Transcript 6.7 Power

6.7 Power
6.7 Power
The idea of power incorporates both the concepts of work
and time.
6.7 Power
The idea of power incorporates both the concepts of work
and time.
Power is work done per unit time.
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:
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:
Units
TABLE 6.3
Units of Measurement for Power
System Work
SI
CGS
BE
joule (J)
erg
foot · pound (ft ·
lb)
÷ Time
second (s)
second (s)
second (s)
= Power
watt (W)
erg per second (erg/s)
foot ·pound/sec (ft · lb/s)
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
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.
Energy Transformations
Q: Give an example where gravitational potential energy is
converted into kinetic energy?
Energy Transformations
Q: Give an example where gravitational potential energy is
converted into kinetic energy?
A: Falling object.
Energy Transformations
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.
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).
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:
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.
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.
Force versus Distance
Graph
Work = Area under the Force versus Distance graph
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.
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.
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.
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?