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Chapter 3: Energy
Energy:
It’s a gas!
• Moving objects have energy. (Think about
flying snowballs or speeding cars.)
• Objects at rest also contain energy. (Think
about pizza or firewood.)
• In Physics, Energy can be defined as the
ability to do work.
• So, what exactly is work?
• Introduction to Work
– A force was simply defined as a push or pull on
an object.
– Work is the result of the force applied to an
object and the distance the object moves as a
result of the force.
– Note: the distance moved must be in the same
direction as the force applied.
– The work done (on an object) is the amount of
applied force multiplied by the parallel
distance through which the force acts.
• Words: Work = force times distance
• Formula: W = Fd
• Note:
- Movement must occur for work to be done.
- The movement must be in the same direction as the
applied force.
• Units of Work in the
Metric and English Systems
– Newton is the unit of force.
– Meter is the unit of distance.
– So, the unit of work is the
(Newton) (Meter).
– A Nm is called a joule (J).
– The unit of work in the
English system is the ftlb.
The force on the book
moves it through a
vertical distance and
work is done.
W = Fd.
• Is work being done?
A force is exerted simply to
hold the books, but the
books do not move through a
distance. Therefore the
distance moved is 0, and the
work accomplished is also 0.
•Work is done against
gravity when lifting an
object. Work is measured
in joules or foot-pounds.
Distance in meters or feet
Force in Newtons or pounds
W = Fd
Work in Nm or foot-lb
– Sample Problem:
• How much work is needed to lift a 2,000 kg
car 1.0 m above the ground?
• First calculate the weight (force) necessary
to lift the car.
• w = mg
• (2,000 kg)(9.8m/s2)
• (2,000 X 9.8)(kg X m/s2)
• 19,600 kg m/s2
• 19,600 N (The weight of the car; a force)
• Now find the work done.
-Work = Force X distance W = Fd
- (19,600N) (1.0m)
- 19,600 Nm or 19,600 J or 19.6 kJ
• Recall: A joule is a Nm
• Power
– Power is the rate at which work is done.
– Power is defined as work per unit of time.
• Power = work/time
• P = W/t
– If the same amount of work is done by two
machines, and one does it in half the time, that
machine used twice the power.
The work
accomplished in
climbing a stairway is
the person's weight
times the vertical
distance.
The power level is the
work accomplished
per unit of time.
Units of Power: That’s Watt I’m talking about
One horsepower is
equal to a power
rating of 550 ft-lb/s.
A watt is defined as a
newton-meter per
second, or joule per
second.
If moving a book from
the floor to a high shelf
requires 10 J of work
then the book will do
10 J of work on an
object of the same mass
when the book falls
from the shelf.
• Motion, Position, and Energy
• Energy is the ability to do work.
• Potential Energy
– Potential energy is the energy that an object
has due to its position.
– Most potential energy is actually gravitational
potential energy, since it is due to the
gravitational attraction of the Earth for an
object.
– For the metric unit of mass, weight is the
product of the mass of an object times g, the
acceleration due to gravity.
– Potential energy = weight X height
• PE = mgh
– Example
• What is the potential energy (PE) of a 2000
lb rock 12260 ft above sea level, perched
above the city of Denver (elevation 5260 ft)
• PE = mgh = wh (note: w = weight)
= (2,000 lb)( 12260 ft - 5260 ft)
= (2,000 lb) (7,000 ft)
= 14,000,000 ftlb
= 1.4 X 10 7 ftlb
• Kinetic Energy
– Kinetic energy is the energy that an object
contains due to its motion.
– Kinetic energy can be measured:
• In terms of the work done to put the object
in motion.
• In terms of the work the moving object will
do in coming to rest (transfer of energy to
another object).
– Kinetic energy is proportional to the mass of a
moving object, but the velocity of the object
has a greater influence.
(A) Work is done on the bowling ball as a force (FB)
moves it through a distance. (B) This gives the ball a
kinetic energy equal to the amount of work done on it.
(C) The ball does work on the pins and has enough
remaining energy to crash into the wall behind the
pins.
– Kinetic energy is proportional to the square of
the velocity.
• The kinetic energy of an object is equal to
1/2 (mass) (velocity)2
• KE = 1/2mv2
• The unit of mass is the kg and the unit of
velocity is m/s.
• Therefore, the unit of kinetic energy is:
• KE = (kg)(m/s)2
• = (kg)(m2/s2)
• = kgm2/s2
• Which is the same as
–(kgm/s2)(m) or
–Nm or
–Joule (J)
– Example
• A 3500 kg automobile is moving down the
interstate with a velocity of 83 km/hr, what
is the kinetic energy of the automobile?
• 83 km/hr X 1 hr/60min X 1 min/60s X 1000
m/km = 23.056 m/s
• Use KE = 1/2mv2
• KE = 1/2(3500.0 kg)(23.056m/s)2
• KE = 1/2 (3500.0 kg)(531.56 m2/s2)
• KE = 930227.62346
• KE = 9.3 X 10 5 kgm2/s2
• KE = 9.3 X 10 5 J
• Energy Flow
• Work and Energy
– Energy is used to do work on an object,
exerting a force through a distance.
