Q&A - P3 Science

[atutor2001]

peggy:

Can anyone help with this P4 qtn ?


John was running around the field during recess before his teacher brought the class into an air-conditioned room. He felt cold after he entered the room.

A) Explain why John felt cold in the air conditioned room.

The key point to note in this question is that he was running around before going into the air-con room, i.e. he is hot with perspiration.


In the air-con room, he will feel colder when he is hot & wet than if he is dry. The reason for feeling colder are :

1. The difference in his body temp and the surrounding temperature increases causing the body to lose heat faster.

(second reason - not in syllabus but sometimes tested)
2. The perspiration on his body evaporates faster in an air-con room because the humidity is lower (air-con remove water vapour in the air). Faster evaporation means heat is lost by the body to the sweat at a faster rate i.e. feeling colder.


In my opinion, the most kiasu answer will be :

Running causes his body temperature to increase while the air-conditioner causes the room temperature to decrease. Therefore, the difference in temperature between his body and the surrounding air increases causing his body to lose heat at a faster rate which is why he feels cold.

(the underlined words should be what the marker is looking for)

[tianzhu]

peggy:

Thanks Tianzhu. Very well explained.

Hi

You're welcome.

Best wishes

[peggy]

Many thanks to all who has responded to my qtn !

[elkniwt]

Hi,


I refer to Qn 10 in this paper,
http://www.orlesson.org/orp/09Sc/P4-09-Sc-SA2-HenryPark.pdf

For this type of question, is it necessary to draw the outlines of transparent objects for the shadows?

Regards

[tianzhu]

elkniwt:

Hi,


I refer to Qn 10 in this paper,
http://www.orlesson.org/orp/09Sc/P4-09-Sc-SA2-HenryPark.pdf

For this type of question, is it necessary to draw the outlines of transparent objects for the shadows?

Regards

Hi

The objects must be translucent or opaque to make a shadow. Translucent object will give a lighter shadow.

If you see from the position of a person holding the torch, option (1) is a logical answer.

Best wishes

[elkniwt]

Dear tianzhu,


Thank you for your quick reply. But may I know how come for the clear plastic which is transparent, it is necessary to draw the outline? My dd said teacher taught that transparents objects do not cast shadow.

Thanks for help. It is kind of confusing for me and the child cos for some papers, the outlines of transparent objects are not drawn, but for this qn the correct ans is the one with the outline drawn.

Regards.

tianzhu:

Hi

The objects must be translucent or opaque to make a shadow. Translucent object will give a lighter shadow.

If you see from the position of a person holding the torch, option (1) is a logical answer.

Best wishes

[elkniwt]

Hi Tianzhu,


My dd's ans for the qn i asked is 4 (the one without the outline for the transparent plastic).

Here's another qn Q19 for this paper
http://www.orlesson.org/orp/09Sc/P4-09-Sc-SA1-RGS.pdf

The third shadow (triangle) only shows triangle cos the circle object is transparent and does not cast shadow. If need to draw outline for transparent object, then around the triangle, there should be a circle outline.

So, I am very confused, teach her until I also blur.

Regards.

[tianzhu]

elkniwt:

Dear tianzhu,


Thank you for your quick reply. But may I know how come for the clear plastic which is transparent, it is necessary to draw the outline? My dd said teacher taught that transparents objects do not cast shadow.

Thanks for help. It is kind of confusing for me and the child cos for some papers, the outlines of transparent objects are not drawn, but for this qn the correct ans is the one with the outline drawn.

Hi elkniwt

The outline shows the transparent object (clear plastic) as one looks through it to see the shadow formed by the mini hoop and frosted glass.

Option (1) is a logical answer if one stands at the position of the torch and look through the clear plastic.

For this question, we should take note that the clear plastic is of much bigger size as compared to the frosted glass.Thus one needs to look through the plastic when the eyes are at the same level as the the position of the torch.

Please take a piece of clear glass/plastic of the same size as a piece of A4 paper and place it before a piece of smaller opaque cardboard, you should be able to see why.You have to look through the glass/plastic as it is much bigger.

Try to look at the context of the question and choose the appropriate answer.

Best wishes

[tianzhu]

Hi elkniwt


For Q19,try to relate to the context of the question.

In this question, the types of shadows formed are shown in the question, and we should draw conclusion from them without additional inputs.

If you want to look for a reason, you may look at it this way, but it’s not needed to answer this question.

The sizes of the objects are small, about 2cm, about the size of a 20 cents coin; it should not block the angle of the eyes while looking at the shadows. Try looking at a 20 cents coin.

Option(4) is a logical answer.

Best wishes

[atutor2001]

elkniwt:

Hi,


I refer to Qn 10 in this paper,
http://www.orlesson.org/orp/09Sc/P4-09-Sc-SA2-HenryPark.pdf

For this type of question, is it necessary to draw the outlines of transparent objects for the shadows?

Regards

I will choose 4 as the answer.