Lecture 13
Work and the Work/Kinetic Energy
Theorem
Motivation to go beyond Newton
’s laws
s laws
Loop the loop
R
mg
N
The normal force has different
direction and magnitude at
every point on the track!!
Writing and solving Newton’s laws can be a nasty experience…
(see appendix of lecture 15)
22
net,
11
x
i
f
F
mv
∆=
−
Playing around with Newton
Playing around with Newton
’s 2nd law.
s 2nd law.
WORK (
W
)
done by force
over
displacement
∆
Consider the motion of a bead of mass
m
on a straight wire
pushed by a constant net force
x
parallel to the wire, along a
displacement
:
net,x
xx
Fma
=
Force produces
acceleration:
∆= −
2
axv v
v
i
f
Acceleration produces
change in speed:
2
xv v
Change in
KINETIC
ENERGY (
K
)
of the bead
This is an expression of the “effectiveness” of the force.
net
WK
=∆
Of how a force applied
over a distance…
…changes something in the
system
Work
The “external agent” that
changes the amount of kinetic
energy (the state) in the
system
W
Fx
=
∆
Kinetic energy
The “internal” quantity (state)
of the system
2
1
2
Km
=
net,
−
Work/Kinetic
Energy Theorem
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∆
x
Initial kinetic energy
K
i
Final kinetic energy
f
>
i
Positive work
W
> 0
Energy
source
Work is energy being transferred
F
net,x
Units for work and energy:
SI:
Not to be confused with:
Calorie (or food calorie)
1 Cal = 1000 cal
calorie
1 cal = 4.184 J
Kilowatthour
KWh
Other common units:
Joule
1 J = 1 N · m
Energy
Many types of energy:
•
kinetic energy
•
electric energy
•
internal –thermal energy
•
elastic energy
•
chemical energy
•E
t
c
.
Energy is transferred and
transformed
from one type to another and is never
destroyed or created.
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 Fall '06
 Johnson
 Physics, Energy, Force, Kinetic Energy, Work, Wnet

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