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E
Energy
Energy
Calculating energy
Example: Calculating
landing speed
Energy
We say someone has a lot of energy when they run and
do lots of things.
Scientists define energy as ‘the ability to do work’.
Work is done when a force moves something.
You should already be familiar with the following eight
forms of energy:
 kinetic (movement) energy
 sound energy
 radiant (light) energy
 heat energy
 electrical energy
 gravitational
potential energy
 elastic potential
energy
 chemical potential
energy
Energy can be changed from one form to another.
Here elastic potential
energy in the spring
is converted to kinetic
energy
and then to
gravitational potential
energy.
This heater changes
electrical energy into heat
energy and radiant energy.
The spirit burner changes
chemical potential energy
into heat energy and
radiant energy.
When this tuning fork is hit, kinetic energy from the
hammer is converted to sound energy.
This rollercoaster
converts gravitational
potential energy into
kinetic energy and back
into gravitational potential
energy again.
A small amount of energy
is converted into heat due
to friction with the rails
and the air.
Energy can neither be created or destroyed, it just
changes form.
Unless the car has been given an extra push, it cannot
rise higher than its starting point.
Energy
Energy is the ability to do work. Work is done when a
force moves something.
Different kinds of energy are: kinetic (movement)
energy, sound energy, heat energy, radiant (light) energy,
electrical energy, gravitational potential energy, elastic
potential energy and chemical potential energy.
Energy can be transformed or changed from one form
into another. For example, an electric heater converts
electrical energy into heat energy and radiant energy.
Most forms of energy end up being converted into heat
energy in the environment.
Energy cannot be created or destroyed, only changed
from one form into another.
1E 1 Energy transformations
1E 2 Energy
Calculating energy
Gravitational potential energy
If someone says you have the potential
to get good marks, they mean you have
the ability to do well, but haven’t
actually done it yet. Potential means
‘stored up’, ‘yet to be used’.
Objects have gravitational potential
energy when they are high up. Energy
was put in to lift them up, and this
energy will be transformed again when
the object falls.
This climber has a lot of gravitational
potential energy.
What factors determine the climber’s
gravitational potential energy (Ep) in
joules?
His height (h) in metres – the higher
he climbs, the more energy he had to
put in, and the greater his Ep.
His mass (m) in kilograms – the heavier
he is, the more energy is needed to
overcome gravity for the climb.
The acceleration due to gravity (g) in
m s–2 – it’d be much easier to do this
climb on the Moon.
Thus:
ΔEp = m g Δh
Kinetic energy
Kinetic energy (Ek) is the energy of moving objects.
The greater an object’s speed, the greater its kinetic
energy.
If two objects are travelling at the same speed, the
one with the greater mass has more kinetic energy.
Ek
 ½mv 2
Where Ek is the kinetic energy in J, m is mass in kg,
v is velocity in m s-1.
Notice that when mass is doubled, Ek doubles, but
when speed is doubled, Ek goes up by a factor of four
(22).
Calculating energy
Potential energy is stored energy.
Objects have gravitational potential
energy when they have been lifted
up. The amount of energy depends on
their height, their mass, and the
strength of the gravitational field.
Objects have kinetic energy when
they are moving. The amount of
kinetic energy depends on their mass
and speed.
When mass doubles, Ek doubles, but
when speed doubles, Ek goes up by a
factor of four.
1E 3 Kinetic energy trials
E p
 mg h
m (mass) in kg
h (height) in m
Ep in J
E k
 ½mv 2
m (mass) in kg
v (speed) in m s–1
Ep in J
Example: Calculating landing speed
A 3.5 kg cat climbs 1.8 m onto a fence then jumps off. What
is its speed when it hits the ground? (Ignore air resistance,
assume g = 10 m s−2.)
Calculate the Ep at the top of
the fence.
Ep
 mgh
Calculate the speed from
this Ek.
Ek  ½mv 2
 3.5 kg × 10 m s –1 × 1.8 m
63 J
This value is equal to its
kinetic energy just before it
lands.
Ek = 63 J
1E 4 Crash!
1E 5 Experimental design: stopping distances
2Ek
m
2Ek
m
 v2
 v
2 × 63 J
3.5 kg
 v
6 m s –1
 v
End of chapter
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