Q&A - PSLE Science

[atutor2001]

cimman:

there is this assessment book, Challenging Mathematics Test Papers for Top Primary Schools. Publisher: math456.com. The questions there lend themselves to algebraic and simultaneous equation solutions. Very useful for students to sharpen their algebra skills.

Thank you very much. Next time when my friends ask, I can tell them to look for it.

[ivanhoo99]

atutor2001:

Verysuperkiasu:

1) Tom has a metal sieve with holes. He put some scraps of paper on it and place a flame below the sieve. Then he drill the holes bigger on the same sieve and repeat the experiment. Which set of scraps of paper will be more brown?

2) Mr Leong uses two metal sieves L and M (same no. of holes). He wants to find out which of the two sieves is a better conductor of heat. He place same scraps of paper on top of the 2 sieves. He observes that the paper inside sieve M looked more burnt than those inside L.

(i) Which sieve is made of a material that is a better conductor of heat.

(ii) Explain your answer in (i).

I heard the sieve conduct the heat away
It appears that M is a better conductor of heat because paper inside it turns browner means more heat get conducted through M. but answer is L. Why? same surface area in contact because same no of holes.

These 2 questions are very similar question to the burning of paper cup question. Understanding heat transfer alone is not enough. We also need to know :
1. Burning and flash point.
2. Temperature distribution.

My reasoning is as follows (sorry very lengthy) :

1. A material will burn only when its TEMPERATURE reaches a level we call \"flash point\". As long as the temperature on the sieve is less than the flash point of the paper, it will not turn brown. Flash point of paper is 232 degree C.

2. The temperature distribution of a metal sieve should roughly be highest at the centre where the flame is directly below and drops radially outwards. The temperature of a candle flame can be as high as 1400 degree C. However, the part of the metal sieve placed directly above the flame will be much lower because being a good conductor of heat, the metal sieve can transfer the heat away quickly from the source.

3. Therefore, a good conductor will have a lower temperature at the point where the flame source is. With lower temperature, the paper above it will not get burnt so easily.

4. A mesh with smaller holes will also have a lower temperature at the point where the flame source is because it has more metal surface and will be able to transfer heat away more quickly than one with big holes.

5. Therefore, if the same flame is used, the peak temperature of a good conductor will be lower compared to one that is a lousier conductor of heat. The temperature of a metal sieve with more metal surface will also have a lower temperature. They will cause less of the paper to get burnt.

Actually this is out of the syllabus but what to do.

[atutor2001]

ck123:

Hi


What energy was possessed by a elastic band when it was stretched?

Ans : it has elastic potential energy

My question is answer just elastic energy correct?

Thanks

Sorry, the word \"Potential\" is more important than the word \"elastic\" because it means \"stored energy.\"

[ivanhoo99]

ck123:

Hi


What energy was possessed by a elastic band when it was stretched?

Ans : it has elastic potential energy

My question is answer just elastic energy correct?

Thanks

Yes. It's wrong. Elastic energy have to come with the word potential. The and is correct.

[HDistinction]

Personally think that it is more important to let kids have strong foundation in Sc by referring to syllabus and using assessment books with typical qns, rather than using school past yr exam papers which hv some beyond syllabus qns (of latest craze? not to allow kids to score 100?!!)

If parents were continue to spoon feed kids with sch past yr papers of the latest craze, sch teachers may think that the kids can cope with these qns (but in fact, the kids hv done the qns bef their exam!!). That’s why some kids sc results were like ‘roller coaster’ --- scored high as they came across the qns bef exam but scored low if otherwise!!!
Always believe that as long as the kids have a good understanding of sc basic concepts, they can also handle the tougher qns in PSLE.
May be we parents should stop pushing kids to do a lot of past yr sch papers just bec of the few 'latest craze' qns which are beyond or out of syllabus but may also appear in their sch exam papers... in order to stop the craze???!!!
Good luck to all p6 kids. All the best.

[andante]

Hi,

For the question below, shouldn't the answer be (4) which is fleshing eating bird ?
http://i52.tinypic.com/2ij1a37.jpg\">

[Verysuperkiasu]

Weights hung progressively to two springs Y and Z and their lengths recorded as shown


Mass of weights Length Y (cm) Length Z (cm)
0 5 4
5 6 6
10 7 8
15 8 10
20 9 12
25 11 14

Qn then ask to plot graph… Based on the graph, explain why the extension of Y becomes greater when 25g of weight was added.

