Units of Measurement

SI Units •  There was a need to report data that

could be reproduced by others o Metric system was developed in

1795 o Systeme Internationale d’Unites

•  By using one system of units, scientists can readily interpret the results of one another

Base Units • A defined unit in

a system of measurements that is based on an object or event in the physical world

Quantity Unit Symbol Length meter m

Mass kilogram kg

Temperature kelvin K

Time second s

Amount of Substance

mole mol

Electric Current

ampere A

Luminous Intensity

candela cd

Derived Units • Are a combination of base

units

Quantity Unit Symbol Area square meter m2

Volume cubic meter m3

Liquid volume cubic decimeter dm3 = L

Density kilograms per cubic meter

kg/m3

Pressure pascal (kg/m*s2) Pa

Energy Joule (kg*m2/s2) J

Frequency hertz (1/s) Hz

Electric Charge coulomb (A*s) C

Speed meters per second m/s

Metric Prefixes • Indicates how many times

a unit should be multiplied or divided by ten

Prefix Symbol Meaning Multiply Unit By

giga- G billion (109) 1,000,000,000

mega- M million (106) 1,000,000

kilo- k thousand (103) 1,000

deci- d tenth (10-1) 0.1 centi- c hundredth (10-2) 0.01

milli- m thousandth (10-3) 0.001

micro- µ millionth (10-6) 0.000001

nano- n billionth (10-9) 0.000000001

Conversion Factors •  Is a ratio of equivalent

measurements that is used to convert a quantity expressed in one unit to another unit • Will be used to complete your

dimensional analysis hw worksheet

Dimensional Analysis •  A problem-solving method that uses the fact

that any number or expression can be multiplied by one without changing its value.

•  Also called factor-label method or the unit factor method

•  The unit factors are conversion factors o  Example any two terms that describe the same or

equivalent “amounts” of what we are interested in o  Ex: 1 inch = 2.54 centimeters o  1 meter = 100 centimeters o  1 km = 1,000 meters

•  10 megawatts is equal to how many watts? •  10 MW x 1,000,000 W = 10,000,000 W 1 MW •  20,000 milligrams is equivalent to how many grams? •  20,000 mg x 1g = 20 g 1,000 mg •  Convert the height of Mount Everest (8,848 m) into

kilometers •  8,848 m x 1km = 8.848 km or 8.8 km 1,000 m

Precision & Accuracy •  Precision is a gauge of how exact a

measurement is o The more significant figures

(reported digits) there are, the more precise the measurement

•  Accuracy is the closeness of a measurement to the actual value of what is being measured