6.7

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6.7 Power
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6.7 Power
The idea of power incorporates both the concepts
of work and time.
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6.7 Power
The idea of power incorporates both the concepts
of work and time. Power is work done per unit
time.
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6.7 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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6.7 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
                                                                                                                                                                                                                  
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Metabolic Rates for a young 70-kg male
Activity Metabolic Rate (W)
Running (15 km/h) 1340
Skiing 1050
Biking 530
Walking (5 km/h) 280
Sleeping 77
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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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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.
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Force versus Distance Graph
Work Area under the Force versus Distance graph
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Work Done by a Variable Force
The work done by a variable force in moving an
object is equal to the area under the graph of F
cosq   versus s.
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Work and the Compound Bow
Find the work that the archer must do in drawing
back the string of the compound bow in Figure
6.22 from 0 to 0.500 m.
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Problem 65
The drawing shows the force-versus-displacement
graph for two different bows. These graphs give
the force that an archer must apply to draw the
bowstring. (a) For which bow is more work
required to draw the bow fully from S 0 to S
0.5m? Give your reasoning. (b) Estimate the
additional work required for the bow identified
in part (a) compared to the other bow.
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Conceptual Question 17
The drawing shows an empty fuel tank being
released by three different jet planes. At the
moment of release, each plane has the same speed
and each tank is at the same height above the
ground. However, the directions of travel are
different. Air resistance is neglected. Q1
Which tank will reach the ground first? Q2 What
can you say about the speed of the tanks at
ground level?