 (Informal Definition of a Unit)  Unit- label used to give meaning to a measurement or number  All numbers in physics must have a unit with them  10kg20V3.4m  Unit written after the number, usually abbreviated  7 meters -- 7m5 kilograms -- 5kg

Over time it became necessary to come up with standard units Length use to be determined by measuring with fingers, hands, the distance between the tip of the elbow to the extended finger (cubit) Mass was determined by using stones, grain, seeds (carob seed- became basis of measure called carat Time was measured by periods of sun and moon

Systems of Measurement Of course, each of these measurements were not uniform and could be problematic By the 1700s, every country used its own system of weights and measures. England had three different systems just within its own borders! Why do we need a standardized system of measurement? – Scientific community is global. – An international “language” of measurement allows scientists to share, interpret, and compare experimental findings with other scientists, regardless of nationality or language barriers.

Metric System & SI The first standardized system of measurement: the “Metric” system – Developed in France in 1791 – Named based on French word for “measure” – based on the decimal (powers of 10) Systeme International d'Unites (International System of Units) – Modernized version of the Metric System – Abbreviated by the letters SI. – Established in 1960, at the 11th General Conference on Weights and Measures. – Units, definitions, and symbols were revised and simplified.

Components of the SI System In this course we will primarily use SI units. The SI system of measurement has 3 parts: – base units – derived units – prefixes Unit (scientific definition): measure of the quantity that is defined to be exactly 1; example- meter Prefix: modifier that allows us to express multiples or fractions of a base unit; example- centi centimeter As we progress through the course, we will introduce different base units and derived units.

SI: Base Units Physical QuantityUnit NameSymbol lengthmeterm masskilogramkg timeseconds electric currentampereA temperatureKelvinK amount of substancemolemol luminous intensitycandelacd

SI: Derived Units Physical QuantityUnit NameSymbol areasquare meterm2m2 volumecubic meterm3m3 speed meter per second m/s acceleration meter per second squared m/s 2 weight, forcenewtonN pressurepascalPa energy, workjouleJ