Q&A - PSLE Science

[Dharma]

tianzhu:

Another question about gears in bicycle(Q38).

http://www.wendykoh.com/08/primary6-acssa1-science.pdf

b) How does the relative sizes of the two gears affect the eventual/final speed of the movement of the back wheel?
What's your view on the answer in the worksheet?(click at link to see)

b) The ratio of size of Gear A : Gear B will determine the speed of rotation of the rear/back wheel.

Size of Gear A/ Size of Gear B = No. of Teeth in Gear A/No. of Teeth in Gear B

For example ....
No. of teeth in Gear A = 16
No. of teeth in Gear B = 8

Gear Ratio = 16/8 = 2

This means that everytime the cyclist pedals to turn Gear A to make 1 revolution, Gear B has to make 2 revolutions. Gear B turns faster than Gear A.

The answer should be as follows
b) The greater the size of Gear A as compared to Gear B, the faster the rear/back wheel will turn.

# The answer given by the publisher is too \"chim\"for me to comprehend

[tianzhu]

Dharma:

b) The greater the size of Gear A as compared to Gear B, the faster the rear/back wheel will turn.

# The answer given by the publisher is too \"chim\"for me to comprehend

Thank you for your answer,Dharma
The answer given in the WS is correct to state that gear B would turn faster when there is increase of speed in gear A.However, it's incomplete as it did not compare the relative sizes of the two gears. In P5 Simple Machines, students are taught that smaller gear turns faster than bigger gear.

As long as the answer goes along this principle, I think it should be fine.

[tianzhu]

Dharma:

Thanks for the compliment. Actually I am learning from you guys.When you post something that tickles the mind, it gives me an opportunity to share new knowledge that I learn with my daughter who is P6 this year. I find the PSLE Science requires the child to have strong concepts that requires a lot of application to real life situations....especially for Section B -Open Ended Qns.

I managed to familiarize a bit on maths and science when my older daughter sat for her PSLE 2 years ago. Fortunately, she did well enough to get into her dream school.

I must thank you Tianzhu for keeping this science thread going!!

Hi Dharma,
With your active participation, I see \"glimmers of hope\" to sustain PSLE 2009 Science's flame burning till Ocober 09.Hopefully, kids taking PSLE Science,especially for those doing without external tuition classes can benefit from the discussion in this thread.I hope it is also useful for parents helping out their kids.Your experience with your older daughter will come in handy.

[tianzhu]

Please click at link to see question Q37, ACS 2007 SA2.

http://www.wendykoh.com/07/primary6-acs-science.pdf

Can a freshly plucked leaf carry out life processes when immersed in hot water?

[Dharma]

tianzhu:

Please click at link to see question Q37, ACS 2007 SA2.

http://www.wendykoh.com/07/primary6-acs-science.pdf

Can a freshly plucked leaf carry out life processes when immersed in hot water?

I don't think it able to for too long. Hot water contains low levels of dissoved oxygen and will be insufficient for respiration.

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

b) There are more stomata on the underside of the leaf which leads to more number of bubbles seen there.

[tianzhu]

Dharma:

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

Hi
Will hot water kill a leaf?

[Dharma]

tianzhu:

Dharma:

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

Hi
Will hot water kill a leaf?

Tianzhu,

Boiling hot water will kill the leaf as the dissolved air in the water will be removed, preventing the leafs ability to make food.

Here, we have a leaf has been freshly cut, air (oxygen) that is still in the leaf from its process of photosynthesis will be forced out of the stomata as air bubbles into the hot water.

Back to your question on whether hot water will kill the leaf ...

Here, the question just says hot water but hot water will lose heat if left to cool. Furthermore, there is nothing to prevent the oxygen in the air to re-dissolve in the water to provide life to the leaf. Maybe, if a layer of oil was poured on the surface of the hot water, we can be more sure ... I guess it all depends how fast the dissolved air is removed when leaf is placed in the hot water as compared to the rate at which air gets re-dissolved in water again.

[corneyAmber]

tianzhu:

Hi
Will hot water kill a leaf?

Sorry my simple brain tells me hot water(i assume boiling hot?) will cook the leaf right? :? cook = dead?

But I do appreciate everything has to be explained in very scientific terms in science, just wondering if children use their common sense first before going into the scientific explanation.

[tianzhu]

Thank you for your time.Another question about rides in theme park.

http://farm4.static.flickr.com/3409/3446771458_b9ae1ca727_o.jpg\">

[Dharma]

tianzhu:

Thank you for your time.Another question about rides in theme park.

http://farm4.static.flickr.com/3409/3446771458_b9ae1ca727_o.jpg\">

a) Point X

b) At Point V, kinetic energy will be greater than the gravitational potential energy while at Point Y the gravitational potential energy will be greater than the kinetic energy