Energy, Work,

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Chapter 10 Energy, Work, Simple Machines
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10.1 Energy Work
A force that causes a Displacement of an object
does Work on that object.
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Work is the product of the Magnitudes of the
component Of a force along the direction And the
displacement.
W Fd
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Work has the dimensions of Force times length.
Thus the units are N m Or the Joule (J)
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Work is only done when Components of a force are
Parallel to a displacement.
W Fd (cos ?)
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Problem...
A tugboat pulls a ship with a force Of 5.00 X 103
N. How much work Is done by the tugboat on the
Ship if it moves 3.00 km?
1.50 X 107 J
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Problem...
How much work is done on a Vacuum cleaner pulled
3.0 m by A force of 50 N at an angle of 30.0
above the horizontal?
130 J
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Energy is The capacity to Do work or to Produce
heat.
Kinetic energy is the energy Of motion
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The formula for kinetic energy Will be used often
KE ½mv2
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Problem...
A 7 kg bowling ball moves at 3 m/s. How much
kinetic energy does my Ball have? How fast must a
Ping-pong ball (4.5) move to have The Same
kinetic energy as the Other ball?
31.5 J 160 m/s
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Problem...
Calculate the KE of an 8.0 X 104 kg airliner
Flying at 600 km/h.
1.1 X 109 J
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The work-kinetic energy theorem Says that the net
work done on An object is equal to the change In
the kinetic energy of The object.
The turns into a formula
W ?KE
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PROBLEMS...
A 105 g hockey puck is sliding Across the ice.
A player exerts a Constant 4.5 N force over a
Distance of 0.15 m. How much Work is does the
player do on The puck? What is the change In the
pucks energy?
0.68 J
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Power is the rate at which Energy is transferred.
Or better stated as
W ?t
P Fv
P
or
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The SI unit for power is the Watt.
A watt is defined as 1 J/s.
Horsepower can also be used, And is equal to 746
W.
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Problem...
A 193 kg curtain needs to be raised 7.5m in as
close to 5 s as possible. Three motors are
available. The Power rating on each motor is 1
kW, 3.5 kW and 5.5 kW. Which Motor is best for
the job.
The 3.5 kW motor is best.
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Problem...
A 1000 kg elevator carries a Max load of 800 kg.
A constant Frictional force of 4000N retards The
motion. What minimum power Must the motor deliver
to lift the Fully loaded elevator at a Constant
speed of 3 m/s ?
66 kW
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10.2 Machines
There are 6 different types of Simple machines.
A lever and Fulcrum An inclined plane Wheel and
axle Wedge Pulley Screw
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Inclined Plane
Wheel Axle
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Screw
Wedge
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Lever
Pulley
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The point of a machine is to Make work easier.
The amount that the work Gets easy is called
Mechanical Advantage.
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Fout Fin
Output Force Input Force
MA

IMA is the Ideal Mechanical Advantage
de distance exerted
de dr
IMA
dr distance Of the object
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Efficiency is a measure of How well a machine
works.
Wout Win
MA IMA
eff
eff
A gasoline car is only about 30 efficient,
where as an electric Car is about 90 efficient!!
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Problem...
A bike wheel has a radius of 35.6 cm and a gear
with radius of 4 cm. When the chain is pulled
155N The rim moves 14 cm. the efficiency Is 95.
What is the IMA? What Is the MA? What is the
resistance Force? How far was the chain pulled?
0.112 0.106 16.4 1.57
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THE END