Chapter 11: Motion
Download
Report
Transcript Chapter 11: Motion
C HAPTER 15:
E NERGY
S ECTION 15–1:
E NERGY AND I TS F ORMS
O BJECTIVES
Describe the relationship between work and energy.
Relate kinetic energy to mass and speed and calculate
these quantities.
Analyze how potential energy is related to an object’s
position and give examples of gravitational and elastic
potential energy.
Solve equations that relate an object’s gravitational
potential energy to its mass and height.
Give examples of the major forms of energy and explain
how each is produced.
E NERGY
AND
W ORK
Remember from chapter 14 that work is done when a
force moves an object through a distance.
Energy is the ability to do work. It is transferred by a
force moving an object through a distance.
Work and energy are closely related. Work is a transfer
of energy. Both work and energy are measured in
joules (J), and a joule is a newton-meter.
K INETIC E NERGY
Many forms of energy can be classified into two general
types: kinetic energy and potential energy.
The energy of motion is called kinetic energy. The
Greek word kinetos means “moving.”
The kinetic energy of any moving object depends upon
its mass and speed.
To calculate the kinetic energy of an object in joules,
multiply the object’s mass in kilograms, the square of its
velocity (speed) in meters per second, and ½.
KE = ½mv2
S AMPLE P ROBLEM – K INETIC E NERGY
A 0.10 kilogram bird is flying at a constant speed of 8.0
m/s. What is the bird’s kinetic energy?
Given:
Mass: 0.10 kg
Speed: 8.0 m/s
Solve:
KE = ½mv2
KE = ½(0.10 kg)(8.0 m/s)2
KE = ½(0.10 kg)(64 m2/s2)
KE = 3.2 kgm2/s2
KE = 3.2 J
P OTENTIAL E NERGY
Potential energy is the energy that is stored as a result
of position or shape.
Pulling a string tightly before plucking it and holding a
book in the air are examples of objects with potential
energy.
There are two forms of potential energy: gravitational
potential energy and elastic potential energy.
G RAVITATIONAL P OTENTIAL E NERGY
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.
An object’s gravitational potential energy depends on
its mass, its height, and the acceleration due to gravity.
Remember that on Earth, g is always 9.8 m/s2.
GPE = mgh
S AMPLE P ROBLEM – P OTENTIAL E NERGY
Suppose a diver is at the top of a 10.0-meter-high diving
platform has a mass of 50.0 kilograms. What is her
gravitational potential energy?
Given:
Height: 10.0 m
Mass: 50.0 kg
Gravity: 9.8 m/s2
Solve:
GPE = mgh
GPE = (50.0 kg)(9.8 m/s2)(10.0 m)
GPE = 4900 kgm2/s2
GPE = 4900 J
E LASTIC P OTENTIAL E NERGY
The potential energy of an object
that is stretched or compressed is
known as elastic potential energy.
Something is said to be elastic of it
springs back to its original shape
after it is stretched or compressed.
When you drop a slice of bread on
the ground, it may barely bounce.
This is because it is not very elastic.
When you drop a golf ball, it
bounces almost back to where it
was dropped from.
F ORMS
OF
E NERGY
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,
electromagnetic energy, and nuclear energy.
M ECHANICAL E NERGY
The energy associated with motion and position of
everyday objects is mechanical energy.
Mechanical energy is not limited to machines.
Mechanical energy is the sum of an object’s potential
energy and kinetic energy.
Speeding trains, bouncing balls, and sprinting athletes
all have mechanical energy.
T HERMAL E NERGY
Almost all of the matter around you contains atoms.
These particles are always in random motion and thus
have kinetic 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. If they get
hot enough, they may emit visible light.
C HEMICAL E NERGY
Chemical energy is the energy stored
in chemical bonds. When bonds are
broken, the released energy can do
work.
All chemical compounds, including
fuels like coal and gasoline, store
energy.
Burning wood at a campfire is
considered chemical energy because
the chemical bonds in the wood are
being broken. This provides thermal
energy to roast marshmallows!
E LECTRICAL E NERGY
Electrical energy is the
energy associated with
electric charges.
Batteries convert
chemical energy into
electrical energy.
Lightning bolts are
produced by electrical
energy.
E LECTROMAGNETIC E NERGY
Electromagnetic energy is a
form of energy that travels
through space in the form
of waves.
X-rays and visible light are
examples of
electromagnetic energy.
Because they can travel
long distances through air
and space, they are often
used in communication.
N UCLEAR E NERGY
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 and fusion are
processes that create and use
nuclear energy.
V OCABULARY
Energy
Thermal energy
Kinetic energy
Chemical energy
Potential energy
Electrical energy
Gravitational potential
energy
Electromagnetic energy
Nuclear energy
Elastic potential energy
Mechanical energy