Why Do We Need a System of Measurement ?

Which would you prefer? 2lbs of gold or 1 kilogram of gold? 2 lbs of Gold 1 kilogram of Gold How do you know if you are getting the same amount as everyone else without a standard to compare it to?

A “standard” is something used as a comparison for measurement 2 Important factors for standards: Must be available for everyone and something in nature that is the same all over the earth The standard must never vary

English System of Measurement The English System of Measurement is commonly used as the day to day system in the United States. –Pounds –Feet –Gallons But what are the standards for these units? –there are no standards in nature for these units, the rest of the world has moved to another measurem ent system. The English System of Measurement is commonly used as the day to day system in the United States. –Pounds –Feet –Gallons But what are the standards for these units? –there are no standards in nature for these units, the rest of the world has moved to another measurem ent system.

International System of Units (SI) Modern Form of the Metric System Units of measurement devised around 7 base units Works on a scale of 10 Prefixes are used to distinguish between base units Use of conversion factors

Know these SI prefixes: Conversion Factors: ratio of equivalent measurements 1 m = 10 dm 1 m = 100 cm 1 m = 1000 mm

SI Units of Measurement QuantitySI UnitSymbol LengthMeterM VolumeCubic meter Liter m3Lm3L MassGramg DensityGrams per cubic centimeter g/cm 3

An approximate comparison of the two systems: English SystemMetric System 1 yard0.9 meter 1 mile1.5 kilometer 1 pound0.5 kilogram 1 quart 4 quarts = 3.8 liters ~1 liter There are exact conversion factors that can be used to change units in one system to units in the other. The table above might help you begin to visualize the size of the units of the metric system.

SIGNIFICANT FIGURES

RuleExamples The following are always significant  non zero digits  zeros between non zero digits  zero to the right of a non zero digit and left of a written decimal  finishing zeros to the right of the decimal point 673 has has has has 4 The following are NEVER significant  zeros to the left of the first non zero digit 0.67 has has 1 EXCEPTIONS  counting numbers  Exact conversion factors 30 days in June 100cm in 1m

LET’S TRY A FEW a) ________sig figs b) 45.0 ________sig figs c) 154 ________sig figs d) ________sig figs e) 54,000,000________sig figs a) ________sig figs b) 45.0 ________sig figs c) 154 ________sig figs d) ________sig figs e) 54,000,000________sig figs