Q&A - PSLE Science

[ADoc]

Verysuperkiasu:

Thank you for your clear explanation, ADoc!

Initially, the confusion my child has is this - If dry ice goes straight to gaseous (which should be invisible) then why we still see the mist which suggests liquid state. Correct me if I'm wrong - that means to say the mist (liquid) we see is from the condensed water vapour from the air and is not the gaseous state of dry ice. Am I right?

hi! That's right, just as we can't observe CO2 in the air mixture around us.

Just to elaborate further. Some kids may have played with dry ice and they observe that \"white bubbles\" are formed when water is added to dry ice. And so wrongfully conclude that carbon dioxide is \"white\".

cheers!

Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\".

[Verysuperkiasu]

ADoc:

Verysuperkiasu:

Thank you for your clear explanation, ADoc!

Initially, the confusion my child has is this - If dry ice goes straight to gaseous (which should be invisible) then why we still see the mist which suggests liquid state. Correct me if I'm wrong - that means to say the mist (liquid) we see is from the condensed water vapour from the air and is not the gaseous state of dry ice. Am I right?

hi! That's right, just as we can't observe CO2 in the air mixture around us.

Just to elaborate further. Some kids may have played with dry ice and they observe that \"white bubbles\" are formed when water is added to dry ice. And so wrongfully conclude that carbon dioxide is \"white\".

cheers!

Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\".

Thank you for clarifying our confusion....:)

[Nebbermind]

ADoc:

Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\".

while I readily agree that the white vapour in the air is from the water vapour, I'm still trying to figure out where does the water vapour come from if the dry ice is inside the water.

[mom-of-2 0]

http://i49.tinypic.com/2ur8rrq.jpg\"> Can I ask if anyone know the answer to this question. The answer is 4 but I think it's wrong.

[G Tan]

mom-of-2:

http://i49.tinypic.com/2ur8rrq.jpg\"> Can I ask if anyone know the answer to this question. The answer is 4 but I think it's wrong.

mom-of-2:

http://i49.tinypic.com/2ur8rrq.jpg\"> Can I ask if anyone know the answer to this question. The answer is 4 but I think it's wrong.

The answer is 1

[fightingmom]

Hi,


Can help with the following question:

In an Archery lesson, John was holding a bow and arrow and trying to hit the bull's eye. Name the force(s) that is/are acting on the moving arrow towards the bull's eye.

My DD's answer is gravitational force and air resistance.

Answer sheet is gravity and friction.

Can anyone cfm whether friction is correct and my DD's answer on air resistance is correct or wrong?

TIA

[atutor2001]

fightingmom:

Hi,


Can help with the following question:

In an Archery lesson, John was holding a bow and arrow and trying to hit the bull's eye. Name the force(s) that is/are acting on the moving arrow towards the bull's eye.

My DD's answer is gravitational force and air resistance.

Answer sheet is gravity and friction.

Can anyone cfm whether friction is correct and my DD's answer on air resistance is correct or wrong?

TIA

The problem is \"what is air resistance?\", i.e. is air resistance a type of force? I recalled forces being classified into push, pull, magnetic, elastic spring, gravitational.... but no air resistance force.

Therefore, probably the syllabus does not recognise air resistance as a type of force. The most kiasu answer will be.

\"Air resistance caused by frictional force.\" I remembered science is \"positive marking\" so as long as there is no major conceptual error, once keyword is there, mark has to be given.

[fightingmom]

atutor2001:

fightingmom:

Hi,


Can help with the following question:

In an Archery lesson, John was holding a bow and arrow and trying to hit the bull's eye. Name the force(s) that is/are acting on the moving arrow towards the bull's eye.

My DD's answer is gravitational force and air resistance.

Answer sheet is gravity and friction.

Can anyone cfm whether friction is correct and my DD's answer on air resistance is correct or wrong?

TIA

The problem is \"what is air resistance?\", i.e. is air resistance a type of force? I recalled forces being classified into push, pull, magnetic, elastic spring, gravitational.... but no air resistance force.

Therefore, probably the syllabus does not recognise air resistance as a type of force. The most kiasu answer will be.

\"Air resistance caused by frictional force.\" I remembered science is \"positive marking\" so as long as there is no major conceptual error, once keyword is there, mark has to be given.

Eh... pardon me - so my DD answer is wrong right in this case ? can only secure 1/2 mark right ? :scratchhead:

[atutor2001]

fightingmom:

Eh... pardon me - so my DD answer is wrong right in this case ? can only secure 1/2 mark right ? :scratchhead:

Your DD is correct. Air resistance is a force, a frictional force between the surface of the arrow and air. Frictional force between a moving object through liquid is called drag or drag force. Therefore, both air resistance and drag are Frictional Force. However, I am not sure if it is covered in primary school syllabus and some schools are silly enough to considered them as wrong if used.

For such cases, I used to coach my kids to be super kiasu by putting in both key words, i.e. \"air resistance\" and \"frictional force\" in the answer. The kiasu answers can be :

\"Air resistance, which is a frictional force between the surface of contact between the arrow and air.\"

or

\"Frictional force between the surface of the arrow and air which is also called air resistance.\"

However, knowing how school's answer keys are like, there is no point arguing with them. However, I believe it will be marked as correct in PSLE.

[fightingmom]

atutor2001:

fightingmom:

Eh... pardon me - so my DD answer is wrong right in this case ? can only secure 1/2 mark right ? :scratchhead:

Your DD is correct. Air resistance is a force, a frictional force between the surface of the arrow and air.

For such cases, I used to coach my kids to be super kiasu by putting in both key words, i.e. \"air resistance\" and \"frictional force\" in the answer. The kiasu answers can be :

\"Air resistance, which is a frictional force between the surface of contact between the arrow and air.\"

or

\"Frictional force between the surface of the arrow and air which is also called air resistance.\"

However, knowing how school's answer keys are like, there is no point arguing with them. However, I believe it will be marked as correct in PSLE.

ok ok ... :thankyou: so much for your explanation and clarification ! Appreciate your help.