Transcript 15.1 Energy and Its Forms

15.1 Energy and Its Forms
In an avalanche, a
mass of loose snow,
soil, or rock suddenly
gives way and slides
down the side of a
mountain.
The avalanche
releases a great
amount of energy.
15.1 Energy and Its Forms
Energy and Work
How are energy and work related?
Energy is the ability to do work.
Work is a transfer of energy.
15.1 Energy and Its Forms
Energy and Work
Work and energy are closely related.
• Energy is known by the changes it causes.
• Work is done when a force moves an object
through a distance. Energy is transferred by a
force moving an object through a distance.
• Both work and energy are typically measured in
joules (J).
15.1 Energy and Its Forms
Energy and Work
Energy has different forms.
A. The sun gives off energy in the form of heat and
light.
15.1 Energy and Its Forms
Energy and Work
Energy has different forms.
A. The sun gives off energy in the form of heat and
light.
B. Plants convert sunlight into food.
15.1 Energy and Its Forms
Energy and Work
Energy has different forms.
A. The sun gives off energy in the form of heat and
light.
B. Plants convert sunlight into food.
C. People convert food energy into muscle
movement.
15.1 Energy and Its Forms
Kinetic Energy
What factors does the kinetic energy of an
object depend on?
The energy of motion is called kinetic energy.
The kinetic energy of any moving object
depends upon its mass and speed.
15.1 Energy and Its Forms
Kinetic Energy
• Doubling the mass in the formula doubles the
kinetic energy.
• Doubling the speed quadruples the kinetic
energy.
15.1 Energy and Its Forms
Kinetic Energy
Calculating Kinetic Energy
A 0.10-kilogram bird is flying at a constant speed
of 8.0 m/s. What is the bird’s kinetic energy?
15.1 Energy and Its Forms
Kinetic Energy
Read and Understand
What information are you given?
What unknown are you trying to calculate?
15.1 Energy and Its Forms
Kinetic Energy
Read and Understand
What information are you given?
What unknown are you trying to calculate?
15.1 Energy and Its Forms
Kinetic Energy
Plan and Solve
What equation contains the given quantities and
the unknown?
Substitute the known values in the formula for KE.
15.1 Energy and Its Forms
Kinetic Energy
Plan and Solve
What equation contains the given quantities and
the unknown?
Substitute the known values in the formula for KE.
15.1 Energy and Its Forms
Kinetic Energy
Look Back and Check
Is your answer reasonable?
15.1 Energy and Its Forms
Kinetic Energy
Look Back and Check
Is your answer reasonable?
It seems reasonable, because the bird has a
low mass, so it would not have much kinetic
energy.
15.1 Energy and Its Forms
Kinetic Energy
1. A 70.0-kilogram man is walking at a speed of
2.0 m/s. What is his kinetic energy?
Answer:
15.1 Energy and Its Forms
Kinetic Energy
1. A 70.0-kilogram man is walking at a speed of
2.0 m/s. What is his kinetic energy?
Answer:
KE = (0.50)(70.0 kg)(2.0 m/s)2 = 140 J
15.1 Energy and Its Forms
Kinetic Energy
2. A 1400-kilogram car is moving at a speed of
25 m/s. How much kinetic energy does the car
have?
Answer:
15.1 Energy and Its Forms
Kinetic Energy
2. A 1400-kilogram car is moving at a speed of
25 m/s. How much kinetic energy does the car
have?
Answer:
= (0.50)(1400 kg)(25 m/s)2
= 440,000 J
15.1 Energy and Its Forms
Kinetic Energy
3. A 50.0-kilogram cheetah has a kinetic
energy of 18,000 J. How fast is the cheetah
running? (Hint: Rearrange the equation to solve
for v.)
Answer:
15.1 Energy and Its Forms
Kinetic Energy
3. A 50.0-kilogram cheetah has a kinetic
energy of 18,000 J. How fast is the cheetah
running? (Hint: Rearrange the equation to solve
for v.)
Answer:
15.1 Energy and Its Forms
Potential Energy
How is gravitational potential energy
determined?
Potential energy is energy that is stored as a
result of position or shape.
An object’s gravitational potential energy
depends on its mass, its height, and the
acceleration due to gravity.
15.1 Energy and Its Forms
Potential Energy
When this musician pulls the
string of her cello to one side,
the string is stretched and
gains potential energy.
15.1 Energy and Its Forms
Potential Energy
The musician adds energy to the cello string
by plucking it.
• The energy stored in the stretched string is
potential energy.
• The stored energy is converted into kinetic
energy when the string is released and it
vibrates.
15.1 Energy and Its Forms
Potential Energy
Gravitational Potential Energy
Potential energy that depends upon an
object’s height is called gravitational
potential energy.
This type of potential energy increases
when an object is raised to a higher level.
15.1 Energy and Its Forms
Potential Energy
This diver has gravitational
potential energy as she stands
at the end of a diving board.
She gained the potential
energy by doing work—by
climbing up the steps to the
diving board.
15.1 Energy and Its Forms
Potential Energy
• The unit for mass is kilograms.
• The unit for height is meters.
• Acceleration due to gravity, g, has a value in
SI units of 9.8 m/s2 on Earth.
• The unit for gravitational potential energy is
joules.
15.1 Energy and Its Forms
Potential Energy
Height is measured from the ground or floor or
some other reference level.
Doubling either the mass of the object or its
height doubles its gravitational potential
energy.
15.1 Energy and Its Forms
Potential Energy
What is the potential energy relative to the
water surface of a diver at the top of a 10.0meter-high diving platform. Suppose she has a
mass of 50.0 kilograms.
