held captive between two non zero numbers are significant.

o    The number 10009 has five Sig Figs; the number 905 has three Sig Figs.

3.     Leading Zeros- Zeros to the left of the first non zero digit are not significant; they simply indicate the position of the decimal point.

o    The number 0.0069 has two Sig Figs; the number 0.05 has one Sig Fig.

4.    Trailing Zeros-Zeros to the right of the decimal point are significant if they are at the end of the number.

o    The number 0.012000 has three Sig Figs

5.    Rounding- Expressed or designated as a whole number or integer; not fractional.

Although rounding is generally straightforward, one point requires special emphasis.  Suppose that the number 9.348 needs to be rounded to two significant figures.  Look only at the first number to the right of the 3 (i.e., 4).  Since this number is less than 5 the number is rounded to 9.3 because 4 is less than 5.  It is incorrect to round sequentially.  For example, do not round the 4 (because the 8 is greater than 5) to 5 to give 9.35 and then round the 3 to 4 to give 9.4.  When rounding, use only the first number to the right of the last significant figure.

1.    When adding or subtracting given amounts, the result should be reported with the same number of decimal places as that of the number with the least number of decimal places.

Example-3.45 + 2.1124 = 5.56; the number 3.45 has the least number of decimal places two and thus limits the final answer to two decimal places. 

2.    When multiplying or dividing measured quantities, the product (or quotient) should contain no more digits than the measurement with the least number of significant figures involved in the calculation. 

Examples-If you drove your car 315 miles on 11.33 gallons of gasoline, you can calculate your gas mileage by dividing the number of miles by the number of gallons of gasoline; 315 miles/11.33 gallons = 27.80229479 mpg.  Although this answer on a calculator, the answer is not really as precise as it appears and should properly be rounded off to 27.8 mpg.  The final answer has three significant figures because 315 contain fewer significant figures than 11.33 and this limits the accuracy of the final answer.


Links

Mr. Young's Page

www.sciencebyjones.com

Ananda's Force Diagram Page

www.google.com search for Significant Figures

www.accessexcellence.org

Mr Youngs worksheet for Sig Figs

www.coker.edu

Other Information

About the Car in my back ground, this is a 1969 Dodge Charger 440. If you ever watched the show "The Dukes of Hazzard" you know about this car. The car featured on the show was named The General Lee, and was known for jumping anything in it's path. This car has a 440 4-Barrel Carb with 375 Horsepower