Today’s Agenda

• 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 we’ll 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) length– kilogram (kg) weight– second (s) time– ampere (A) current– kelvin (K) temperature– candela (cd) luminosity– mole (mol) 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 m/s velocity– meter / second squared m/s2 acceleration– newton (N) kg·m / s2 force– pascal (Pa) N / m2 pressure– joule (J) N·m energy– volt (V) kg m⋅ 2 s⋅ −3 A⋅ −1 voltage– watt (W) J / s power– ohm (Ω) V / A resistance– farad (F) C / V capacitance– henry (H) V s / A ⋅ inductance– hertz (Hz) 1 / s frequency

Commonly used prefixes

T tera 1012

G giga 109

M mega 106

k kilo 103

d deci 10-1

c centi 10-2

d milli 10-3

μ micro 10-6

n nano 10-9

p pico 10-12

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 secondii.) in meters per secondiii.) 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 Chapter 1