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Measurement Uncertainty

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Police arresting man

How do police officers identify criminals?

After a bank robbery has been committed, police will ask witnesses to describe the robbers. They will usually get some answer such as “medium height.” Others may say “between 5 foot 8 inches and 5 foot 10 inches.” In both cases, there is a significant amount of uncertainty about the height of the criminals.

Measurement Uncertainty

Some error or uncertainty always exists in any measurement. The amount of uncertainty depends both upon the skill of the measurer and upon the quality of the measuring tool. While some balances are capable of measuring masses only to the nearest 0.1 g, other highly sensitive balances are capable of measuring to the nearest 0.001 g or even better. Many measuring tools such as rulers and graduated cylinders have small lines which need to be carefully read in order to make a measurement. The figure below shows two rulers making the same measurement of an object (indicated by the blue arrow).

Uncertainty in measurement on a ruler

Uncertainty in measurement.

With either ruler, it is clear that the length of the object is between 2 and 3 cm. The bottom ruler contains no millimeter markings. With that ruler, the tenths digit can be estimated and the length may be reported as 2.5 cm.  However, another person may judge that the measurement is 2.4 cm or perhaps 2.6 cm. While the 2 is known for certain, the value of the tenths digit is uncertain. 

The top ruler contains marks for tenths of a centimeter (millimeters). Now the same object may be measured as 2.55 cm. The measurer is capable of estimating the hundredths digit because he can be certain that the tenths digit is a 5. Again, another measurer may report the length to be 2.54 cm or 2.56 cm. In this case, there are two certain digits (the 2 and the 5), with the hundredths digit being uncertain. Clearly, the top ruler is a superior ruler for measuring lengths as precisely as possible.

Summary

  • Uncertainty exists in all measurements.
  • The degree of uncertainty is affected in part by the quality of the measuring tool.

Practice

Read the material at the link below and answer the questions on the web site:

http://www2.southeastern.edu/Academics/Faculty/rallain/plab194/error.html

Review

Questions

  1. What is uncertainty in measurements?
  2. Why is the top ruler more reliable in measuring length than the bottom ruler?
  3. How could the top ruler be made more accurate?

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