My question : My son says it has this to do with elastic limit? But I am not sure becoz elastic limit means it cannot go back to original length which is not shown in the table above.
Answer key : Spring Y has reached its maximum elasticity hence the increase after 25g weight was added was not constant anymore. Do you agree with this? And I’ve only heard of elastic limit not maximum elasticity. Are they the same?

[chrisu]

Both terms maximum elasticity and elastic limit are correct terms but different in meaning and usage.


Maximum elasticity is the limit of the spring that it stretch and yet still can go back to its original length, in this case should be less than 25g. So the text is not wrong to say it has reached its max. elasticity.

If deformation set in after 25g which is not shown in the experiment, then we can say 25g is the elastic limit of the spring.

So I would use the book answer for this question.

Elasticity and Elastic Limit
Elasticity of a body is the property of the body by virtue of which the body regains its original size and shape when the deformation force is removed. Most materials are elastic in nature to a lesser or greater extend, even though perfectly elastic materials are very rare.

From http://www.eduresourcecollection.com/civil_sm_Elasticity.php;
\"The maximum stress upto which a material can exhibit the property of elasticity is called the elastic limit. If the deformation forces applied causes the stress in the material to exceed the elastic limit, there will be a permanent set in it. That is the body will not regain its original shape and size even after the removal of the deforming force completely. There will be some residual strain left in it.\"

Verysuperkiasu:

Weights hung progressively to two springs Y and Z and their lengths recorded as shown

Mass of weights Length Y (cm) Length Z (cm)
0 5 4
5 6 6
10 7 8
15 8 10
20 9 12
25 11 14

Qn then ask to plot graph.. Based on the graph, explain why the extension of Y becomes greater when 25g of weight was added.

My question : My son says it has this to do with elastic limit? But I am not sure becoz elastic limit means it cannot go back to original length which is not shown in the table above.
Answer key : Spring Y has reached its maximum elasticity hence the increase after 25g weight was added was not constant anymore. Do you agree with this? And I've only heard of elastic limit not maximum elasticity. Are they the same?

[Verysuperkiasu]

chrisu:

Both terms maximum elasticity and elastic limit are correct terms but different in meaning and usage.


Maximum elasticity is the limit of the spring that it stretch and yet still can go back to its original length, in this case should be less than 25g. So the text is not wrong to say it has reached its max. elasticity.

If deformation set in after 25g which is not shown in the experiment, then we can say 25g is the elastic limit of the spring.

So I would use the book answer for this question.

Elasticity and Elastic Limit
Elasticity of a body is the property of the body by virtue of which the body regains its original size and shape when the deformation force is removed. Most materials are elastic in nature to a lesser or greater extend, even though perfectly elastic materials are very rare.

From http://www.eduresourcecollection.com/civil_sm_Elasticity.php;
\"The maximum stress upto which a material can exhibit the property of elasticity is called the elastic limit. If the deformation forces applied causes the stress in the material to exceed the elastic limit, there will be a permanent set in it. That is the body will not regain its original shape and size even after the removal of the deforming force completely. There will be some residual strain left in it.\"

Thank u for sharing!

[Verysuperkiasu]

Anyone can share about the swasthikaa answer keys for section A prelim 2011? My son didn’t agree with quite a few of them. Hope someone could verify…


Unfortunately I don’t have a scanner. One of them is as follows

Which of the following statements about the organism below are correct? Picture shows a frog.

A. It has many cells of various shapes and sizes.
B. It has cells which contain nuclei that pass hereditary materials down.
C. It has different cells to carry out different functions.
D. Cell division takes place as it grows.
E. It has some cells that are surrounded by cellulose walls.

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

Son’s answer is (3) but answer key is (2). Which is correct?

Qn 12 : Stephanie counted the no of plants & animals in her garden. Her findings are shown in the graph below.

No. of plants that grow on the ground = 10
No. of plants that grow on trees = 7
No of grasshoppers = 5
No of Lime Butterflies = 6
No of caterpillars of lime butterflies = 8

Based on the graph above (supposed to be a graph), which of the following statemets are definitely true?

A: There are at least 3 populations of animals
B: There are more plants than animals in Stephanie’s garden.
There are at least 2 populations of plants.
There is only one garden community

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

Son’s answer is (3) but answer key is (4)