Download Ch 2.7T Class Notes 2016

Class Notes:
What is money?
Work, Energy and Power
(Write your definition)
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What is energy?
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Work is done on an object by a force
when a FORCE is applied which
moves that object through a
DISTANCE.
(Write your definition)
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Work is done on an object by a force
when a FORCE is applied which
moves that object through a
DISTANCE.
Work is done on an object by a force
when a FORCE is applied which
moves that object through a
DISTANCE.
Is work being done on the car?
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Since both force and displacement
are vectors, the amount of work
done equals the distance moved
times the component of the force
that acts in the direction of motion.
in other words... or symbols...
W= FdCos!
W= FdCos!
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How much work is done?
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How much work is done?
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Work can also be found by
calculating the area under a Force
vs. displacement graph.
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Work can also be found by
calculating the area under a Force
vs. displacement graph.
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Units
kg•m2
s2
Since W = FdCos!
or
the units of work are
N•m
kg•m2
s2
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or
Joules
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The sign of work is important!
The work done on an object is
positive when the component of
force is in the same direction as
the displacement.
(! < 90°)
See Figure 5-3 in Holt page 170.
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Energy can be defined as “the
capacity to do work”.
The sign of work is important!
The work done on an object is
negative when the component of
force is in the opposite direction of
the displacement.
(! > 90°)
Work is a measure of the energy
converted from one form to
another.
See Figure 5-3 on page 170.
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Energy is a conserved quantity.
Kinetic energy is the energy of
motion.
Energy can be converted from one
form to another but the total
amount of energy will stay
constant.
Kinetic energy is defined as:
1
KE = mv
2
2
Stop Day
One
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First, let’s define net work.
The Work-Kinetic Energy Theorem.
Sometimes called the
Work-energy principle:
W net = Fnet d
“The net work done on an object is
equal to the object’s change in
kinetic energy.”
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The Work-energy principle
The Work-energy principle
W = !KE
When work is done on an object, the
object’s kinetic energy will change.
net
W net = KE f ! KE i
1
1
W = mv ! mv
2
2
2
net
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f
If W is positive, KE will increase. If
W is negative, KE will decrease.
2
i
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Potential Energy:
Gravitational Potential Energy:
The energy associated with an
object due to the position of the
object.
The Potential Energy associated
with an object due to the position
of the object relative to the Earth
or some other gravitational source.
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PE
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gravitational
= mgh
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Elastic Potential Energy:
The Potential Energy stored in a
stretched or compressed elastic
object.
Where h is the height above some
arbitrary reference point ... usually
the ground.
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PE
elastic
1
= kx
2
The Law of Conservation of Energy
2
Energy can be transferred from
one form to another, but the total
energy remains constant.
Where k is the “spring constant”
measured in newtons per meter and
x is the distance that the spring is
stretched or compressed.
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For objects affected only by
gravity and in the absence of
friction, we have a simplified
version of conservation of energy:
Power is the rate at which work
is done.
W FdCos"
P= =
!t
!t
or
2
i
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f
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Power
PEi + KEi = PEf + KEf
1
1
mgh + mv = mgh + mv
2
2
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f
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Power is measured in Joules per
second.
Power is not a measure of how
much work is done.
Power is a measure of how
quickly work is being done.
One Joule per second is also
called 1 Watt.
J
1 = 1W
s
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Another way to calculate Power:
A commonly needed conversion:
Fd
d
P=
= F = Fv
!t
!t
1 Horsepower (HP) = 746 Watts
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The End
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