PE = mgh
= (50.0 kg)(9.8 m/s2)(10.0 m)
= 4900 kg•m2/s2 = 4900 J
15.1 Energy and Its Forms
Potential Energy
Elastic Potential Energy
The potential energy of an object that is
stretched or compressed is known as
elastic potential energy.
Something that is elastic springs back to
its original shape after it is stretched or
compressed.
15.1 Energy and Its Forms
Potential Energy
A compressed bicycle shock absorber and a
wound-up toy robot both have elastic potential
energy.
15.1 Energy and Its Forms
Forms of Energy
What are the major forms of energy?
The major forms of energy are mechanical
energy, thermal energy, chemical energy,
electrical energy, electromagnetic energy,
and nuclear energy.
15.1 Energy and Its Forms
Forms of Energy
All energy can be considered to be one of
three forms:
• kinetic energy,
• potential energy, and
• energy in fields such as those produced by
electromagnetic waves.
Each of these forms of energy can be
converted into other forms of energy.
15.1 Energy and Its Forms
Forms of Energy
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.
15.1 Energy and Its Forms
Forms of Energy
Thermal Energy
The total potential and kinetic energy of
all the microscopic particles in an object
make up its thermal energy.
When an object’s atoms move faster, its
thermal energy increases, and the object
becomes warmer.
15.1 Energy and Its Forms
Forms of Energy
This molten metal is
extremely hot. It
contains a great deal
of thermal energy.
15.1 Energy and Its Forms
Forms of Energy
Chemical Energy
Chemical energy is the energy stored
in chemical bonds.
When bonds are broken, the released
energy can do work. All chemical
compounds, including fuels such as coal
and gasoline, store energy.
15.1 Energy and Its Forms
Forms of Energy
This family is using the chemical energy of
burning wood to produce thermal energy for
heating marshmallows.
15.1 Energy and Its Forms
Forms of Energy
Electrical Energy
Electrical energy is the energy
associated with electric charges.
Electric charges can exert forces that do
work.
15.1 Energy and Its Forms
Forms of Energy
Electromagnetic Energy
Electromagnetic energy is a form of
energy that travels through space in the
form of waves.
Visible light and X-rays are examples of
electromagnetic energy.
15.1 Energy and Its Forms
Forms of Energy
A. Lightning bolts transfer
electric charge.
B. Galaxies are giant
structures in space that
typically contain billions
of stars. The stars give
off enormous amounts
of electromagnetic
energy.
15.1 Energy and Its Forms
Forms of Energy
Nuclear Energy
The nucleus of an atom is held together by
strong and weak nuclear forces, which can
store an enormous amount of potential energy.
• The energy stored in atomic nuclei is known as
nuclear energy.
• Nuclear fission releases energy by splitting nuclei
apart.
• Nuclear fusion releases energy when less
massive nuclei combine to form a more massive
nucleus.
15.1 Energy and Its Forms
Assessment Questions
1. How are work and energy related?
a.
b.
c.
d.
Energy is the rate of doing work.
Work is a form of energy.
Work is the transfer of energy.
Energy is created by work.
15.1 Energy and Its Forms
Assessment Questions
1. How are work and energy related?
a.
b.
c.
d.
Energy is the rate of doing work.
Work is a form of energy.
Work is the transfer of energy.
Energy is created by work.
ANS: C
15.1 Energy and Its Forms
Assessment Questions
2. A moving object with a mass of 10 kg has 320 J of
kinetic energy due to its motion. How fast is the
object moving?
a.
b.
c.
d.
64 m/s
32 m/s
8 m/s
10 m/s
15.1 Energy and Its Forms
Assessment Questions
2. A moving object with a mass of 10 kg has 320 J of
kinetic energy due to its motion. How fast is the
object moving?
a.
b.
c.
d.
64 m/s
32 m/s
8 m/s
10 m/s
ANS:
C
15.1 Energy and Its Forms
Assessment Questions
3. Which of these is an example of elastic potential
energy?
a.
b.
c.
d.
a bow prepared to release an arrow
a rubber ball thrown into the air
a book about to fall from a table
a truck pulling a trailer
15.1 Energy and Its Forms
Assessment Questions
3. Which of these is an example of elastic potential
energy?
a.
b.
c.
d.
a bow prepared to release an arrow
a rubber ball thrown into the air
a book about to fall from a table
a truck pulling a trailer
ANS: A
15.1 Energy and Its Forms
Assessment Questions
4. A small airplane and a helicopter have identical
masses. If the airplane’s altitude compared to the
ground is three times that of the helicopter, how
much more gravitational potential energy does the
airplane have than the helicopter?
a.
b.
c.
d.
0.333 times as much
3 times as much
6 times as much
9 times as much
15.1 Energy and Its Forms
Assessment Questions
4. A small airplane and a helicopter have identical
masses. If the airplane’s altitude compared to the
ground is three times that of the helicopter, how
much more gravitational potential energy does the
airplane have than the helicopter?
a.
b.
c.
d.
0.333 times as much
3 times as much
6 times as much
9 times as much
ANS: B
15.1 Energy and Its Forms
Assessment Questions
5. The energy stored in the bonds between atoms of
a compound is called
a.
b.
c.
d.
electromagnetic energy.
chemical energy.
atomic energy.
thermal energy.
15.1 Energy and Its Forms
Assessment Questions
5. The energy stored in the bonds between atoms of
a compound is called
a.
b.
c.
d.
electromagnetic energy.
chemical energy.
atomic energy.
thermal energy.
ANS: B