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Uncertainty in Multiplication and Division

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Uncertainty in Multiplication and Division

Who should report the numbers - you or your calculator?

Calculators do just what you ask of them, no more and no less.  However, they sometimes can get a little out of hand.  If I multiply 2.48 times 6.3, I get an answer of 15.687, a value that ignores the number of significant figures in either number.  Division with a calculator is even worse. When I divide 12.2 by 1.7, the answer I obtain is  7.176470588.  Neither piece of data is accurate to nine decimal places, but the calculator doesn’t know that.  The human being operating the instrument has to make the decision about how many places to report.

Uncertainty in Multiplication and Division

The density of a certain object is calculated by dividing the mass by the volume.  Suppose that a mass of 37.46 g is divided by a volume of 12.7 cm 3 .  The result on a calculator would be:

D=\frac{m}{V}=\frac{37.46 \ \text{g}}{12.7 \ \text{cm}^3}=2.949606299 \ \text{g}/\text{cm}^3

The value of the mass measurement has four significant figures, while the value of the volume measurement has only three significant figures.  For multiplication and division problems, the answer should be rounded to the same number of significant figures as the measurement with the least number of significant figures.  Applying this rule results in a density of 2.95 g/cm 3 , for three significant figures – the same as the volume measurement.

Sample Problem:  Significant Figures in Calculations

Perform the following calculations, rounding the answers to the appropriate number of significant figures.

  1. 0.048 \ \text{m}  \times  32.97 \ \text{m}
  2. 14,570 \ \text{kg}  \div  5.81 \ \text{L}

Step 1:  Plan the problem.

Analyze each of the measured values to determine how many significant figures should be in the result.  Perform the calculation and round appropriately.  Apply the correct units to the answer. When multiplying or dividing, the units are also multiplied or divided.

Step 2:  Calculate

  1. 0.048 \ \text{m} \times 32.97 \ \text{m}  =  1.6 \ \text{m}^2   Round to two significant figures because 0.048 has two.
  2. 14,570 \ \text{kg} \div 5.81 \ \text{L}  =  2510 \ \text{kg}/ \text{L}  Round to three significant figures because 5.81 has three.

Summary

  • For multiplication and division problems, the answer should be rounded to the same number of significant figures as the measurement with the least number of significant figures.

Practice

Read the information and work the problems on the following web site:

http://sigfigscalculator.appspot.com/tutorial/multiply_and_divide

Review

  1. What is the basic principle involved in working with multiplication and division?
  2. What happens to units in multiplication and division problems?

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