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Chapter 4
Energy
The Nature of Energy
What is energy?
• Energy is the ability to do work.
• Energy is the ability to cause a change.
• Examples:
– Baseball flying through the air, a rollercoaster, CD
player, lights, etc.
• There are 5 major forms of energy:
– Mechanical, Heat, Chemical, Electromagnetic,
and Nuclear.
The Nature of Energy
1.
•
•
Mechanical Energy:
Energy associated with motion.
Examples:
–
Water in a water fall, wind, automobiles, planes, walking, riding a
bike, hitting a ball, sound, etc.
2. Heat Energy:
•
The internal motion of particles of matter.
•
The faster the particles move, the more heat energy is
present.
•
Heat energy often causes changes in the temperature
and phase (solid, liquid, or gas) of any form of matter.
•
Examples:
–
Rubbing hands together turns mechanical energy into heat energy.
The Nature of Energy
3. Chemical Energy:
• The energy that bonds atoms or ions together.
• When bonds are broken, atoms are rearranged,
and new bonds are formed. During this process,
stored chemical energy is released.
• Examples:
– Rocket fuel being ignited changes chemical energy into
heat energy, BBQ grills, throwing a ball (muscles:
chemical energy is used in your body in order to throw the
ball).
The Nature of Energy
4. Electromagnetic Energy:
• Moving electrical charges.
• Power lines carry electromagnetic energy
into your home in the form of electricity.
• Examples:
– Electric motors, light (each color = a different
amount of energy), x-rays, radio waves, laser
light, etc.
The Nature of Energy
5. Nuclear Energy:
• Energy released by the splitting of the nucleus of an
atom; energy locked within the nucleus by the strong
force.
• Nuclear energy is the most concentrated form of
energy.
• When a nucleus splits, nuclear energy is released in
the form of heat energy and light energy.
• Examples:
– The sun’s energy is produced from a nuclear fusion
reaction in which hydrogen nuclei fuse to form helium
nuclei.
The Nature of Energy
The five forms of energy can be classified
as either kinetic or potential energy.
Kinetic Energy (KE):
The energy a moving object has
because of its motion.
The faster an object moves, the more
kinetic energy it has.
COPY THIS SLIDE IN THE MARGIN
OF THE HANDOUT
Kinetic energy depends on both the mass of the
object and the speed at which the object is
moving.
Kinetic Energy = ½ × (mass) × (velocity)2
KE = ½mv2
According to this equation, an increase in either
mass or velocity of an object will increase
the kinetic energy of the object.
Measured in Joules (J).
The Nature of Energy
Example:
• A jogger whose mass is 60-kg is moving at
a speed of 3-m/s. What is the jogger’s
kinetic energy?
KE = ½ mv2
KE = ½ (60-kg)(3-m/s)2
KE = 270-J
The Nature of Energy
Example:
• What is the kinetic energy of a baseball
moving at a speed of 40-m/s if the baseball
has a mass of 0.15-kg?
KE = ½ mv2
KE = ½ (0.15-kg)(40-m/s)2
KE = 120-J
The Nature of Energy
Example: You Try it
• A car moving at a speed of 20-m/s has a
kinetic energy of 300,000-Joules. What is the
car’s mass?
KE = ½ mv2
300,000-J = ½ (m)(20-m/s)2
Mass = 1500-kg
The Nature of Energy
Example:
• A sprinter has a mass of 80-kg and a kinetic
energy of 4000-Joules. What is the sprinter’s
speed?
KE = ½ mv2
4000-J = ½ (80-kg)(v)2
Velocity = 10-m/s
The Nature of Energy
Potential Energy (PE):
• Energy stored in an object due to its
position.
• Example:
–As you stretch a rubber band, energy
is put into it. In its new position, the
stretched rubber band contains
potential energy.
2.
Question?
• If you are standing on the edge of a 1-m high
diving board, do you have any energy?
• Yes – Potential energy; due to your position
above the water below.
Panther drill: Write the number and then state
whether the picture shows potential or kinetic energy
1
2
3
P
4
P
K
5
K
P
Choose one of the skate
boarder # 1,2, or 3, and
calculate the KE and GPE for
him. Make sure to circle the
one you choose and show
your work
GPE= mph
KE = ½ x M
3m
xV2
Exit Ticket
Choose one of the skate
boarder # 1,2, or 3, and
calculate the KE and GPE for
him. Make sure to circle the
one you choose and show
your work
GPE= mph
KE = ½ x M
3m
xV2
Exit Ticket
Review
What is potential Energy?
Stored Energy
What is kinetic energy?
Moving energy
What unit is energy measured in?
Joules (J)
If you increase mass or velocity of an object
what happens to the KE?
It increases
KE = ½ mv2
How do you figure out KE?
The Nature of Energy
Energy that is dependent on the height above
the earth’s surface is called gravitational
potential energy (GPE).
Weight also determines the amount of GPE an
object has.
GPE = Weight × Height
GPE = Mass × Gravity × Height
GPE = mgh
The Nature of Energy
GPE = Weight × Height
W = m x ag
GPE = Mass × Gravity × Height
GPE = mgh
The Nature of Energy
The greater the weight and height of an
object, the greater the GPE.
Measured in Joules (J).
