Q&A - PSLE Science

[atutor2001]

Verysuperkiasu:

Question:

Cutting down of trees will cause the phenomenon 'global warming'. Which of the following are involved in global warming?
A: burning of trees causes the carbon in the tree to combine with oxygen in the air to form carbon dioxide.
B: Carbon dioxide in the atmosphere acts as a blanket to trap heat in the Earth.
Without trees, Sun heats up te Earth's surface directly.
Soil erosion will destroy aquatic life forms, which in turn will enhance global warming subsequently.

(1) A and B only
(2) A, B and C only
(3) A, B and D only
(4) A, B, C and D

Answer is (4). My son doesn't think C and D are involved in global warming. Is he correct or is he wrong. His answer is (1).

The difficulty is in the interpretation of \"involved in global warming\". Many activities (C & D included) will involve global warming although they are not the main contributing factors. So I guess (4) is the answer because the question did not asked for the \"main cause\" of global warming.

[atutor2001]

Nebbermind:

atutor2001:

But option (4) specifically stated \"....but not in flask A\".

For science, this is a very strong statement and it would mean that not a tiny bit of milkiness is allowed in A.\"

I go for (1)

CO2 is a trace gas. While there may be precipitation (is that the right word?), I believe it's not enough to turn the water milky. I'll got for (4)

The duration of that experiment was not mentioned (missing). If the experiment is conducted for a few hours, I think A will definitely turn chalky. The reason is as follows :

Percentage of CO2 in exhaled air is about 4% and we can easily turn lime water chalky by blowing into it for 1 minute.

Percentage of CO2 in air is about 0.03. So if we increase the length of time by 270 times i.e. 270 minutes (4.5 hours), the lime water should also turn chalky. (270 times of 0.03 = 8.16 - although this is not a correct mathematical model it can provide a crude indication)

[atutor2001]

Wish all students who are taking the PSLE science paper tomorrow all the Very Best.


Section A’s mark is easier to get than Section B - don’t waste them.

Keep at least 1h 15mins for section B. Please explain even if the question did not ask for it.

Good night and good luck!

[Nebbermind]

BOOMZ:

starlight1968sg:

May I ask if amount of frictional force depends on amount of surface area ie the bigger the surface area in contact, the larger the frictional force?

Yes. The larger the surface area in contact with another surface, the greater the amount of frictional force.

IIRC, when we calculate the frictional force by getting the product of the coefficcient of friction and the force apply between the 2 surfaces.

P.S. That was 30 yrs ago....could have remember wrongly. :oops:

[Verysuperkiasu]

atutor2001:

Verysuperkiasu:

Question:

Cutting down of trees will cause the phenomenon 'global warming'. Which of the following are involved in global warming?
A: burning of trees causes the carbon in the tree to combine with oxygen in the air to form carbon dioxide.
B: Carbon dioxide in the atmosphere acts as a blanket to trap heat in the Earth.
Without trees, Sun heats up te Earth's surface directly.
Soil erosion will destroy aquatic life forms, which in turn will enhance global warming subsequently.

(1) A and B only
(2) A, B and C only
(3) A, B and D only
(4) A, B, C and D

Answer is (4). My son doesn't think C and D are involved in global warming. Is he correct or is he wrong. His answer is (1).

The difficulty is in the interpretation of \"involved in global warming\". Many activities (C & D included) will involve global warming although they are not the main contributing factors. So I guess (4) is the answer because the question did not asked for the \"main cause\" of global warming.

Thanks....!

[Verysuperkiasu]

http://i55.tinypic.com/35a0ny1.png\">

Above picture shows a small square cardboard, behind which is a metal ball (sorry don't know how to make it 3D), and behind that is another square cardboard which is the same height as the metal ball. There is a small pinhole light source facing the small square cardboard (not in the picture).

Below is the 3D version but the circle is supposed to be a metal ball. What do you see on the screen? Screen is supposed to be behind the bigger square. Is the shadow a square or circle?

http://i51.tinypic.com/2dlnknc.png\">

[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

[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?