Challenging Question on Energy Conversion
Remembering the science knowledge on energy conversion is easy. However, to apply the science concepts to the exam question, that requires some practice.
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Question
The diagram below shows a slingshot. Mary uses four identical slingshots to create set-ups W, X, Y and Z.
The mass of the stone in each set-up and the length to which the elastic band was stretched in each set-up are shown in the table below.
| Set-up W | Set-up X | Set-up Y | Set-up Z | |
| Mass of the stone (g) | 90 | 120 | 90 | 120 |
| Length to which the elastic band was stretched (cm) | 15 | 25 | 25 | 15 |
Based on the above information, which of the following statements is/are true?
A The stone in set-up Y has more kinetic energy than the stone in setup W when the elastic bands are released.
B Comparing set-up X and set-up Y, the stone in set-up Y will travel a greater distance when the elastic band is released.
C Mary can compare set-up X and Z to investigate how the amount of elastic potential energy of the stretched elastic bands affects the amount of kinetic energy of the stone after she released the elastic band.
D Set-ups W and Z can be used to investigate how the mass of the stone affects the amount of elastic potential energy of the stretched elastic band.
Which sentence(s), A, B, C and/or D, is/are correct?
Please try out the question first before looking at the answer.
Once you have completed this question, let’s investigate each sentence.
Sentence A
The stone in set-up Y has more kinetic energy than the stone in setup W when the elastic bands are released.
Scientific Explanation
As the band is stretched longer in setup Y than W, the elastic band has more elastic potential energy in setup Y than W.
When the bands are released, there is more elastic potential energy is converted to more kinetic energy in setup Y than W.
As a result, there is a greater transfer of kinetic energy from the released elastic band to the stone.
Stone in set-up Y has more kinetic energy than in set-up W.
Conclusion: Sentence A is correct.
Sentence B
Extracted from the question:
Comparing set-up X and set-up Y, the stone in set-up Y will travel a greater distance when the elastic band is released.
Table taken from the question
| Set-up W | Set-up X | Set-up Y | Set-up Z | |
| Mass of the stone (g) | 90 | 120 | 90 | 120 |
| Length to which the elastic band was stretched (cm) | 15 | 25 | 25 | 15 |
What could have caused the stone to travel a greatest distance?’
The clue lies in the word, “travel”.
This is an action word.
For this question, to travel the greatest distance, the stone needs to have the greatest speed.
You got ask yourself: How do I know which stone has the greatest speed?
Science concept
Be warned: the following explanation requires mathematics.
WHAT??? You need maths to do this question???
Yup. You need that.
But…
It is a very, very simple maths formula. Don’t worry. *Phew* 😄
When the stretched elastic band is released, the elastic potential energy is converted to kinetic energy.
In other words, we can summarise using a simple formula here:
Elastic potential energy (EPE) = kinetic energy (KE)
Why do you see an “=” sign?
Apply the laws of energy conversion. Energy can neither be created nor destroyed. Energy is converted from one form to another.
What the above formula means is when the stretched elastic band is released, the elastic potential energy in the band (Let’s assume is 10 units) is converted to the KE (10units). The total energy conversion from elastic potential energy to kinetic energy is 10 units.
The kinetic energy (KE) of the released band is then transferred to the ball. The ball has 10units of KE.
10 units of EPE = 10 units of KE in the ball.
In your Primary science syllabus, since KE is affected by mass and speed, let’s use this simple formula.
KE = Mass X speed
Take note: “X” means multiply
This is where you need to understand the maths formula. To achieve the same KE, there are two situations.
1) When the mass of the ball is smaller 🔽, the speed has to be greater 🔼.
Same KE = mass 🔽 X speed 🔼
Take note: “X” means multiply
2) When the mass of the ball is larger 🔼, the speed has to be smaller 🔽.
Same KE = mass 🔼 X speed 🔽
Take note: “X” means multiply
Still don’t understand?
Try this example:
1) Assume 15 is the total KE of the ball.
To get 15, you can do this: 15 = 7.5 X 2
(Where 7.5 is the mass and 2 is the speed)
2) Assume 15 is the total KE of the ball.
To get the same number 15, you can also do this: 15 = 5 X 3
(where 5 is the mass, 3 is the speed)
What pattern can you see here?
The greater the mass, the speed must be lower to achieve the same kinetic energy of the ball in both setups X and Y.
