Download Kinetic Energy

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© Boardworks Ltd 2009
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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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
= ½ x 1,500 x 400
=
750
x
400
= 300,000 J = 300 kJ
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KE, mass and velocity calculations
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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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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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A long way down…
How would you describe a
bungee jumper? As
someone who is:
 brave
 insane
 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 acceleration x height
 Mass is measured in kilograms (kg).
 Gravitational acceleration is the same for all objects
(ignoring air friction) and is equal to 9.8m/s2. (For our
calculations we can round up and just use “10”.)
 Height is measured in meters (m).
 GPE is measured in joules (j).
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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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Factors affecting GPE
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GPE, mass and height calculations
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What is conservation of energy?
There are many different forms of energy, such as kinetic,
sound, thermal and light energy.
Each form of energy can be transferred or converted into
an another form. All energy transfers follow the law of
conservation of energy:
Energy cannot be created or destroyed,
just changed in form.
This means that energy never just ‘disappears’.
The total amount of energy always stays the same,
i.e. total input energy = total output energy.
In most energy transfers, the energy is transferred to
several different forms, which may or may not be useful.
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Gerald the Human Cannonball
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The relationship between GPE and KE
The law of conservation of energy means that as an object
falls, the GPE it loses must turn into a different form.
GPE lost = KE gained
This is only true if air resistance
and friction are ignored.
In reality, GPE would also be
transferred into heat and sound
energy, so the KE of a
rollercoaster would be less than
the GPE lost.
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Energy transfer of rollercoasters
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What is a useful energy transfer?
Many household objects are designed to transfer energy from
one form into another useful form.
What energy transfer is an electric fan designed to carry out?
electrical
energy
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kinetic
energy
© Boardworks Ltd 2009
What is a useful energy transfer?
What energy transfer are these speakers designed to
carry out?
electrical
energy
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sound
energy
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What is a useful energy transfer?
What energy transfer are wind turbines in a wind farm
designed to carry out?
kinetic
energy
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electrical
energy
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What is a useful energy transfer?
What energy transfer is a hydroelectric power station
designed to carry out?
gravitational
potential energy
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electrical
energy
© Boardworks Ltd 2009
Energy transfer in a television
An energy transfer diagram shows the input and output
energies for a device. This includes all the useful and wasted
forms of energy.
For example, in a television:
light
electrical
sound
heat
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Energy transfer in a car engine
What are the main energy transfers in a car engine?
(Don’t forget the wasted energy.)
kinetic
chemical
sound
heat
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How can we represent energy transfers?
All the energy transfers
(useful and wasted) that
are associated with a
device can be represented
by a Sankey diagram.
A Sankey diagram uses
arrows to represent all
the output energies.
Filament light bulb
100 J
10 J
electrical
light
energy
energy
(input)
(output)
90 J
heat energy
(wasted)
The thickness of each
arrow is proportional to
the amount of energy
involved at that stage.
Energy efficient light bulb
20 J
10 J
electrical
light
energy
energy
(input)
10 J (output)
heat energy
How does the energy use in
(wasted)
these light bulbs compare?
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Useful energy transfers
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