Q&A - PSLE Science

[atutor2001]

Hi Verysuperkiasu


The shadow on the screen depends on the relative distances between the objects and also the distance of the light source from the small square. Below are some of the scenarios :

1. Light source is very close to small square and the ball is quite a distance from the small square - shadow will be that of the small square

2. Light source is very close to small square and the ball is also very close to small square - shadow will be that of the ball.

3. Light source is a distance from small square and ball very close to small square and big square is a distance from the ball - shadow will be that of the ball.

… and so on.

The only way to find out is to draw light rays from the light source on the actual picture and see which object blocks the light rays. That object will give the shadow.

[atutor2001]

candy2011:

dear atutor2001,


Can you please help with this question and explain?

The question is as followed:

Aim of the experiment: To find out whether plants need carbon dioxide to photosynthesise

(Set-up shows a bell jar containing a plant and a substance that removes carbon dioxide)

What is the control of the experiment?
(1) [Set-up shows a plant]
(3) [Set-up shows a plant in a bell jar]

Thanks

This is a very tricky question which really stretch our understanding on aim and control set up. The difficulty lies with our understanding on the purpose of the \"bell jar\".

The bell jar is part of the \"device\" used to prevent carbon dioxide from reaching the plant. It is used in combination with the substance that removes carbon dioxide. Therefore, we need to view the \"bell jar and the substance\" as a single part - solely for the purpose of preventing carbon dioxide from reaching the plant.

Once that concept is clear, the control set up must be a set up that allows carbon dioxide to reach the plant and the bell jar is definitely not required. I will put (1) as my answer.

[Verysuperkiasu]

atutor2001:

Hi Verysuperkiasu


The shadow on the screen depends on the relative distances between the objects and also the distance of the light source from the small square. Below are some of the scenarios :

1. Light source is very close to small square and the ball is quite a distance from the small square - shadow will be that of the small square

2. Light source is very close to small square and the ball is also very close to small square - shadow will be that of the ball.

3. Light source is a distance from small square and ball very close to small square and big square is a distance from the ball - shadow will be that of the ball.

...... and so on.

The only way to find out is to draw light rays from the light source on the actual picture and see which object blocks the light rays. That object will give the shadow.

Thanks....unfortunately my son didn't notice the distance and he can't remember now. It was today's science mcq question. He put 'square' as the answer.

Logically if u see from the front, since the length of the square is the same as the diameter of the circle, then you should see a square because of this four right angle corners. But u have a point about the relative distance though. Think my son overlooked that factor.

But the question given is the side profile. Can he still draw lines to find out if the ball indeed blocks the square? Because the 4 corners cannot be seen from the side profile?

[atutor2001]

Verysuperkiasu:

atutor2001:

Hi Verysuperkiasu


The shadow on the screen depends on the relative distances between the objects and also the distance of the light source from the small square. Below are some of the scenarios :

1. Light source is very close to small square and the ball is quite a distance from the small square - shadow will be that of the small square

2. Light source is very close to small square and the ball is also very close to small square - shadow will be that of the ball.

3. Light source is a distance from small square and ball very close to small square and big square is a distance from the ball - shadow will be that of the ball.

...... and so on.

The only way to find out is to draw light rays from the light source on the actual picture and see which object blocks the light rays. That object will give the shadow.

Thanks....unfortunately my son didn't notice the distance and he can't remember now. It was today's science mcq question. He put 'square' as the answer.

Logically if u see from the front, since the length of the square is the same as the diameter of the circle, then you should see a square because of this four right angle corners. But u have a point about the relative distance though. Think my son overlooked that factor.

But the question given is the side profile. Can he still draw lines to find out if the ball indeed blocks the square? Because the 4 corners cannot be seen from the side profile?

I think the chances of the 4 corners of the big square casting shadow on the screen is very slim. That can only happen if the ball and the big square are very close to each other and in addition, the light source must be very far from the ball cum big square.

Probably the shadow is that of the small square (their usual trick), by placing the small square very near to the light source.

[chrisu]

As it's Science question, unless the question specified that the diagram shown is actual size, I think it's unfair to expect the students to draw lines to see the shadow cast. If the diagram shown is actual size or in good ratio proportion, then drawing straight lines from the light source is a good way to see the shadow cast. Otherwise we can only guess the actual distance between objects and light source.