Conclusion:
Both setups have the same amount of KE transferred to the stone because the elastic bands are stretched equally between set-up X and Y.
The stone in setup Y has a smaller mass compared to the stone in setup X. In order to have the same KE in both setups, the speed of stone in setup Y has to increase to compensate for the smaller mass.
With a greater speed, stone Y will travel a longer distance.
Sentence B is correct.
Sentence C
Extracted from the question:
Mary can compare set-up X and Z to investigate how the amount of elastic potential energy of the stretched elastic bands affects the amount of kinetic energy of the stone after she releases the elastic band.
Analyse the part highlighted in yellow.
Does it ring any familiar bells in your mind?
That sentence is the aim of the experiment.
This is a very important clue!
The aim of the experiment tells you the changing variable and dependent variable.
Must-know science knowledge
What is a “changing variable”?
It is a variable that is changed in a scientific experiment (the person conducting the experiment is free to change the variables.)
That means this changing variable does not depend on another variable.
What is a “Dependent variable”?
It is the variable that is affected by the other variable.
In other words, the changing variable affects the measured variable.
OR
The measured variable is affected by the changing variable.
Let’s apply the science knowledge to Sentence C.
We know that sentence C shows you the aim of the experiment.
To properly investigate the aim of the experiment, we need to ensure the experiment is fair.
For an experiment to be fair, there is only one changing variable.
What is the changing variable in the highlighted part of Sentence C?
Changing variable: the length in which the elastic band is stretched.
Why is this a changing variable?
As the band is stretched longer and released, this results in a greater transfer of kinetic energy from the elastic band to the stone.
The analysis then brings you to the next part:
What is the dependent variable in the highlighted part of Sentence C?
Dependent variable: The amount of kinetic energy in the stone after Mary has released the elastic band.
Table taken from the question
| Set-up W | Set-up X | Set-up Y | Set-up Z | |
| Mass of the stone (g) | 90 | 120 | 90 | 120 |
| Length to which the elastic band was stretched (cm) | 15 | 25 | 25 | 15 |
Is the experiment conducted fairly?
Yes!
Evidence: Based on the table above, we can see set-up X was stretched 25cm while set-up Z was stretched 15cm. There is only one changing variable as only the elastic band is stretched. All other variables are kept the same (we can see the mass of the stone is kept the same). The experiment is fair.
Science concepts:
As the band is stretched longer, the stretched band has more elastic potential energy
When the stretched band is released, there is energy conversion of elastic potential energy to kinetic energy
This results in a greater transfer of kinetic energy from the released elastic band to the stone.
Conclusion:
After analysing the evidence and the science concepts, Sentence C is true because the amount of elastic potential energy of the stretched bands will affect the amount of kinetic energy of the stone after she released the elastic band.
Sentence C is correct.
Sentence D
Extracted from the question:
Set-ups W and Z can be used to investigate how the mass of the stone affects the amount of elastic potential energy of the stretched elastic band.
Again, does this sound familiar?
This is the aim of the experiment. Your immediate next step is to identify the changing variable and dependent variable.
What is the changing variable?
Changing variable: mass of the pebble.
What is the dependent variable?
Dependent variable: amount of elastic potential energy of the stretched elastic band
Table taken from the question above
| Set-up W | Set-up X | Set-up Y | Set-up Z | |
| Mass of the stone (g) | 90 | 120 | 90 | 120 |
| Length to which the elastic band was stretched (cm) | 15 | 25 | 25 | 15 |
Evidence: Based on the table above, we can see set-up W uses 90g of stone while set-up Z uses 120g of stone. This is only changing variable as only the mass of stone has changed. All other variables are kept the same (we can see the length of the elastic band is kept the same). That means the experiment is fair.
So, is Sentence D the correct answer?
NO!
You must consider the science concept too!
Remember sentence C?
We analyse both the evidence and science concept.
Science concept:
Sentence D does not make sense.
Simply changing the mass of the stone does not affect the elastic potential energy of the band.
Why?
Recall this science knowledge:
Elastic potential energy is affected by the length to which the elastic band is stretched.
Hence,
Mass of the stone does not affect the amount of elastic potential energy of the stretched elastic band.
OR
The amount of elastic potential energy of the stretched band is not affected by the mass of the stone.
Conclusion: Sentence D is wrong as the mass of the stone does not affect the elastic potential energy of the band.
Final answer:
Sentences A, B and C only.
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Andy Ling
Simplifying science concepts for your child
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