Post on 31-May-2020
1 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
2 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
3 of 31 © Boardworks Ltd 2016
What is a system?
A system is an object or a group of objects. It is a useful way
of thinking about some physics problems.
When something about a system changes, the way that
energy is stored in it can also change from one form
to another.
Energy can take many different forms. For example:
• moving objects have kinetic energy
• stretched or compressed objects have elastic
potential energy
• objects in a gravitational field have gravitational
potential energy.
4 of 31 © Boardworks Ltd 2016
Changes in energy
Can you describe the way that energy is changing in
these systems?
An arrow fired from a bow:
kinetic energy
gravitational
potential energy
elastic potential energy
An archer draws back the bow
string, storing elastic potential
energy. When they release it, the
arrow flies forwards and upwards.
5 of 31 © Boardworks Ltd 2016
Changes in energy
Can you describe the way that energy is changing in
these systems?
kinetic energygravitational
potential energy
elastic potential energy
A basketball hitting the floor:
An athlete scores a basket. As the
basketball falls, its gravitational
potential energy is converted to
kinetic energy. It compresses
(squashes) when it hits the floor,
and the kinetic energy is stored as
elastic potential energy.
6 of 31 © Boardworks Ltd 2016
Changes in energy
Can you describe the way that energy is changing in
these systems?
A car braking at a traffic light:
sound energy
heat (thermal) energy
kinetic energy
When the brakes are applied,
friction increases. The kinetic
energy of the wheels is transferred
into sound energy and heat
(thermal) energy.
7 of 31 © Boardworks Ltd 2016
Changes in energy
Can you describe the way that energy is changing in
these systems?
Water boiling in an electric kettle:
sound energy
heat (thermal) energy
electrical energy
When the kettle is switched on,
current flows through the heating
element. Electrical energy is
converted into sound energy and
heat (thermal) energy.
8 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
9 of 31 © Boardworks Ltd 2016
When an object is lifted upwards, the force doing
the lifting does work against the force of gravity.
The work done is stored as gravitational
potential energy (Ep). It is called potential
energy because it has the potential to be
transferred into other forms of energy later.
For example, this crane does work lifting this box.
For example, if the hook broke and the box
fell, the stored gravitational potential energy
would be converted to kinetic energy.
What is gravitational potential energy?
10 of 31 © Boardworks Ltd 2016
How is GPE calculated?
The GPE of an object can be calculated using this equation:
GPE = mass × gravitational field strength × height
Height (h) is measured in metres (m).
Mass (m) is measured in kilograms (kg).
Gravitational potential energy (Ep) is measured in joules (J).
Gravitational field strength (g) is measured in newtons per
kilogram (N/kg), and is usually assumed to be 10N/kg
close to the surface of Earth.
Ep = mgh
or
11 of 31 © Boardworks Ltd 2016
Factors affecting GPE
12 of 31 © Boardworks Ltd 2016
A plane with a mass of
5,000kg travels at a height
of 10km above the ground.
Calculating GPE question 1
How much gravitational
potential energy does
the plane have?
= 5,000 ×10 × 10,000
GPE = mass × gravitational field strength × height
= 500,000,000J
Assume a value of 10N/kg
for g.
= 500MJ
13 of 31 © Boardworks Ltd 2016
An orange with a mass of 250g
falls 2.5m from its branch to
the ground.
Calculating GPE question 2
How much GPE will the orange
have lost when it hits the ground?
= 0.25 × 10 × 2.5
= 6.25J
= mass × gravitational field strength ×GPE
lost
change
in height
Assume a value of 10N/kg for g.
14 of 31 © Boardworks Ltd 2016
GPE, mass and height calculations
15 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
16 of 31 © Boardworks Ltd 2016
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 might affect the kinetic energy of
a moving object?
17 of 31 © Boardworks Ltd 2016
How is kinetic energy calculated?
The kinetic energy (Ek) of an object can be calculated
using this equation:
Velocity, v, is measured in metres per second (m/s).
Mass, m, is measured in kilograms (kg).
Kinetic energy, Ek, is measured in joules (J).
Ek = ½ × mass × velocity2
Ek = ½mv2
or
18 of 31 © Boardworks Ltd 2016
kinetic energy = ½ × mass × velocity2
A car with a mass of 1,000kg
travels at a velocity of 20m/s.
Calculating kinetic energy question
What is the kinetic energy
of the car?
= 200,000J = 200kJ
= ½ × 1,000 × 202
19 of 31 © Boardworks Ltd 2016
If you know the kinetic energy of an object, you can rearrange
the equation in order to calculate its mass or velocity.
Ek = ½mv2
Rearranging the kinetic energy equation
v = 2Ek
mm =2Ek
v2
Rearranged to
calculate mass:
Rearranged to
calculate velocity:
20 of 31 © Boardworks Ltd 2016
A lorry has a mass of 20,000kg.
If its kinetic energy is 2.25MJ,
at what velocity is it travelling?
Calculating velocity question
Ek = ½mv2
= 15m/s
velocity =
=
21 of 31 © Boardworks Ltd 2016
Using the kinetic energy equation
22 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
23 of 31 © Boardworks Ltd 2016
Energy transfers within a system
Imagine a tennis ball with a mass of 60g dropped from
a height of 50cm.
h = 0.5m
m = 0.06kgInitially, the tennis ball has
some gravitational
potential energy (Ep):
Ep = mgh
= 0.06 × 10 × 0.5
= 0.3J
And no kinetic energy (Ek),
because it is not moving.
The total energy in the system is 0.3J.
24 of 31 © Boardworks Ltd 2016
total energy = 0.3J
Ek = 0J
Ep = 0.3J
Ek = 0.1J
Ep = 0.2J
total energy = 0.3J
Ek = 0.2J
Ep = 0.1J
total energy = 0.3JEk = 0.3J
Ep = 0J
total energy = 0.3J
Imagine a tennis ball with a mass of 60g dropped from
a height of 50cm.
Energy transfers within a system
When it is dropped, Ep is
converted to Ek.
When it hits the ground, Ep
is reduced to zero. All of
the gravitational potential
energy has been transferred
to kinetic energy.
As it falls, Ep decreases
and Ek increases, but the
total energy of the system
stays the same.
25 of 31 © Boardworks Ltd 2016
Energy transfers in rollercoasters
What happens to the Ek and Ep of a rollercoaster?
26 of 31 © Boardworks Ltd 2016
As the rollercoaster loses height, the stored gravitational
potential energy is transferred into kinetic energy.
This equation is only true if air
resistance and friction are ignored.
Ep lost = Ek gained
The relationship between GPE and KE
In reality, Ep would also be
transferred into heat and
sound energy, so the Ek of
the rollercoaster would be
a bit less than the Ep lost.
27 of 31 © Boardworks Ltd 2016
Energy transfer of roller coasters
28 of 31 © Boardworks Ltd 2016
Terminal velocity
29 of 31 © Boardworks Ltd 2016
Potential and Kinetic Energy
30 of 31 © Boardworks Ltd 2016
Glossary
31 of 31 © Boardworks Ltd 2016
Multiple-choice quiz