Chapter 3-4 Energy
Horsepower
• James Watt
3-1. The Meaning of Work
3-2. Power
3-3. Kinetic Energy
3-4. Potential Energy
3-5. Energy
Transformations
3-6. Conservation of
Energy
3-7. The Nature of Heat
3-8. Linear Momentum
3-9. Rockets
3-10. Angular Momentum
3-11. Special Relativity
3-12. Rest Energy
3-13. General Relativity
3-14. The Energy Problem
3-15. The Future
– Perfected the steam engine 200 years ago
– Had to provide a comparison to the work output of a
horse. He found that:
• Typical horse could perform 497 W of work for as much
as 10 hours a day
• Watt increased the standard to 746 W
– 1 horsepower (hp) = 746 W = 0.746 kW
– 1 kilowatt (kW) = 1.34 hp
– Early steam engines ranged from 4-100 hp
3-1. Work
Work equals force times distance. W = Fd
The SI unit of work is the joule.
1 joule (J) = 1 newton-meter (N · m)
W=Fd=(100N)(8m)=800N·m=800J
3-2. Power
Power is the rate at which work is being done:
P = W/t
SI unit of power is the watt.
1 watt (W) = 1 joule/second (J/s)
3-3. Kinetic Energy
Energy is that property something has that
enables it to do work.
The energy of a moving object is called kinetic
energy (KE):
KE = ½mv2
where m = mass and v = speed.
KE increases very rapidly with speed because of the v2 factor.
3-4. Potential Energy
Potential energy (PE)
is the energy an object
has by virtue of its
position.
Gravitational Potential
Energy:
PE = mgh
The kilowatt (kW) is a convenient unit of
power for many applications.
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3-5. Energy Transformations
Energy can be transformed or converted from
one form to another.
3-6. Conservation of Energy
The law of conservation of energy states that
energy cannot be created or destroyed, although it
can be changed from one form to another.
Matter can be considered as a form of energy;
matter can be transformed into energy and energy
into matter according to the law of conservation of
energy.
Eo = moc2
where Eo = rest energy, mo = rest mass, and c = speed
of light (3x108m/s or 186,000 miles/sec).
3-7. Nature of Heat
Count Rumford supported the British in the
Revolutionary War and supervised the
making of cannons. He observed that
during the boring process heat was given
off (frictional heat) that could be used to
boil water and could be produced over and
over again from the same piece of metal.
Heat must be energy.
3-5. Energy Transformations
Types of Energy
1. Kinetic energy
2. Potential energy
3. Chemical energy
4. Heat energy
5. Electric energy
6. Radiant energy
3-8. Linear Momentum
Linear Momentum is a measure of the tendency of
a moving object to continue in motion along a straight
line:
p = mv
3-8. Linear Momentum
The law of
conservation of
momentum states: In
the absence of
outside forces, the
total momentum of a
set of objects remains
the same no matter
how the objects
interact with one
another.
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3-8. Linear Momentum
Newton’s Cradle-an
example of the
conservation of
linear momentum.
3-9. Rockets
3-10. Angular Momentum
Angular momentum
is a measure of the
tendency of a
rotating object to
continue spinning
about a fixed axis
L=mvr
L= angular Momentum
m=mass circling
v=velocity of rotation
r=distance from center
The smaller the “r” the faster
the “v”. Angular momentum
is conserved.
3-10. Angular Momentum
• Definition:
The momentum of
the exhaust gases is
balanced by the
rocket's upward
momentum.
– The more angular momentum an
object has, the greater its
tendency to continue to spin
(and be stable)
• Toy tops
• The earth
• Footballs
• Bullets
– Defining angular momentum is
complicated; depends on…
Multistage rockets are more
efficient than single-stage, and
so are widely used.
3-9. Rockets
Rockets are a version of Newton’s
third law of motion as well as the
conservation of linear momentum.
• How fast the object is turning
• Mass of the object
• How the mass is distributed (the
further the mass is from the center
of the object, the greater the
angular momentum)
3-10. Angular Momentum
Gyroscopes
Due to angular
momentum, when
a force is applied
in one direction,
the combined
forces, including
the angular
momentum, will be
in a perpendicular
direction.
http://www.youtube.com/
watch?v=8H98BgRzpOM
Torpedo
Gyroscope
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3-10. Angular Momentum
3-11. Special Relativity
Naval Gyroscopes used to
stabilize ships and guns
mo = m γ heavier
to = t / γ slower
lo = l / γ shorter
Twin Paradox
Muon
Experiment
••
3-10. Angular Momentum
3.12 Rest Energy
Naval Gyroscopes used to
stabilize ships and guns
3-11. Special Relativity
Albert Einstein (1879-1955) published the special
theory of relativity in 1905.
Special relativity is based on two postulates:
1. The laws of physics are the same in all frames of
reference moving at constant velocity.
2. The speed of light (c ) in free space has the same
value for all observers (c = 3 x 108 m/s)
3.12 Rest Energy
• E = mc2 or Energy and Mass are the same!
• Example 3.8 p 91
– How much mass is converted to energy
in a 100MW nuclear power plant?
T=(60)(60)(24)= 86,400 s/day
E=Pt=108W(86,400 s/day)=8.64 x 1012J
m = E/c2 = 8.64 x 1012J/(3 x 108m/s)2
m = 9.6 x 10-5kg or about 0.000013 oz
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3-13. General Relativity
General theory of relativity was developed by
Einstein in 1916, which related gravitation to the
structure of space and time and showed that even
light was subject to gravity.
3-14. The Energy Problem
1. Oil and natural gas reserves will last about another
century..
2. Although coal reserves will last several hundred more
years, mining coal is dangerous, and burning coal creates
environmental problems such as acid rain, air pollution,
and enhanced global warming.
3. The potential for a large-scale nuclear accident is
present.
4. Discharge of radioactive wastes into the environment
from badly run nuclear power plants has occurred.
5. An unsolved disposal problem of radioactive nuclear
waste exists.
Fig. 3.39
Fig. 3.38
Fig. 3.40
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3-14. The Energy Problem
Chernobyl Nuclear Accident
Fig. 3.41
http://www.ems.psu.edu/~radovic/Chernobyl.html
3-15. The Future
Fig. 3.42
3-14. The Energy Problem
Chernobyl Nuclear Accident
3-14. The Future
Energy consumption 2003
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Lecture Quiz 3-4
1. What is the equation that defines work?
Power?
2. How many horsepower is there in one
kilowatt?
3. What kinds of energy are involved in a
windmill electric generator?
4. Which theory of relativity involves
gravity?
5. What is the major fuel used for energy?
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