# Physical Quantities and Units - UMass D Todays Agenda on Units Primary and Secondary Physical Quantities and Units Unit Conversion Standard We Will Use We will follow the International System of Units also known as the SI standard It includes: Distance measured in meters (m) Mass measured in kilograms (kg) Time measured in seconds (s) Charge measured in Coulombs (C) Temperature measured in degrees Kelvin (o K or K) That means well need to convert other measures to our standard

Fundamental or Primary Quantities 7 SI base units: those from which all others are derived meter (m) kilogram second (s) ampere (A) kelvin (K) candela(cd) mole (mol)

length (kg) weight time current temperature luminosity molecular quantity Quick in-class exercise: Identify physical examples, and electrical examples Derived or Secondary Quantities SI derived unit: derived from base units Unlimited number of derived units! Some examples:

meter / second meter / second squared newton pascal joule volt watt ohm

farad henry hertz (N) (Pa) (J) (V) (W) ()) (F) (H) (Hz) m/s m/s2 kgm / s2 N / m2

Nm kgmm2ms3mA1 J/s V/A C/V Vms / A 1/s velocity acceleration force pressure energy voltage power resistance capacitance inductance

frequency Commonly used prefixes T tera 1012 G giga M mega k kilo 103 d deci10-1 c centi m milli 10-3 micro n nano p pico10-12 109 106 10-2

10-6 10-9 What Units Should We Use? Example distance units: Example time units: What Happens If We Mix Units? Consider the Mars Climate Orbiter that was launched in 1998 (see handout). It turns out that someone at NASA forgot to convert English units (pounds) to metric units (Newtons) when calculating the necessary rocket thrust to put the craft in orbit around Mars Guess what happened?

Class Activity #1 a.) A tractor-trailer truck is traveling at 60 miles per hour eastbound on Interstate 195 towards the North Dartmouth exit. Determine the speed of the truck i.) in feet per second ii.) in meters per second iii.) in kilometers per hour. b.) What is the maximum legal speed in centimeters per hour in a 55 miles per hour posted speed zone? Homework for next class In the textbook, Read Chapter I (Introduction) Read Chapters 1 and 2