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
Fundamental or Primary
Quantities
• 7 SI base units: those from which all others are derived
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–
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–
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meter
kilogram
second
ampere
kelvin
candela
mole
(m)
(kg)
(s)
(A)
(K)
(cd)
(mol)
length
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:
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–
–
–
–
–
–
–
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meter / second
meter / second squared
newton
pascal
joule
volt
watt
ohm
farad
henry
hertz
(N)
(Pa)
(J)
(V)
(W)
(Ω)
(F)
(H)
(Hz)
m/s
m/s2
kg·m / s2
N / m2
N·m
kg⋅m2⋅s−3⋅A−1
J/s
V/A
C/V
V⋅s / A
1/s
velocity
acceleration
force
pressure
energy
voltage
power
resistance
capacitance
inductance
frequency
Commonly used prefixes
T
G
M
k
tera
giga
mega
kilo
1012
109
106
103
d
c
d
μ
n
p
deci
centi
milli
micro
nano
pico
10-1
10-2
10-3
10-6
10-9
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 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 Chapter 1