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Energy
Chapter 15
Chapter 15 Pretest
1. How much work is done when a
weightlifter holds a barbell motionless
over his head?
– No work is done!
2. Calculate the work done on a 2-N mass
when it is lifted to a height of 2 m.
–4J
3. Calculate the average speed of a bicycle
that travels 100 m in 20 s.
– 5 m/s
Chapter 15 Pretest
4. Is weight a force?
– yes
5. What is the formula for calculating
weight?
– W = mg
6. How does the temperature of an object
change when it is acted on by friction?
– The temperature increases.
Chapter 15 Pretest
7. True or false: In a closed system, the
loss of momentum of one object equals
the gain in momentum of another object.
– True
8. How is power related to work?
– Power is the rate at which work is done
9. True or false: The amount of work done
on a machine (work in) always equals the
amount of work done by the machine
(work out).
– False
How is Energy Related to Work?
• Energy is defined as the ability to do work.
Recall that work is the product of force and
distance. If a force acts through a greater
distance, it has done more work. You can use
work to measure changes in energy.
• Place two identical books on the table so there is
a gap of about 8 cm between books. Place a
sheet of notebook paper on the books so it
covers the gap shown. Now drop a penny from
a height of 10 cm onto the paper above the gap.
Note what happens. Next, drop the penny from
a height of 30 cm and observe the results.
How is Energy Related to Work?
1. How did the height of the penny affect the
distance the paper moved?
– The paper moved farther when the penny was
dropped from a greater height.
2. How did lifting the penny affect the work it
did on the paper?
– Lifting the penny allowed it to do more work on the
paper.
3. How did lifting the penny affect its
energy?
– Lifting the penny increased the penny’s energy.
15.1- Energy and Its Forms
• Energy is the ability to do work.
– Energy is transferred by a force moving an object
through a distance.
• Work and Energy are closely related.
– When work is done on an object, energy is
transferred to that object.
– Work is a transfer of energy.
• Energy is typically measured in joules (J) just
like work.
15.1 – Kinetic Energy
• The energy of motion is called kinetic energy.
• The kinetic energy of any moving object
depends upon its mass and speed.
KE = ½ mv2
• KE = joules (J), mass (m) = kg, speed (v) = meters/sec (m/s)
– Double the mass, double the KE
– Double the speed, quadruple the KE
Kinetic Energy Math Practice
1. A 70.0 kilogram man is walking at a speed of
2.0 m/s. What is his kinetic energy?
– KE = 1/2mv2 = 0.5(70kg)(2 m/s)2= 140 J
2. A 1400 kilogram car is moving at a speed of 25
m/s. How much kinetic energy does the car
have?
– KE = 1/2mv2 = 0.5(1400kg)(25 m/s)2= 437,500 J
3. A 50.0 kilogram cheetah has a kinetic energy of
18,000 J. How fast is the cheetah running?
– v = √2(KE)/m = √2(18,000 J)/50.0 kg = 27 m/s
15.1 – Potential Energy
• Potential energy is the energy that is
stored as a result of position or shape.
– PE has the potential to do work
– Examples: plucking a guitar string, lifting a
book
• Two forms of potential energy are:
– Gravitational Potential Energy
– Elastic Potential Energy
15.1 – Gravitational Potential Energy
• Potential energy that depends on an object’s
height is called gravitational potential energy.
– Gravitational Potential Energy increases when an
object is raised to a higher level.
• An object’s gravitational potential energy
depends on its mass, its height, and the
acceleration due to gravity.
PE = mgh
• PE = joules (J), mass (m) = kilograms (kg), acceleration due to
gravity (g) = 9.8 m/s2 on Earth, height (h) = meters
• Note: height is measured from the ground or floor, and GPE is
measured relative to that same reference level.
15.1 – Elastic Potential Energy
• The potential energy of an object that is
stretched or compressed is known as elastic
potential energy.
• Something is said to be elastic if it springs back
to its original shape after it is stretched or
compressed.
• Examples: rubberband, spring, basketball,
shock absorber, wind-up toy
15.1 – Forms of Energy
• All energy can be considered to be kinetic
energy, potential energy, or the energy in fields
such as those produced by electromagnetic
waves.
• The major forms of energy are:
–
–
–
–
–
–
mechanical energy
thermal energy
chemical energy
electrical energy
electromechanical energy
nuclear energy.
15.1 – Mechanical Energy
• The energy associated with the motion and position
of everyday objects is mechanical energy.
• Mechanical energy is the sum of an object’s
potential energy and kinetic energy.
• Examples: speeding trains, bouncing balls, sprinting
athletes
• Mechanical energy is NOT limited to machines.
• Mechanical energy does not include thermal energy, chemical
energy, or other forms of energy associated with the motion or
the arrangement of atoms or molecules.
15.1 – Thermal Energy
• The total potential and kinetic energy of all the
microscopic particles in an object make up its
thermal energy.
– Almost all of the matter around you contains atoms.
– These particles are always in random motion and
thus have kinetic energy.
