Download Kinetic energy - Sackville School

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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.
GPE is the amount of energy an object has because of
its position above the ground, i.e. its height.
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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 on Earth.
 Height is measured in metres (m).
 GPE is measured in joules (j).
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Factors affecting GPE
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Calculating GPE question 1
An osprey with a mass of
2 kg flies at a height of
200 m above the ground.
How much gravitational
potential energy does the
osprey 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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Feeling energetic?
Who has the most kinetic energy: Rita or the cat?
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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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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 metres per second (m/s).
 KE is measured in joules (j).
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Calculating kinetic energy question
A car with a mass of
1,500 kg travels at a
velocity of 20 m/s.
What is the kinetic
energy of the car?
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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Calculating velocity question
A lorry 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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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 travelling 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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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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Energy transfer of falling objects
What happens to the KE and GPE of a rollercoaster?
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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
rollercoaster would be less than
the GPE lost.
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Energy transfer of rollercoasters
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Terminal Velocity
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Identifying forces
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What are resultant forces?
There are usually several different forces acting on an object.
The overall motion of the object will depend on the size and
direction of all the forces.
The motion of the object will depend on the resultant force.
This is calculated by adding all the forces together, taking
their direction into account.
50 N
30 N
Resultant force on the crate = 50 N – 30 N
= 20 N to the left
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Calculating resultant forces
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What is friction?
Friction is a resistive force that slows things down and tries
to stop objects sliding past each other.
friction
pulling force
Friction always acts in the opposite direction to which an
object is moving or trying to move.
What would happen if friction didn’t exist?
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Different types of friction
Friction is most obvious when it acts between two solid
objects, but it also acts between solid objects and gases,
and between solid objects and liquids.
 Friction caused by an
object moving through air
is called air resistance.
 Friction caused by an
object moving through a
liquid, such as water, is
called drag.
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Introducing balanced forces
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Terminal velocity of a skydiver
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Velocity–time graph of skydiver
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Glossary
 conservation of energy – The law that states that






energy cannot be created or destroyed, just transferred into
different forms.
energy transfer – A process in which one form of energy
is transferred into another form.
gravitational field strength – The strength of gravity in
a particular location.
gravitational potential – The energy an object has
because of its position in a gravitational field.
joule – The unit of energy.
kinetic – The energy an object has because it is moving.
velocity – A measure of the speed and direction of a
moving object.
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Anagrams
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Multiple-choice quiz
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