A long way down…

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A long way down…

How would you describe a high diver? As someone who is:

full of gravitational potential energy (GPE).

brave

insane

full of beans

GPE is the amount of energy an object has because of its position above the ground, i.e. its height.


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.

The gravitational potential energy (GPE) of an object on Earth depends on its mass and its height above the Earth’s surface.

What is gravitational potential energy?


How is GPE calculated?

The GPE of an object can be calculated using this equation:

GPE = mass x gravitational field strength x height

Height is measured in metres (m).

Mass is measured in kilograms (kg).

GPE is measured in joules (j).

Gravitational field strength is measured in newtons per kilogram (N/kg), usually taken as 10 N/kg on Earth.


Factors affecting GPE


An osprey with a mass of2 kg flies at a height of200 m above the ground.

Calculating GPE question 1

How much gravitational potential energy does the osprey have?

= 2 x 10 x 200

GPE = mass x gravitational field strength x height

= 4,000 J


An apple with a mass of 200 g falls 3 m from its branch to the ground.

Calculating GPE question 2

How much GPE will the apple have lost when it reaches the ground?

= 0.2 x 10 x 3

= 6 J

= mass x gravitational field strength xGPElost

changein height


GPE, mass and height calculations



Who has the most kinetic energy: Rita or the cat?

Feeling energetic?


Kinetic energy is the energy an object has because it is moving.

What is kinetic energy?

The word ‘kinetic’ comes from the Greek word ‘kinesis’, meaning motion.

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?


How is kinetic energy calculated?

The kinetic energy (KE) of an object can be calculated using this equation:

Velocity is measured in metres per second (m/s).

Mass is measured in kilograms (kg).

KE is measured in joules (j).

KE = ½ x mass x velocity2

= ½mv2


kinetic energy = ½ x mass x velocity2

A car with a mass of 1,500 kg travels at a velocity of 20 m/s.

Calculating kinetic energy question

What is the kinetic energy of the car?

= 300,000 J = 300 kJ

= ½ x 1,500 x 202


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 = 2KE v2

v = 2KE m

Rearranging the KE equation


A lorry has a mass of 20,000 kg. If its kinetic energy is 2.25 MJ, at what velocity is it travelling?

Calculating velocity question

KE = ½ x mass x velocity2

= 15 m/s

velocity = 2KE mass

=

2 x 2,250,000 20,000


KE, mass and velocity calculations


The kinetic energy of cars


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.

What factor – mass or velocity – has the greatest effect on the kinetic energy of a moving object?

Dangerous speeding?

KE = 160 kJ KE = 320 kJ

1,000 kg 2,000 kg

20 mph

40 mph

KE = 40 kJ KE = 80 kJ


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:

Too much kinetic energy

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.



This means that energy never just ‘disappears’. The total amount of energy always stays the same, i.e. total input energy = total output energy.

There are many different forms of energy, such as kinetic, sound, thermal and light energy.

Energy cannot be created or destroyed,just changed in form.

What is conservation of energy?

Each form of energy can be transferred or converted into an another form. All energy transfers follow the law of conservation of energy:

In most energy transfers, the energy is transferred to several different forms, which may or may not be useful.


Gerald the Human Cannonball


Energy transfer of falling objects

What happens to the KE and GPE of a rollercoaster?


The law of conservation of energy means that as an object falls, the GPE it loses must turn into a different form.

This is only true if air resistance and friction are ignored.

GPE lost = KE gained

The relationship between GPE and KE

In reality, GPE would also be transferred into heat and sound energy so the KE of rollercoaster would be less than the GPE lost.


Energy transfer of rollercoasters



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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