Gravity = 9.81-m/s2 (on Earth)
How it works
GPE=
mxgxh
5 kg
9.8
m/s2
5 x 9.8 x 2
2m
98 joules (j)
The Nature of Energy
Example:
• What is the GPE of a ceiling fan that has a
mass of 7-kg and is 4-m above the ground?
GPE = mgh
GPE = (7-kg)(9.81-m/s2)(4-m)
GPE = 274-Joules
The Nature of Energy
Example:
• Find the GPE of a coffee mug with a mass of
0.3-kg on a 1-m high counter top.
GPE = mgh
GPE = (0.3-kg)(9.81-m/s2)(1-m)
GPE = 2.94-Joules
The Nature of Energy
Example: YOU TRY
• Find the GPE of a baseball with a mass of
0.15-kg that has been thrown to a height of 50
m.
GPE = mgh
GPE = (0.15-kg)(9.81-m/s2)(50m)
GPE = 73.5-J
What if you need other info?
What if you need to find mass or height?
Switch the formula around! GPE = m x g x h
Mass =
Height =
GPE
gxh
GPE
mxg
The Nature of Energy
Example: You Try This time switch the
formula around to find mass
• What is the mass of a hiker 200-m above the
ground if her GPE is 117,600-J?
GPE = mgh
117,600-J = (m)(9.81-m/s2)(200-m)
Mass = 60-kg
The Nature of Energy
• Other types of Potential Energy:
• Elastic potential energy is the energy stored
by something that can stretch or compress.
– Examples: Rubber band or spring.
• Chemical potential energy is energy stored
in chemical bonds.
– Examples: Gasoline or food.
A-B Partners
1. Each partner will do a problem and either A
or B
2. Then you will explain to your partner how you
got your answer
3. Each partner will sign of on the others work.
4. WHEN YOU ARE TOLD TO TURN IT IN
THE BASKET
Problems
Partner A
Partner B
What is the GPE of a What is the GPE of a
ball with a mass of
person standing at
0.5 kg and a height
the top of a ladder
of 10 meters above
with a height of 15
the ground?
meters? The
person has a mass
of 80 kg.
Individual Practice
On your OWN!!!
Choose 2 of the 3 problems on the small sheets
of paper I give you. Answer the question,
show your work, and bring them to me so I
can see them when you are done when you
are done.
You have 5-10 minutes to complete this activity
QUIETLY
Choose one of the skate
boarder # 1,2, or 3, and
calculate the KE and GPE for
him. Make sure to circle the
one you choose and show
your work
GPE= mph
KE = ½ x M
3m
xV2
Exit Ticket
Extra Examples
Use this equation: GPE = m x 9.8 m/s2 x h
• What is the GPE of a bowling ball with a mass
of 1.5 kg and held 2 meters above the
ground?
• What is the GPE of a car with a mass of 505
kg and a height of 40 meters above the
ground? (It’s falling off of a cliff)
Monday
Panther Drill
In your owns words describe how energy is
changed from when you turn a key in the
car to after get to where you are going.
DO YOUR BEST DON’T SWEAT IT IF IT’S
NOT PERFECT.
Conservation of Energy
• Energy is always conserved, it can never
be destroyed, only changed from one type
of energy to another.
• The changes in the forms of energy are
called energy transformations.
• The most common energy conversion:
changing of potential energy to kinetic
energy
EXAMPLES
• Examples:
– A lightbulb converts electrical energy to light and
heat energy.
– A gasoline engine converts chemical energy
eventually into kinetic energy.
Potential energy to kinetic
energy
• Example: Rollercoasters
– At the top of the first hill, the car has a maximum amount
of potential energy and almost zero potential energy.
– As the car rolls down the first hill, its kinetic energy
increases and the potential energy decreases.
– What happens to the cars energy as it goes up the next
hill?
Conservation of Energy
Mechanical energy (ME): The total amount
of potential and kinetic energy in a system
ME = KE + PE
The total ME of an object always remains
constant, it never changes.
Conservation of Energy
Example:
• Suppose a ball is released from a height of
2.00-meters above the ground.
– What is the gravitational potential energy of the
ball before it is released?
– What is the kinetic energy of the ball before it is
released?
– What is the total mechanical energy of the ball
before it is released?
– What is the kinetic energy of the ball just before it
hits the ground after it is released?
Conservation of Energy
Law of Conservation of Energy: energy cannot
be created or destroyed.
On a large scale, this law means that the total
amount of energy in the universe does not
change.
The same amount of energy that was here at the big
bang (15mil+ yrs ago) is here now!!!!
Conservation of Energy
Most famous use of Law
• Albert Einstein dealt with the concept that
mass and energy were interchangable.
E=
2
mc
• Einstein said that matter is another form of
energy, or that mass and energy are two
forms of the same thing and can be converted
into each other.
E = mc2
Resulted in Nuclear Bomb that ended what war
when the USA used it?
WWII
Energy Transformations
What happens when turn on the TV?
Potential Electric to Electric to Light energy
& Heat energy
How about when you run track?
Chemical Potential in your body turns to kenetic
energy in muscles & heat energy (sweat)
1-5 about Energy conversions
1-2 Still need a lot of help
3 I am ok w/ it
4 I am good with it
5 Take the day of Mr P I will
teach it!!!