Verysuperkiasu:

atutor2001:

Hi Verysuperkiasu

The shadow on the screen depends on the relative distances between the objects and also the distance of the light source from the small square. Below are some of the scenarios :

1. Light source is very close to small square and the ball is quite a distance from the small square - shadow will be that of the small square

2. Light source is very close to small square and the ball is also very close to small square - shadow will be that of the ball.

3. Light source is a distance from small square and ball very close to small square and big square is a distance from the ball - shadow will be that of the ball.

...... and so on.

The only way to find out is to draw light rays from the light source on the actual picture and see which object blocks the light rays. That object will give the shadow.

Thanks....unfortunately my son didn't notice the distance and he can't remember now. It was today's science mcq question. He put 'square' as the answer.

Logically if u see from the front, since the length of the square is the same as the diameter of the circle, then you should see a square because of this four right angle corners. But u have a point about the relative distance though. Think my son overlooked that factor.

But the question given is the side profile. Can he still draw lines to find out if the ball indeed blocks the square? Because the 4 corners cannot be seen from the side profile?

[atutor2001]

chrisu:

As it's Science question, unless the question specified that the diagram shown is actual size, I think it's unfair to expect the students to draw lines to see the shadow cast. If the diagram shown is actual size or in good ratio proportion, then drawing straight lines from the light source is a good way to see the shadow cast. Otherwise we can only guess the actual distance between objects and light source.

Maybe you have not noticed. In math geometry question, all questions will have the print \"Not draw to scale\". However, we seldom see such wordings in Science questions. Hope you understand what I mean.

[chrisu]

You are right, they don't put that in Science. Does that mean drawn to scale? Haha, what can I say then? It makes me think of nowadays a lot of ads also quote \"picture shown only for reference. Actual item may differ from picture shown\"...so what is shown may or may not be what is shown, it's difficult to assume.


BTW Tianzhu, the Science teacher had informed the boy in my daughter's class about the 2 balls collision answer just before they sat for the PSLE Science exam yesterday. Now the teacher said the answer is (1) after confirmation with the Science HOD without explanation. Answer (1) is;
http://img.photobucket.com/albums/v160/guppiesonli/Misc/F1.jpg\">

I've read the link you've given last Sunday, thanks. I played snooker but to achieve elastic collision needs certain level of skill.

atutor2001:

chrisu:

As it's Science question, unless the question specified that the diagram shown is actual size, I think it's unfair to expect the students to draw lines to see the shadow cast. If the diagram shown is actual size or in good ratio proportion, then drawing straight lines from the light source is a good way to see the shadow cast. Otherwise we can only guess the actual distance between objects and light source.

Maybe you have not noticed. In math geometry question, all questions will have the print \"Not draw to scale\". However, we seldom see such wordings in Science questions. Hope you understand what I mean.

[Verysuperkiasu]

atutor2001:

Probably the shadow is that of the small square (their usual trick), by placing the small square very near to the light source.

Thank you for your reply.

[Cheerfuldad]

Verysuperkiasu:

atutor2001:

Hi Verysuperkiasu


The shadow on the screen depends on the relative distances between the objects and also the distance of the light source from the small square. Below are some of the scenarios :

1. Light source is very close to small square and the ball is quite a distance from the small square - shadow will be that of the small square

2. Light source is very close to small square and the ball is also very close to small square - shadow will be that of the ball.

3. Light source is a distance from small square and ball very close to small square and big square is a distance from the ball - shadow will be that of the ball.

...... and so on.

The only way to find out is to draw light rays from the light source on the actual picture and see which object blocks the light rays. That object will give the shadow.

Thanks....unfortunately my son didn't notice the distance and he can't remember now. It was today's science mcq question. He put 'square' as the answer.

Logically if u see from the front, since the length of the square is the same as the diameter of the circle, then you should see a square because of this four right angle corners. But u have a point about the relative distance though. Think my son overlooked that factor.

But the question given is the side profile. Can he still draw lines to find out if the ball indeed blocks the square? Because the 4 corners cannot be seen from the side profile?

Hi VSkiasu,

My DS put square as well. According to my son, the distance between the circle and the square is 0. i.e. side by side. Therefore, my DS concluded is square.

[Verysuperkiasu]

Cheerfuldad:

Hi VSkiasu,

My DS put square as well. According to my son, the distance between the circle and the square is 0. i.e. side by side. Therefore, my DS concluded is square.

Thanks for your sharing.....but they say just extend the straight lines from the pinhole to the objects - the circle is the one which is blocking the light - the lines didn't touch the bigger square. But i wish the answer was 'square'.