Download Gravitational and Potential Energy

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Warm Up 9/24
Directions: Write which of Newton’s Laws
these sentences describe:
1. I can move the small snowball I made for
the snowman’s head, but I will need help
moving his belly.
2. When I push back with my ski poles, I
move forward.
3. The harder I pull, the faster my dog runs.
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Potential Energy
• Potential Energy is stored energy.
• Ex. A stretched rubber band has the
potential or ability to fly across the room.
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Types of Potential Energy
1. Gravitational Potential Energy
2. Chemical: This type of energy is stored
when you eat food or burn an object
3. Electrical: Used to power houses (also
found in batteries)
4. Elastic: Stored when you stretch a rubber
band (earthquakes too)
5. Nuclear: Produced by splitting the nucleus
of an atom.
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A long way down…
How would you describe a high diver?
As someone who is:
 brave
 insane
 full of beans
 full of gravitational potential energy (GPE).
GPE is the amount of energy an object has
because of its position above the ground,
i.e. its height.
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What is gravitational potential energy?
The gravitational potential
energy (GPE) of an object
on Earth depends on its
mass and its height above
the Earth’s surface.
 When a bungee jumper
starts to fall, he starts to
lose GPE.
 As the elastic cord pulls
the bungee jumper back
up, he gains GPE.
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How is GPE calculated?
The GPE of an object can be calculated using this equation:
GPE = mass x gravitational field strength x height
 Mass is measured in kilograms (kg).
 Gravitational field strength is measured in newtons per
kilogram (N/kg), usually taken as 10 N/kg (9.8) on Earth.
 Height is measured in meters (m).
 GPE is measured in joules (j).
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Factors affecting GPE
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Calculating GPE question 1
An eagle with a mass of
2 kg flies at a height of
200 m above the ground.
How much gravitational
potential energy does the
eagle have?
GPE = mass x gravitational field strength x height
= 2 x 10 x 200
= 4,000 J
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Calculating GPE question 2
An apple with a mass of
200 g falls 3 m from its
branch to the ground.
How much GPE will the
apple have lost when it
reaches the ground?
change
GPE
= mass x gravitational field strength x
in height
lost
= 0.2 x 10 x 3
= 6J
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GPE, mass and height calculations
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What is kinetic energy?
The word ‘kinetic’
comes from the
Greek word ‘kinesis’,
meaning motion.
Kinetic energy is the
energy an object has
because it is moving.
All moving things have kinetic energy, but the amount of
energy they have is not just dependent on how fast they
are moving.
What other factors affect the kinetic energy of a
moving object?
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Kinetic Energy
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Kinetic Energy
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How is kinetic energy calculated?
The kinetic energy (KE) of an object can be calculated using
this equation:
KE = ½ x mass x velocity2
= ½mv2
 Mass is measured in kilograms (kg).
 Velocity is measured in meters per second (m/s).
 KE is measured in joules (j).
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Calculating kinetic energy question
A truck with a mass
of 1,500 kg travels at
a velocity of 20 m/s.
What is the kinetic
energy of the truck?
kinetic energy = ½ x mass x velocity2
= ½ x 1,500 x 202
= 300,000 J = 300 kJ
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Rearranging the KE equation
Sometimes it is necessary to rearrange the kinetic energy
equation in order to calculate the mass or the velocity of
a moving object.
KE = ½mv2
What are the rearranged versions of this equation for
calculating mass and velocity?
m =
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2KE
v2
v =

2KE
m
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KE, mass and velocity calculations
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Too much kinetic energy
Doubling the mass of a moving object doubles its kinetic
energy, but doubling the velocity quadruples its kinetic
energy.
If the velocity of a car is slightly above the speed limit,
its kinetic energy is much greater than it would be at
the speed limit. This means that:
 It is more difficult to stop the
car and there is more chance
of an accident.
 It the car does collide with
something, more energy will
be transferred, causing more
damage.
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The kinetic energy of cars
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Dangerous speeding?
Use the KE = ½mv2 equation to fill in the kinetic energy
values in the table below for two cars each traveling at
two different velocities.
1,000 kg
2,000 kg
20 mph
KE = 40 kJ
KE = 80 kJ
40 mph
KE = 160 kJ
KE = 320 kJ
What factor – mass or velocity – has the greatest effect on
the kinetic energy of a moving object?
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Feeling energetic?
Who has the most kinetic energy: Rita or the cat?
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Calculating velocity question
A truck has a mass of 20,000 kg.
If its kinetic energy is 2.25 mJ,
at what velocity is it travelling?
KE = ½ x mass x velocity2
velocity =
=


2KE
mass
2 x 2,250,000
20,000
= 15 m/s
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