• When an object’s atoms move faster, its thermal
energy increases and the object becomes
warmer.
• Examples: molten metal, toasting
marshmallows
15.1 – Chemical Energy
• Chemical energy is the energy stored in
chemical bonds.
– When bonds are broken, the released energy
can do work.
• All chemical compounds store energy.
• Examples: fuels such as coal or gasoline,
wood
15.1 – Electrical Energy
• Electrical energy is the energy associated
with electric charges.
– Electric charges can exert forces that do work.
• Examples: batteries, lightning bolts
15.1 – Electromagnetic Energy
• Electromagnetic energy is a form of
energy that travels through space in the
form of waves.
• Examples: visible light, x-rays, sun
15.1 – Nuclear Energy
• The energy store in atomic nuclei is known
as nuclear energy.
• Examples: heat and light of the sun,
nuclear power plant operations
– nuclear fission – process that releases energy by
splitting nuclei apart
– nuclear fusion – releases energy when less massive
nuclei combine to form a more massive nuclei
15.2 – Energy Conversion
• Energy can be converted from one form to
another.
• The process of changing energy from one
form to another is energy conversion.
• Examples: wind-up toys, light bulbs,
striking a match
15.2 – Conservation of Energy
• When energy changes from one form to another,
the total energy remains unchanged even
though many energy conversions may occur.
• The law of conservation of energy states that
energy cannot be created or destroyed.
– Energy can be converted from one form to another.
– In a closed system, the amount of energy present at
the beginning of a process is the same as the amount
of energy at the end.
15.2 – Energy Conversions
• One of the most common energy conversions is
between potential energy and kinetic energy.
• The gravitational potential energy of an object is
converted to the kinetic energy of motion as the
object falls.
• Examples: avalanche, compressed spring,
gull/oyster shell
15.2 – Energy Conversion
• A pendulum consists of a weight swinging
back and forth from a rope or a string.
• Pendulum Examples: rope swing, clock
15.2 – Energy Conversion Calculations
• Mechanical Energy = KE + PE
• *Conservation of Mechanical Energy
(KE + PE)beginning = (KE + PE)end
* Friction is neglected
Conservation of Mechanical Energy
• A 10 kg rock is dropped and hits the
ground below at a speed of 60 m/s.
Calculate the gravitational potential energy
of the rock before it was dropped. You
can ignore the effects of friction.
PEbeginning = KEend = 1/2mv2
=(0.5)(10kg)(60m/s)2 = 18,000 J
Conservation of Mechanical Energy
• A diver with a mass of 70.0 kg stands motionless
at the top of a 3.0 m high diving platform.
Calculate his potential energy relative to the
water surface while standing on the platform,
and his speed when he enters the pool. (Hint:
Assume the diver’s initial vertical speed after
diving is zero).
PEbeginning = mgh
At the beginning, KE = 0 and at the end, PE = 0,
so PEbeginning = KEend = 1/2mv2
2100 J = (0.5)(70.0kg)v2
v = 7.7m/s
Conservation of Mechanical Energy
• A pendulum with a 1.0 kg weight is set in motion
from a position of 0.04 m above the lowest point
on the path of the weight. What is the kinetic
energy of the pendulum at the lowest point?
(Hint: Assume there is no friction.)
PEbeginning=mgh
=(1.0 kg)(9.8 m/s2)(0.04m) = 0.4 J
At the beginning, KE = 0; at the lowest point, PE = 0;
PEbeginning = KEend = 0.4 J
15.2 – Energy and Mass
• Special theory of relativity developed by
Albert Einstein in 1905.
• E = mc2 c = 3.0 x 108 m/s
• E = energy, m = mass, c = speed of light
• Einstein’s equation says that energy and
mass are equivalent and can be converted
into each other.
– Energy is released as matter is destroyed
– Matter can be created from energy
15.3 – Energy Resources
• Energy resources can be classified as
renewable or nonrenewable.
• Nonrenewable – exist in limited supply and
cannot be replaced quickly
– Examples: oil, natural gas, coal, uranium
– Fossil Fuels: oil, natural gas, coal
• Usually readily available and inexpensive, but their
use creates pollution
15.3 – Renewable Energy Resources
• Renewable – energy resources that can
be replaced in a relatively short period of
time.
– Most originate directly or indirectly from the
sun
– Examples: hydroelectric, solar, geothermal,
wind, biomass, nuclear fusion
15.3 - Definitions
• Hydroelectric energy – energy obtained from
flowing water
• Solar energy – sunlight that is converted into
usable energy
• Geothermal energy – thermal energy beneath
Earth’s surface
• Biomass energy – chemical energy stored in
living things
• Hydrogen fuel cell – generates electricity by
reacting hydrogen with oxygen.
15.3 – Conserving Energy Resources
• Energy resources can be conserved by
reducing energy needs and by increasing
the efficiency of energy use.
• Things you can do:
– Turning off lights, etc. when not in use
– Walk or bike on short trips
– Carpool or mass transportation
– Fuel-efficient automobiles
– Energy-efficient purchases