Transcript Work_power site

Work & Energy
Energy
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(Some types of energy)
 Chemical
 Nuclear
 Kinetic
 Solar
 Thermal
 Etc...
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Unit of energy is the Joule (J)
Work
Work
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Work is a force acting upon an object to
cause a displacement in the same
direction as the force.
W
Work = force X displacement
F
d
Usually the force we use is Weight when
you are lifting something!!!
Weight= Fg = mg
Mrs. Koonce wants to know….
Is work being done when….??
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….a student holds a heavy chair at arms’ length for
several minutes?
No work
being
done
….a student carries a bucket of water along a
horizontal path while walking at a constant velocity?
SUPRISE: There is no work done on the chair or
bucket even though effort was required in both cases
because no force is applied in the direction of travel.
Example
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When a weightlifter lifts
a barbell, he does work
on the barbell
When he is holding the
barbell over his head, he
does no work
You have to be applying
a force over a distance!!!
Example of Work
How much work is done to a 10 kg mass that is
lifted 2 meters?
G
Fg = 10kg ∙ 9.8m/s2
= 100N
d = 2.0 m
U
E
S
Work = ?
Work = F ∙ d
W = 100N ∙ 2 m
S
W = 200 J
Work
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Example
 Rock
Climber:
 Take
 Trail
a long time to move up a few meters
Hiker:
 Takes
a short amount of time to move up a
few meters
 Both
do same amount of Work just the
hiker does it in less time.
Power
Power
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Power is the rate at which work is done
W
P
t
Power
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Which are you more tired…
 Walking
a box up a flight of stairs or
 Running a box up a flight of stairs?
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You are doing the same amount of work, so why
are you more tired?
POWER
Power is the measure of how fast work is done
Power
Power is also the rate at which energy is changed
from one form to another
The unit of power is the joule per second, called the
watt
One watt (W) of power is used when one joule of
work is done in one second
Which has more power?
Lawnmower
Engine
30
minutes
of work
…but at
different rates
They burn the
same fuel…
Jet
Engine
0.5 s
of work
Practice Problem
Kevin walks 3.5 m up the stairs on the way to his
class. Kevin has a mass of 85 kg. What is his
power output if he climbs the stairs in 8.5 seconds?
G
U
E
S
S
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