Chapter 3 Forms of Energy and Energy Conversions

Gravitational Potential Energy

4. The following objects have gravitational potential energy because of their
positions above the ground.

Ground

r= · ·The · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·\
boy has the greatest gravitational =
'•:::.:::.•·. potential energy when he is at position A.
He has the smallest gravitational potential

••••••••••••••••••••••••••••••••••••••••••••••••••••
• • ener gy when he is at position B.

5. Gravitational potential energy is related to the height of the object above

the ground.
6. The greater the height of an object above the ground, the higher the
gravitational potential energy of the object.
7. Gravitational potential energy is related to the mass of an object.
8. The greater the mass of an object that is above the ground, the greater the
gravitational potential energy.

Kinetic Energy
9. When an object moves, it possesses kinetic energy.
10. Kinetic energy is related to the speed of a moving object.
11. The faster an object moves, the greater its kinetic energy.

Mind Your Language

Another way of writing 'moves foster': 'increases in speed'

12. Kinetic energy is also related to the mass of the moving object.
13. A moving object with a greater mass has a greater kinetic energy than
another object with a smaller mass moving at the same speed.

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Chapter 3 Forms of E_nergy and Energy Conversions

Transferring Energy
14. Energy cannot be destroyed or created but can be transferred from one
object to another.

-+
Water flow A Kinetic energy

In terms of forces,
the water pushes
the water wheel,
making it turn.
• •••••••••••••••••••••••••••••••••••••••••••••••• • t. .............................................................

Conversions of Energy
15. Energy cannot be destroyed or created but can be converted from one
form into another.
16. Examples of conversion of energy:
Example Energy conversion{s)

1 1/ battery
,\
bulb In the battery: chemical potential energy
� electrical energy
In the bulb: electrical energy� heat+ light

Chemical energy in petrol � kinetic energy

of car

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 Chapter 3 Forms of Energy and Energy Conversions

Gravitational potential energy of cart at A

� kinetic energy of cart at B

compressed

zF��
spring ball
Elastic potential energy of compressed
spring � kinetic energy of ball

� --·············································· arrows for energy transfer. For ······� -

Avoid using these ................................................................................
L
··•:::.:: . example, when a fan moves the air, kinetic energy .!
of the blades is transferred to kinetic energy of the
wind. Do not use an arrow to represent this transfer.
••••••••••••••••••••••••••••••••••••• ..•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••

Science Bites
We know that the battery is used to produce electricity. There are also dynamos
and generators that convert kinetic energy into electrical energy.

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Chapter 3 Forms of Energy and Energy Conversions

17. Experiment
Aim:
To find out the relationship between gravitational potential energy and
kinetic energy

Procedure:
Release a ball from the top of an inclined plane at height h. Measure the
horizontal distance travelled by the ball. Repeat the previous steps with
increasing height h of inclined plane.

,--:,- ball is released here

inclined plane

Tabletop

----

Results:
1 15
18

Analysis:
When the ball rolls down the inclined plane, its gravitational potential
energy changes to kinetic energy. From the results, the higher from which
the ball is released, the further the ball travels.

Conclusion:
The greater the gravitational potential energy of the ball, the greater the
kinetic energy of the ball when it reaches the bottom of the inclined plane.

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Chapter 3 Forms of Energy and Energy Conversions

Using Bar Graphs to Show Changes in Energy

22. At the platform, the bungee jumper has the greatest gravitational potential
energy because he is at the highest position from the ground.
23. When he drops from the platform, his gravitational potential energy is
changed to kinetic energy.
24. When the bungee cord starts to stretch, the cord has elastic potential
energy.
P,' a7rm ,,
I
I
Bungee cord I
I
I
I
h hi h h
I
I
I
I
I
Ground I

Position 1 Position 2 Position 3 Position 4

Just before Bungee cord Bungee cord Bungee cord
stepping off at unstretched partially at maximum
platform length stretched stretch
25. The bar graphs below represent the amounts of energy in the four positions
of the bungee jumper above.
Position 1: Position 2:

Potential Elastic Kinetic Total Potential Elastic Kinetic Total

energy energy energy energy energy energy energy energy
Position 3: Position 4:

Potential Elastic Kinetic Total Potential Elastic Kinetic Total

energy energy energy energy energy energy energy energy

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 1. A marble is released at position A in a bowl.

marble
released marble
rolled back

,'
,, �/
'
'
B

The marble moves past the lowest position B and stops momentarily at
position C before rolling back.

(a) Why does the marble reach position C, which 1s lower than
position A? [1]

Process skill Description f

Identifying forms The ball at position A possesses gravitational potential
of energy energy. At position B, it possesses kinetic energy. At C,
it possesses gravitational potential energy.
Analysing diagram Since position C is lower than position A, the
gravitational potential energy of the ball at C is lower
than the gravitational potential energy of the ball at A.
Some energy has been lost.

Answer:
Some kinetic energy of the bal I has been lost as heat due to friction.

(b) Jie Ling applies oil on the bowl so that the marble reaches a higher
position than C before rolling back. Explain whether it will reach a
position higher than position A. [1]

Process skill Description

Recalling knowledge Energy cannot be created.
and concepts Oil is a lubricant that could reduce friction between
two surfaces that rub against each other.

Answer:
No, it will not reach a position higher than position A. The marble only has
the amount of gravitational potential energy to reach the same level as
position A.

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Chapter 3 Forms of Energy and Energy Conversions

2. Swee Lin pulled a rubber band back and launched a stone from
position A. The stone hit a suspended block which moved to B and to the
highest position C.

(a) State the energy conversions that occur from A to C. [ 1]

.. . .

·...
�.·.skill . ..
Description
Recalling knowledge Energy comes in different forms. Energy can be
and concepts converted from one form into another.
Identifying forms of A stretched rubber band possesses elastic potential
energy energy. A moving stone possesses kinetic energy.
The stone that is in its highest position of its swing
possesses gravitational potentiar energy.

Answer:
Elastic potential energy in stretched rubber band at A � kinetic energy
of stone and block � gravitational potential energy of block at C

(b) Without modifying the set-up, suggest a way to make the block
swing to a higher position. Explain your answer. [1]

'Description ••
Generating possibility To make the block swing higher, the stone needs to
have more kinetic energy. This means there should
be more elastic potential energy in the stretched
rubber band.

Answer:
Pull the rubber band back further. The rubber band would stretch more
and store more elastic potential energy.

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