– This force is usually against something and
there are five main groups of resistance.
• Work against inertia
–Since inertia is an object’s resistance to
change of motion, it naturally follows that
this would resist forces acting upon it.
• Work against fundamental forces
–Gravitational attraction.
–Electromagnetic forces.
–Nuclear forces.
• Work against friction
–Friction is always present when two
objects are in contact with each other.
–Friction is always a force in the opposite
direction of the applied force.
• Work against shape
–Work is needed to stretch or compress an
object.
–This is what happens when we work
against the shape of a spring.
• Work against any combination of inertia,
fundamental forces, friction, or shape.
Examples of
working
against:
(A) inertia,
(B) gravity,
(C) friction,
and
(D) shape.
– Some kind of energy change has taken
place, which may include one of the
following:
• Increased kinetic energy
–Work against inertia results in energy of
motion for an object.
• Increased potential energy
–Work against fundamental forces and
work against shape result in an increase
in energy of position (potential energy)
Kinds of Energy Change
continued
• Increased temperature
–Work against friction always results in an
increase in temperature.
• Increased combination of kinetic energy,
potential energy, and/or temperature.
• Energy Forms (five forms)
– Mechanical energy
• Usually associated with the kinetic energy of
everyday objects and potential energy that
results from the effect of gravity.
This boat has energy of
motion, KE.
– Chemical energy
• Chemical energy is the form of energy
associated with chemical reactions.
• Chemical energy is released during the
process known as oxidation.
• Chemical energy is potential energy that is
released when chemical reactions break
bonds in molecules.
– Radiant energy
• Radiant energy is the form of energy that
travels through space.
• Also called electromagnetic radiation.
• Visible light is one small part of the
electromagnetic radiation.
– Electrical energy
• Electrical energy is a form of
energy that comes from
electromagnetic interactions.
• Electrical energy that travels
through the wires in our
homes to light our houses is a
familiar form of electrical
energy.
– Nuclear energy
• This is the form of energy generated in
nuclear power plants.
• Energy Conversion
– Energy can be converted from one form to
another.
– For example, during a fall PE lost = KE gained
– mgh = 1/2mv2
– Solving for vf
– vf = 2gh
– This allows you to calculate the final velocity of
a falling object after its potential energy is
converted into kinetic energy.
This pendulum bob loses potential energy (PE) and
gains an equal amount of kinetic energy (KE) as it
falls through as distance h. The process reverses as the
bob moves up the other side of its swing.
The ball trades potential energy for kinetic energy as it
falls. Notice that the ball had 98 J of potential energy
when dropped and has a kinetic energy of 98 J just as
it hits the ground.
Energy arrives from the sun, goes through a number of
conversions, then radiates back into space. The total
sum eventually equals the original amount that
arrived.
– Example
• a 11.1 kg rock falls from a height of 10.1 m.
What is its velocity as it hits the floor.
• vf = 2gh
• = 2(9.8m/s2)(10.1m)
• = 197.96 m2/s2
• = 14.1 m/s
• Energy Conservation
– Any form of energy can be converted into
another form.
– The total amount of energy remains constant.
– Law of Conservation of Energy:
• Energy is never created or destroyed.
Energy can be converted from one form to
another, but the total energy remains
constant.
• Energy Sources Today
• Introduction
– Petroleum is our most widely used source of
energy.
• Petroleum provides about 40 percent of the
energy used by the US.
– Natural gas provides about 23 percent of our
energy needs.
– Coal provides about 23 percent of our energy
needs.
– Alternative energies (solar, wind, geothermal)
provide less than 0.5 percent of the total.
– Over 99 percent of our energy needs are
supplied by 4 sources:
• Petroleum
• Coal
• Hydropower
• Nuclear
• Petroleum
– Petroleum is extracted from oil bearing rocks.
– Petroleum and natural gas form from
compaction and decay of organic matter that
has settled out of water.
– Most of the organic material comes from
plankton.
– The process of converted organisms into
petroleum/natural gas takes millions of years.
– Natural gas forms under higher temperatures
than petroleum.
• Coal
– Coal forms from an accumulation of plant
materials that collected millions of years ago.
– Carbon rich decayed plant material is called
peat.
– Pressure, compaction, and heating lower the
water content and increase the carbon % in the
materials. The coal formation process has
begun.
– Coal is ranked according to how long it took to
form and how hard it is.
• Lignite is the lowest ranked and is softest,
takes the least time to form, burns quickest
and contains the least amount of usable
energy.
• Bituminous is the next highest raking.
• Anthracite is the hardest, takes the longest to
form and contains the most usable energy.
• Softer coal also has more impurities which
contribute to increased pollution levels.
• Water Power
– Moving water is a source of renewable energy
that has been used for thousands of years.
– The US has built about all of the hydropower
plants that we can.
• Nuclear Power
– Nuclear power plants use the energy that is
released from the splitting of uranium and
plutonium atoms to produce electrical energy.