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Energy Objectives
1. Distinguish between kinetic and potential
energy.
2. Calculate kinetic energy.
3. Describe different forms of potential energy.
4. Calculate gravitational potential energy.
5. Describe how energy can be transferred from one
form to another.
6. Explain how the mechanical energy of a system is
the sum of the kinetic and potential energy.
7. Discuss the law of conservation of energy.
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Nature of Energy
Energy is the ability to cause change
1. kinetic energy (Ek)- energy in the form of motion
-depends on an object’s mass and speed
~increase mass,
increase kinetic energy
~increase speed,
increase kinetic energy
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Nature of Energy
1. Kinetic energy (Ek)
-shown by the relationship
kinetic energy = ½ mass x speed2
Ek = ½ mv2
-SI unit is the Joule (J), or kg•m2/s2, or N•m
-the faster you are moving, the more kinetic energy
you have, helping you overcome gravity
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Which is more important to Ek, speed or mass?
-if you double the mass, you only double that
component
Ek = ½ mv2
Ek = ½ mv2
= ½ (2kg)(1m/s)2
= ½ (4kg)(1m/s)2
= (1kg)(1m2/s2)
= (2kg)(1m2/s2)
-if you double the velocity, you quadruple that
component
Ek = ½ mv2
Ek = ½ mv2
= ½ (1kg)(1m/s)2
= ½ (1kg)(2m/s)2
= (0.5kg)(1m2/s2)
= (0.5kg)(4m2/s2)
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Kinetic Energy: Example 1
A jogger whose mass is 60 kg is moving at a speed of 3 m/s.
What is the jogger’s Ek?
Known variables: m = 60 kg
v = 3 m/s
Unknown variable: Ek = ?
Formula:
Ek = ½ mv2
=1/2(60kg)(3 m/s)2
=30 kg(32 m2/s2)
=30 kg(9 m2/s2)
=270 kg•m2/s2
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Kinetic Energy Practice
Remember: J = kg·m2/s2
1. What is the EK of a 1500 kg car moving at 18 m/s?
2. A 4 kg rock is rolling 10 m/s. Find the EK.
3. A rolling ball has 18 J of energy and is rolling 3 m/s.
What is it’s mass?
4. A 4 kg bird has 8 J of kinetic energy. How fast is it
flying?
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Energy Homework 1
1. Two trucks have the same velocity but different masses.
The truck with the greater mass has ___________ EK than
the other truck.
2. __________ is the ability to cause change.
3. A __________ is a unit of measure of all forms of energy.
4. Which has more kinetic energy: A 25 kg mass or a 30 kg
mass?
5. Which has more kinetic energy: Two 10 kg masses, one
going 75 m/s and one going 45 m/s?
6. Which has more kinetic energy: A car at rest or a car
rolling down a hill?
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2. potential energy- energy in a motionless object, giving
it the potential to cause change.
a. elastic potential energy- energy stored by things
that compress or
stretch
b. chemical potential energy- energy stored in
chemical bonds between atoms
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2. potential energy (EP)- energy in a motionless object,
giving it the potential to cause change.
c. gravitational potential energy (EGP)- energy stored by things
that are above Earth.
-amount depends on its mass, the acceleration due to
gravity and its height above ground
EGP = mass x 9.8 m/s2 x height
EGP = mgh
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Potential Energy: Example 1
Suppose a ceiling fan has a mass of 7 kg and is 4 m above
the ground. What is its gravitational potential energy?
Known variables: m = 7 kg
g = 9.8 m/s2
h=4m
Unknown variable: EGP = ? J
Formula:
EGP = mgh
=(7 kg)(9.8 m/s2)(4 m)
=274 kg·m2/s2
=274 J
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Potential Energy Practice
Remember: J = kg·m2/s2
1. Calculate the EGP of a 0.52 kg bird flying at an altitude of
550 m.
2. A diver has 3400 J of EGP after stepping up onto a
platform that is 6.0 m above the water. What is the
diver’s mass?
3. Before ascending the cliff, a 65 kg rock climber has a EGP
of 22295 J. How high was he from the base of the cliff?
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Energy Homework 2
1. Two baseballs have the same mass. The ball closer to the
ground has __________ GPE than the other ball.
2. The _________ energy of an object depends on its mass
and height.
3. The energy of food and other fuels is __________ potential
energy.
4. What is the difference between kinetic and potential
energy?
5. Which has more potential energy: a 25 kg mass or a 30 kg
mass at the top of a hill?
6. Which has more potential energy: a car at the top of the
hill or the bottom of a hill?
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Kinetic and Potential Energy Practice
1.
2.
3.
4.
5.
