1 of 10© Boardworks Ltd 2009. 2 of 10© Boardworks Ltd 2009 Kinetic energy is the energy an object...

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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 meters 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 truck 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 truck?

= 300,000 J = 300 kJ

= ½ x 1,500 x 202

= ½ x 1,500 x 400

= 750 x 400


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 traveling 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.



A long way down…

How would you describe a bungee jumper? As someone who is:

full of gravitational potential energy (GPE).

brave

insane

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 acceleration x height

Height is measured in meters (m).

Mass is measured in kilograms (kg).

GPE is measured in joules (j).

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”.)


An eagle 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 eagle have?

= 2 x 10 x 200

GPE = mass x gravitational field strength x height

= 4,000 J


Factors affecting GPE


GPE, mass and height calculations



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


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 a rollercoaster would be less than the GPE lost.


Energy transfer of rollercoasters

© Boardworks Ltd 200921 of 11


Many household objects are designed to transfer energy from one form into another useful form.

What is a useful energy transfer?

electricalenergy

kineticenergy

What energy transfer is an electric fan designed to carry out?


electricalenergy

soundenergy

What energy transfer are these speakers designed to carry out?



What energy transfer are wind turbines in a wind farmdesigned to carry out?


kineticenergy

electricalenergy


gravitationalpotential energy

electricalenergy

What energy transfer is a hydroelectric power station designed to carry out?



electrical

light

sound

heat

An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy.

Energy transfer in a television

For example, in a television:


chemical

kinetic

sound

heat

What are the main energy transfers in a car engine?

(Don’t forget the wasted energy.)

Energy transfer in a car engine


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.

How can we represent energy transfers?

100 Jelectrical

energy(input)

20 Jelectrical

energy(input)

10 Jlightenergy(output)10 J

heat energy(wasted)

90 Jheat energy

(wasted)

10 Jlightenergy(output)

Energy efficient light bulb

Filament light bulb

How does the energy use in these light bulbs compare?

The thickness of each arrow is proportional to the amount of energy involved at that stage.


Useful energy transfers

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