Work and Energy

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Work and Energy
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Work Done by a Constant Force
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Work Done by a Constant Force
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Work Done by a Constant Force
Work is done when a force F pushes a car through
a displacement s.
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Work Done by a Constant Force
Work is done when a force F pushes a car through
a displacement s. Work Force X Distance.
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Question

• Two men, Joel and Jerry, push against a wall.
  Jerry stops after 10 min, while Joel is able to
  push for 5 min longer. Compare the work against
  the wall they each do. 
• Joel does 50 more work than Jerry.
• Jerry does 50 more work than Joel.
• Joel does 75 more work than Jerry.
• Neither of them do any work.

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Units
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Question

• A 102 kg man climbs a 5.0 meter high stair case
  at constant speed. How much work does he do
  against gravity? 
• 510 J
• 49 J
• 5000 J
• 2500 J

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Bench Pressing
During bench-pressing work is done against gravity
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Kinetic Energy
SI Unit of Kinetic Energy joule (J)
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6.3 Gravitational Potential Energy
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6.3 Gravitational Potential Energy
The gravitational potential energy PE is the
energy that an object of mass m has by virtue of
its position relative to the surface of the
earth. That position is measured by the height h
of the object relative to an arbitrary zero
level
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6.3 Gravitational Potential Energy
The gravitational potential energy PE is the
energy that an object of mass m has by virtue of
its position relative to the surface of the
earth. That position is measured by the height h
of the object relative to an arbitrary zero
level
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6.3 Gravitational Potential Energy
The gravitational potential energy PE is the
energy that an object of mass m has by virtue of
its position relative to the surface of the
earth. That position is measured by the height h
of the object relative to an arbitrary zero
level
SI Unit of Gravitational Potential Energy joule
(J)
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Pile Driver
Gravitational potential energy of the hammer
relative to the ground is,
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A Gymnast on a Trampoline
A gymnast springs vertically upward from a
trampoline. The gymnast leaves the trampoline at
a height of 1.20 m and reaches a maximum height
of 4.80 m before falling back down. All heights
are measured with respect to the ground. Ignoring
air resistance, determine the initial speed v0
with which the gymnast leaves the trampoline.
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The Conservation of Mechanical Energy
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THE PRINCIPLE OF CONSERVATION OF MECHANICAL ENERGY
The total mechanical energy (E KE PE) of an
object remains constant as the object moves,
provided that the net work done by external
nonconservative forces is zero.
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Conservation of Mechanical Energy
If friction and wind resistance are ignored, a
bobsled run illustrates how kinetic and potential
energy can be interconverted, while the total
mechanical energy remains constant.
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Roller Coaster (Ideal)
The tallest and fastest roller coaster in the
world is now the Steel Dragon in Mie, Japan
(Figure 6.20). The ride includes a vertical drop
of 93.5 m. The coaster has a speed of 3.0 m/s at
the top of the drop. Neglect friction and find
the speed of the riders at the bottom.
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Power
The idea of power incorporates both the concepts
of work and time. Power is work done per unit
time. Average power, P is the average rate at
which work W is done, and it is obtained by
dividing W by the time t required to perform the
work
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Units
Horsepower, hp 1 hp 550 ft.lb/s 746 W
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Metabolic Rates for a young 70-kg male
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Forms of Energy
So far we have considered the following forms of
energy Kinetic energy, Gravitational potential
energy, and Mechanical energy. Some of the
other forms of energy are Electrical energy,
Chemical energy, Nuclear energy, Thermal energy,
and Radiant energy.
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Energy Transformations
Q Give an example where gravitational potential
energy is converted into kinetic energy?
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Energy Transformations
Q Give an example where gravitational potential
energy is converted into kinetic energy? A
Falling object.
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Energy Transformations
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Energy Transformations in the Human body
Part of the chemical energy stored in food is
transformed into the kinetic energy of physical
activities and into the thermal energy needed to
keep our bodies at a temperature near 98.6 F.
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Energy Transformations in an Automobile
In an automobile chemical energy of gasoline is
converted into kinetic energy, as well as
electrical energy (to operate the radio,
headlights, and air conditioner), and heat (to
warm the car during the winter).
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Energy Transformations in a Nuclear Power Station
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The CONSERVATION OF ENERGY
Whenever energy is transformed from one form to
another, it is found that no energy is gained or
lost in the process the total of all the
energies before the process is equal to the total
of the energies after the process. This
observation leads to the conservation of energy
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The CONSERVATION OF ENERGY
Whenever energy is transformed from one form to
another, it is found that no energy is gained or
lost in the process the total of all the
energies before the process is equal to the total
of the energies after the process. This
observation leads to the conservation of energy
Energy can neither be created nor destroyed, but
can only be converted from one form to another.
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The CONSERVATION OF ENERGY
Whenever energy is transformed from one form to
another, it is found that no energy is gained or
lost in the process the total of all the
energies before the process is equal to the total
of the energies after the process. This
observation leads to the conservation of energy
Energy can neither be created nor destroyed, but
can only be converted from one form to another.
Learning how to convert energy from one form to
another more efficiently is one of the main goals
of modern science and technology.