Remainder of 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 Refer to Appendix A in text for conversions and units tables Going back to our previous derived units, In the SI standard, Velocity is in m/s F = m x a is in kg x m/s2 = 1 Newton (N) Work = energy = F x d = N-m = 1 Joule (J) Current = i = charge/time = C/s = 1 Ampere or Amp (A) Power = work/time = J/s = 1 Watt (W) = I x voltage i.e. J/s = C/s x V => V = J/C = work/charge 2

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 Fundamental examples: Time units: (s): second (must be lower case). Mass (kg): kilogram (must be lower case). Length (m): meter (must be lower case). Temperature (K): Kelvin (must be capital). For electrical systems we add: Charge (C) Coulomb (must be capital)

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⋅m2⋅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 Physical examples: Speed (velocity) = distance/time (m/s) or (ms-1)(must be lower case). Acceleration = distance/time2 (m/s2) or (ms-2)(must be lower case). Force = mass x acceleration (N): Newton (must be capital) 1N = 1 kg m/s2; . Work = energy = force x distance (N.m): Joule; (Joule must be capital). Electrical examples: Current = charge/time (A) Ampere 1A = 1C/s Power = work/time (W): Watt; 1W = 1J/s . Voltage = work/charge (V) Volt; 1V = 1J/C Resistance = resistivity*length/cross-sectional area (Ohm) Ohm

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 5

What Units Should We Use? Example distance units: Example time units: Feet, miles, meters, microns etc Seconds, years, hours

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? Hand out CNN article

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? Allow about 5 min for this activity a) i)60 mi/h * 1/60 h/min * 1/60 min/sec *5280 ft/mi = 88 ft/sec ii) 60 mi/hr * 1/60 hr/min * 1/60 min/sec *1609 meters/mi = 26.8 m/s iii) 60 mi/hr * 1.609 km/mi = 96.54 km/h b) 55 mi/hr * 1.609x105 cm/mi = 8,849,500 cm/hr

Homework for next class In the textbook, Read Chapter I (Introduction) Read Chapter 1