Q&A - PSLE Science

[charsen]

Alarmchain:

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Hope someone can help with this.

Attached was a past PSLE question which my daughter did as part of her revision.

For the 2nd reason in part (b) of the attached, I explained to my daughter that she did not elaborate how more friction caused the snake to move over a shorter distance. I suggested to her that she should have added the following: \"As friction creates heat, the snake will gain more heat compared to the lizard. As such, the lizard can move over a longer distance than the snake.\"

I also suggested to her that since part (a) of the question was about heat gain / loss and surface area, she should use that as a clue to answer part (b).

HOWEVER, the answer given by her teacher was:
\" For the lizard, at each time, only 2 of its feet touches the ground but for the snake, the whole body is in contact with the hot sand.\"

Like my daughter, I am confused! How does having only 2 feet help the lizard move a longer distance? If it is about having less surface area in contact, wouldn't that be a repetition of reason 1? Would anyone happen to know how? She is unable to understand her teacher's reasoning, despite asking.

More importantly, would someone let me know if the answer from my daughter and my suggested additions is also an acceptable answer to this question? Or how your child's teacher answered this part of the question.

Thanking you in advance.

Hi
Your answer is definetely an acceptable answer.I wrote a similar answer to your acceptable answerand my teacher accepted it.The only thing that your daughter did not do was she did not explain fully the question.The teacher's answer is totally wrong as you have to compare the friction between the snake body and the sand to the lizard.
My answer is this:The friction between the lizard 's legs to the hot sand is smaller than the friction between the snake and the hot sand,causing the lizard to move a longer distance.
cheers

[charsen]

kitty2:

Series or parallel circuit is brighter?Circuit with series battery is bright or with parallel battery?Circuit with series bulb is brighter or with parallel bulb?


Please help as my nephew is very confused with the above.

Thanks

It depends on the arrangement of batteries or the arrangement of bulbs of the circuit.I can tell what the brightness of the bulb is when you say the arrangement of batteries is in series or parallel or the arrangement of bulbs is in series or parallel.
Cheers

[atutor2001]

Alarmchain:

http://farm8.staticflickr.com/7134/7722111444_208d639f6f_z.jpg\">

The question :
\"...why this lizard can move over a longer distance on hot sand compared to the snake....\"

Analysis :
There is no time factor involved in the question i.e. it has nothing to do with speed. So friction is unlikely to be a consideration.

The focus is on the hot sand i.e. the distance moved by each animal is limited because of the heat from the sand.

At any one time, different pairs of legs of the lizard are in contact with the hot sand while the same part of the body of the snake is continuously in contact with the sand. When the pair of legs of the lizard is lifted up, it can lose heat to the surrounding air.


My answer :
At any one time, only 2 feet of the lizard is in contact with the hot sand. It also alternates/switches between the 2 pairs of feet to be in contact. As such, given the same duration, each pair of feet of the lizard will only spend half the time in contact with the hot sand while the body of the snake will be in contact with the hot sand for the whole duration. The feet of the lizard can also lose heat when it is not in contact with the sand. Therefore, the total amount of heat gained by the lizard is much lesser than the snake and it can move a longer distance over the hot sand compared to the snake.

[Alarmchain]

Thank you so much to both Charsen and atutor, appreciate your inputs!


To atutor:
I finally understand the reasoning of the teacher, thank you!

However, as friction generates heat, which is how my daughter and I had explained in the answer in my original post, wouldn’t that be similar to your last sentence of "Therefore, the total amount of heat gained by the lizard is much lesser than the snake and it can move a longer distance over the hot sand compared to the snake.", since the snake would have gained more heat generated by friction as well?

If we had explained it as friction being an opposing force and therefore it results in a shorter distance travelled, I will then agree that it may not have fully answered the context of this question.

Appreciate your thoughts on this.

Thanks!

[atutor2001]

Alarmchain:

Thank you so much to both Charsen and atutor, appreciate your inputs!


To atutor:
I finally understand the reasoning of the teacher, thank you!

However, as friction generates heat, which is how my daughter and I had explained in the answer in my original post, wouldn't that be similar to your last sentence of \"Therefore, the total amount of heat gained by the lizard is much lesser than the snake and it can move a longer distance over the hot sand compared to the snake.\", since the snake would have gained more heat generated by friction as well?


My thought :
I have simplified the last sentence just for easier understanding at Pr level. The reason for the snake or the lizard to stop moving and seek for shade from the sun is when the heat gained at the part of the body in contact with the hot sand is so high that they will get burnt.

The heat gained through friction is negligible compared to that by conduction from the hot sand. Therefore, it is a minor factor and will not be accepted as an answer.

If we had explained it as friction being an opposing force and therefore it results in a shorter distance travelled, I will then agree that it may not have fully answered the context of this question.

