Chapter 2 Measurement

Standards of Length, Mass, and Time In 1960, an international committee agreed on a standard system of units for the fundamental quantities of science… It is called the SI (Système International), and its units of length, mass, and time are the meter, kilogram, and second.

Length In October 1983, the definition of the meter was changed… It is now defined as the distance traveled by light in a vacuum during a time interval of 1/299,792,458 second. This establishes the speed of light is 299,792,458 meters per second.

Mass The SI unit of mass, the kilogram, is defined as the mass of a specific platinum-iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sèvres, France.

Time Before 1960, the time standard was defined in terms of the average length of the solar day in the year 1900. Scientist now use a high precision device known as an atomic clock. The second is now defined as 9,192,631,700 times the period of oscilation of radiation from the cesium atom.

Principles of Measurement Instruments are tools used to permit more accurate measurements… However…instruments have limitations... They are Man-made and therefore are subject to error.

Accuracy Accuracy is a qualitative term. An instrument is considered accurate if it produces a measurement close to the actual value. To make a quantitative assessment, the experimental results must be compared to the actual or accepted value. This comparison is called percent error.

Calculating Percent Error Percent Error = observed value – actual value X 100% actual value

Precision Precision is a qualitative term that describes the exactness of a number or measured data. The last digit in any measurement always has a level of uncertainty.

Conversion Factors The first step in solving a problem with measurements in it is to convert to similar quantities with the same units. In physics, this usually means we will be using SI units (meter, liter, second, etc,)…but, there is a wide range of units for expressing the same quantities…

Conversion Factors Sometimes it is necessary to convert between these quantities or between one system of measurement to another. Conversion factors are the equivalents between quantities and systems… Example: 1 mile = 1609 m = 1.609 km

Conversion of Units Units can be treated as algebraic quantities that can cancel each other… Convert 15.0 in into cm: 1 inch = 2.54 cm 15.0 in X 2.54 cm/in = 38.1 cm