Units Unit Systems Systems set up fundamental units

Units

Unit Systems ] Systems set up fundamental units ] British system - foot, pound, second Metric system - meter, kilogram, second ]

Time ] ] ] The unit of time originally was based on the day and the year. The second was 1/60 *1/24 of a day. In the 20 th century the second was measured based on the timing of atoms. We now know that the day is getting longer and “leap seconds” are added every few years. The SI unit of time is the second (s)

Length ] ] The oldest standards of length were based on the human body. The metric system defined the meter in terms of the Earth: 1/10, 000 from the pole to the equator. The meter is now defined in terms of the second and speed of light. The SI unit of length is the meter (m)

Mass ] ] Standard weights have been maintained for centuries and were treated as equivalent to mass. The metric system originally defined mass in terms of a standard 1 kilogram mass is kept in Paris. On May 20, 2019 the kilogram was redefined in terms of Planck’s constant, the second and the meter. The SI unit of mass is the kilogram (kg)

SI Prefixes ] Prefixes on units are used to represent powers of ten. ] ] Prefixes denote powers of ten from -18 to +18 in steps of three. Example: a kilometer is 103 meters or 1000 meters. ] ] ] Most Common micro ( ) 10 -6 milli (m) 10 -3 kilo (k) 103 mega (M) 106 Common, but the power is not a factor of three. centi (c ) 10 -2 deci (d ) 10 -1

Other Units ] There are other fundamental units in SI. See NIST. • • ] ampere* (A) kelvin* (K) mole* (mol) candela (cd) All units are based on universal constants. *also redefined on May 20, 2019 ] Derived units are built from the fundamental units. • • • area (m 2) volume (m 3) velocity (m/s) acceleration (m/s 2) force (kg m/s 2) or (N) energy (kg m 2/s 2) or (J)

Special Units 149, 597, 871 km = 1. 5 x 1011 m 1 Astronomical Unit (1 AU) ] Earth to the Sun ] (1. 5 x 1011 m) / (3. 0 x 108 m/s) / (60 s/min) = 8. 3 light-minutes Ø