Appreciate your thoughts on this.

Thanks!

[cimman]

Alarmchain, it is very important to answer in context of the question.

Most students do not understand the essence of the question.
For example, what is the difference between a Red Ferrari and a white Mercedes Benz ?
one can say, the colors are different. However, this would be a wrong answer.
The essence of these 2 cars, is that one is a sports car and the other is a sedan. So while technically, the 2 cars have different colors, the objectives of the 2 cars are very different.
In your case of the lizard and the snake, while there is friction involved, ask yourself this question, if it is night time in the dessert, would the lizard and the snake covered the same distance ? yes, they would, despite the snake having to overcome the greater frictional force. The essence of the question is that it happened in the dessert in the day time. The dessert is known for 2 main things, extreme heat and dryness. We can discard lack of water here, since nothing in the question links to that. You have to link the answer back to the extreme heat in the dessert. Link your answer to the essence of the question, do not look for superficial differences, though it may be technically correct, but it will be marked wrong.

[kitty2]

charsen:

kitty2:

Series or parallel circuit is brighter?Circuit with series battery is bright or with parallel battery?Circuit with series bulb is brighter or with parallel bulb?


Please help as my nephew is very confused with the above.

Thanks

It depends on the arrangement of batteries or the arrangement of bulbs of the circuit.I can tell what the brightness of the bulb is when you say the arrangement of batteries is in series or parallel or the arrangement of bulbs is in series or parallel.
Cheers

Please let me know your answers for the above.Thanks

[Sun_2010]

cimman:

Alarmchain, it is very important to answer in context of the question.

Most students do not understand the essence of the question.
For example, what is the difference between a Red Ferrari and a white Mercedes Benz ?
one can say, the colors are different. However, this would be a wrong answer.
The essence of these 2 cars, is that one is a sports car and the other is a sedan. So while technically, the 2 cars have different colors, the objectives of the 2 cars are very different.
In your case of the lizard and the snake, while there is friction involved, ask yourself this question, if it is night time in the dessert, would the lizard and the snake covered the same distance ? yes, they would, despite the snake having to overcome the greater frictional force. The essence of the question is that it happened in the dessert in the day time. The dessert is known for 2 main things, extreme heat and dryness. We can discard lack of water here, since nothing in the question links to that. You have to link the answer back to the extreme heat in the dessert. Link your answer to the essence of the question, do not look for superficial differences, though it may be technically correct, but it will be marked wrong.

Wonderful explanation cinman. Thanks.

[Alarmchain]

Thank you so much to both atutor and cimman. Really appreciate you guys taking the time to fully elaborate the minor nuances that makes the difference between a right and wrong answer.


I do get the point about superficial and main reasons behind an answer and about answers that fulfil the context of a question.

However, I am indeed very worried about such methods of examining science, especially to kids 12/13 years of age. Unless this is the type of question that separates an A from an A* student, it would indeed not only confuse most kids (as well as parents and teachers, I might add!), it may actually turn them off!

Much as heat from friction is not the main reason and is only technically correct, I do believe that if a child is able to reason it that way, it should not be marked wrong. Unless the reasoning was way off, or concepts were wrongly interpreted, we should be teaching our children this reasoning technique and not give them the impression that there is only 1 answer. Yes, I do agree that in this instance, the main reasoning should be the intense heat from the hot desert sand. However, I wonder if in the actual exam papers, the words \"hot sand\" was in bold to capture the child's attention. Even so, under exam conditions, how does one expect a normal 12 year old to be able to realise that and also be able to phrase the answer in the way we did! Indeed, I may even argue that it is precisely because of the hot sand that the minute heat from friction becomes an even more important factor, versus when the 2 reptiles were moving in the cool of the night.

This question would have been an ideal one for the school science workbooks, which I am sure many will agree, is grossly lacking in such application questions (and yet they expect our children to tackle such questions during exams, to such minute details too!). It would indeed generate good discussions in class on how concepts are applied in various ways. But under exam conditions??? (unless it is one of the few that separates the A from the A*)

It is such narrow interpretations of answers and marking schemes that have now resulted in science being taught in a very narrow manner in schools, unfortunately (we all must know of the keywords way of teaching science today). If science for 12 year olds is about exciting the kids to explore things around them, I wonder if stressing NARROWLY on being scientifically and technically correct answers is the right way to go. We are not talking about being way off, of course.

My apologies for straying off. Just wonder if there are parents out there who feel the same way.

Once again, my gratitude to both for your inputs. At least, I can now \"correctly\" explain this question to my daughter.

Cheers!

[atutor2001]

Hi Alarmchain


In my opinion, such questions are to "prevent" students from getting full mark. Generally, such type of questions will add up to only about 5 marks of the whole paper. So even if they get it wrong, A* is still attainable.