Transcript 3. Energy and Movement

1 of 30
© Boardworks Ltd 2007
2 of 30
© Boardworks Ltd 2007
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.
3 of 30
© Boardworks Ltd 2007
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.
4 of 30
© Boardworks Ltd 2007
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).
5 of 30
© Boardworks Ltd 2007
Factors affecting GPE
6 of 30
© Boardworks Ltd 2007
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
7 of 30
© Boardworks Ltd 2007
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
8 of 30
© Boardworks Ltd 2007
GPE, mass and height calculations
9 of 30
© Boardworks Ltd 2007
10 of 30
© Boardworks Ltd 2007
Feeling energetic?
Who has the most kinetic energy: Rita or the cat?
11 of 30
© Boardworks Ltd 2007
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?
12 of 30
© Boardworks Ltd 2007
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).
13 of 30
© Boardworks Ltd 2007
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
14 of 30
© Boardworks Ltd 2007
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 =
15 of 30
2KE
v2
v =

2KE
m
© Boardworks Ltd 2007
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
16 of 30
© Boardworks Ltd 2007
KE, mass and velocity calculations
17 of 30
© Boardworks Ltd 2007
The kinetic energy of cars
18 of 30
© Boardworks Ltd 2007
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?
19 of 30
© Boardworks Ltd 2007
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.
20 of 30
© Boardworks Ltd 2007
21 of 30
© Boardworks Ltd 2007
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.
22 of 30
© Boardworks Ltd 2007
Gerald the Human Cannonball
23 of 30
© Boardworks Ltd 2007
Energy transfer of falling objects
What happens to the KE and GPE of a rollercoaster?
24 of 30
© Boardworks Ltd 2007
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.
25 of 30
© Boardworks Ltd 2007
Energy transfer of rollercoasters
26 of 30
© Boardworks Ltd 2007
27 of 30
© Boardworks Ltd 2007
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.
28 of 30
© Boardworks Ltd 2007
Anagrams
29 of 30
© Boardworks Ltd 2007
Multiple-choice quiz
30 of 30
© Boardworks Ltd 2007