Remember: J = kg·m2/s2
Calculate the potential energy of a 5 kg object sitting
on a 3 m ledge.
A 10 kg rock is at the top of a 20 m tall hill. How much
potential energy does it have?
How high up is a 3 kg object that has 300 J of energy?
An object with 20 J of energy sits on a cliff that is 15 kg
high. What is the mass of the object?
A skater travels 150 m with 38 J of energy. How fast is he
going?
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Energy Quiz 1
1. Label each of the following as either kinetic or
potential energy:
a. Car traveling 45 mi/hr
b. Rock on a 5m ledge
2. Which has more potential energy: plane on the
ground or a plane in the air?
3. What are the units for energy?
4. What kind of energy is gained from
a. A to C?
b. D to F?
5. An 8 kg object has 64 J of energy. How fast is it
going?
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Conservation of Energy
Law of Conservation of Energy:
energy may change from one form to another, but
the total amount of energy never changes
top = all potential (no kinetic)
middle = ½ potential + ½ kinetic
bottom = all kinetic (no potential)
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Since energy can only be transformed, then for an object
thrown into the air or falling,
EK (at the bottom)= EGP (at the top)
and
Total Energy (ET) = EK + EGP
What happens to the kinetic energy as the potential energy
increases?
-as potential energy increases, kinetic energy decreases
and vice versa
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Energy Transformations
energy conversions-energy changing from one form to
another
1. chemical energy (as seen earlier)
-fuels store energy
-chemical PE
thermal energy and/or kinetic energy
2. electrical energy (aka electricity)
-lightbulbs
light + thermal energy
-alarm clocks
sound + light + thermal energy
-video games
sound + light + thermal energy
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Energy Transformations
3. thermal energy (heat energy)
-heat
burns
-end product of many transformations
4. nuclear energy (from radiation, fission, fusion)
-produces huge amounts of energy
-fusion in the sun
heat + light + radiation
-fission in power plants
heat (steam)
electricity
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Energy Transformations
5. radiant energy (light energy)
-aka electromagnetic radiation
-sun
chemical potential (photosynthesis)
6. mechanical energy (can become work)
-energy of an objects motion (EK) or position (EGP)
-swinging, falling, tossing a ball
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In energy conversions, friction
converts some of the
mechanical energy into
thermal energy (heat) and
sometimes sound.
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Energy Homework 3
Match the following:
Energy increase or decrease?
7. A force pushes something into the air.
8. When your car brakes.
What kind of energy transfer?
9. A nuclear power plant.
10. A battery powered flashlight.
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Conservation of energy in your body.
1. chemical potential energy from food stored in your body
is used to fuel the processes that keep you alive; a few
include
-making your heart beat
-digesting food
-regulate body temperature
-muscle movement
-breathing
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2. The food Calorie is used to measure how much energy
you get from various foods.
-One Calorie is equivalent to about 4180 J of energy
-every gram of fat consumed can supply 9 C of energy
-carbs and proteins can supply 4 C of energy
Discussion
Is energy lost when a car burns gasoline? Explain.
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Energy Review 2
1. The type of energy that is greatest at the bottom of a
swing’s path is ____________.
2. The type of energy due to the position and motion of an
object is _____________.
3. What is the unit for energy?
4. Describe the law of conservation of energy.
5. Identify the energy transformations for the following:
a. A jet plane rapidly accelerates on the runway.
b. A swallow sitting on the fence sings a song.
c. An electric blanket warms a bed on a cold night.
6. A hammer falls off the roof and strikes the ground. If it fell
from twice the height, how would the kinetic energy
compare?
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More Practice Problems – GPE and KE
Remember: J = kg·m2/s2
1. Calculate the gravitational potential energy of a 93.0 kg
sky diver who is 550 m above the ground.
2. What is the kinetic energy of a 0.02 kg bullet traveling 300
m/s?
3. If a 2.0 kg rock has a gravitational potential energy of 850
J, how high above the ground is it sitting?
4. If the 2.0 kg rock in problem #3 above falls off the cliff it is
sitting on has a kinetic energy of 850 J, what is the speed
it is traveling before it hits the ground?
5. What is the speed of a 0.15 kg snowball moving through
the air when it has 49 J of energy?
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Energy Homework 4
Write the Q and A
1.Can an object have potential energy and kinetic
energy at the same time? Why or why not?
3. How can you increase the kinetic energy of an
object without increasing it’s speed?
4. How do mass and height affect gravitational
potential energy?
5. What are the energy transformations that take
place in:
a. photosynthesis b. computer
c. walking
6. True/False: Any two objects on the same shelf of a
cupboard have the same potential energy?
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