1. UNITS OF MEASUREMENT Using and Converting

2. Introduction ■Scientists use the International System of Units (SI), which is an extension of the metric system. ■In the US we commonly use the US customary measurement system, which is similar to the UK Imperial system of units. ■Other specialized fields use units derived from one of these systems. –In astronomy the Astronomical Unit (AU) is defined to be the mean distance between the Earth and the Sun and is equal to 93,000,000 miles. –Also in astronomy the light year (ly) is the distance light travels in a vacuum in one year and is equal to 9,460,730,472,580,800 meters (or 9.46X10 15 m). Note that a light-year is a measure of distance, not time! ■So we need to know how to convert from one system to another.

3. SI Units of Length ■The SI unit of length is the meter (m). The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. –1 kilometer = 1,000 meters =10 3 meters [abbreviation km] –1 centimeter = 0.01 meters = 10 -2 meters [abbreviation cm] –1 millimeter = 0.001 meters = 10 -3 meters [abbreviation mm] ■Conversion factors –1 meter = 3.28 feet = 39.37 inches –1 mile = 1.609 kilometers = 1.609X10 3 meters

4. SI Unit of Time ■The SI unit of time is the second (s). It is defined in terms of the time it takes a 133 Cs atom to make 9,192,631,770 transitions between states. –1 day = 86,400 seconds = 8.64X10 4 s –1 year = 365 days = 31,536,000 seconds = 3.1536X10 7 s

5. Combining Quantities with Units

6. Conversion Example {{{ 111