Energy and Power
Work
• Work is accomplished when energy is
transformed from one for to another
•
work = force ⋅ distance
•
W =
F ⋅ d
•
joules = newtons ⋅ meters
Power
• A small tractor can do as
much work as a large
tractor.
• The large tractor can do
the work faster.
• Power = work / time
• P
= W / t
• watts = joules / second
Practical Units
•
•
•
•
•
•
•
•
•
•
The electric company uses bigger units:
P = W/t
W =
P
⋅
t
kilowatt-hours = kilowatts ⋅ hours
PECO charges about $.18 per kilowatt-hour. What does
it cost to light a 20 watt bulb for a day?
P = 20 watts = .02 kilowatts
t = 1 day = 24 hours
Energy = work =
W = P ⋅ t = .02 ⋅ 24 = .48 kwatt-hours
.48 kwatt-hours x $.18/kwatt-hour = .0864 = $.09
Word, Letter, and Units
•
•
•
•
•
•
•
•
Word
Letter
distance
d or s
mass
m
time
t
force
F
Energy (work) W
Power
P
Unit
Unit Abbrv.
meters
m
kilograms
kg
seconds
s
newtons
N
joules
J
watts
W
miles
mi