Q&A - PSLE Science

[tianzhu]

Hi ChiefKiasu


Going back to the drum's question, I wish to tap your A-level physics's knowledge.
http://farm4.static.flickr.com/3574/3424583787_5d970d6047_o.jpg

PSLE students learn that a lesser effort is required to lift the same load when the fulcrum is moved nearer to the load. In this question, moving the fulcrum towards the wooden disc means the same effort (same ball dropping from the same height) can lift a heavier load. To put in another way, the wooden block is now thrown with a greater force.

I am just wondering if there are any formulae in Physics to substantiate the above explanation. I know this is beyond PSLE Science, just to satisfy my curiosity.

[ChiefKiasu]

tianzhu:

Hi ChiefKiasu


Going back to the drum's question, I wish to tap your A-level physics's knowledge.
http://farm4.static.flickr.com/3574/3424583787_5d970d6047_o.jpg

PSLE students learn that a lesser effort is required to lift the same load when the fulcrum is moved nearer to the load. In this question, moving the fulcrum towards the wooden disc means the same effort (same ball dropping from the same height) can lift a heavier load. To put in another way, the wooden block is now thrown with a greater force.

I am just wondering if there are any formulae in Physics to substantiate the above explanation. I know this is beyond PSLE Science, just to satisfy my curiosity.

Sure. Although most of my Air-level physics knowledge have followed the same exponential decay as the half-life of Polonium-210, a bit of it still remains.

A balance of weights is in equilibrium if the download moment on the left arm L is the same as the download moment on the right arm, ie. L = R. When that happens, the balance is horizontal.

Now, L = WL * DL
and R = WR * DR
where WL is the download force applied on the left arm and DL is the distance from the centre of gravity of WL from the fulcrum,
and WR is the download force applied on the right arm and DR is the distance from the centre of gravity of WR from the fulcrum.

So, in equilibrium, WL * DL = WR * DR
Now, if the total length of the system remains constant, and DL increases, then DR will decrease accordingly. To maintain equilibrium, it is necessary that WL decrease while WR increase.

If WL remains the same, then the difference in moments would be L'-R' , where L' and R' are the new moments of the system. This difference is translated into net upward force on the right side of the balance.

So if WL remains the same, and DL continues to increase while DR decreases, then the net upward force on the right side will become greater and greater.

Makes sense? :?

[tianzhu]

Thank you for your help.

http://farm3.static.flickr.com/2167/3530920904_2a982cebce_o.jpg\">

I think all the options for this MCQ are incorrect.Is it appropriate to classify as \"reproduce from roots\" ?

African Tulip, radish and carrot -----seeds
palm ------ suckers
yam ----- underground stem.

What's your view?

[lizawa]

tianzhu:

Thank you for your help.

http://farm3.static.flickr.com/2167/3530920904_2a982cebce_o.jpg\">

I think all the options for this MCQ are incorrect.Is it appropriate to classify as \"reproduce from roots\" ?

African Tulip, radish and carrot -----seeds
palm ------ suckers
yam ----- underground stem.

What's your view?

Hi tianzhu,

think you are right. There is no \"reproduction by roots\". carrot reproduce from seeds but it is the root that we eat.

[tianzhu]

Going back once again to the diving’s question, it’s a trendy one being crafted during the year of Beijing Olympics 2008.


First be mesmerised by the beauty, grace and elegance of China’s Guo Jing Jing.
http://www.youtube.com/watch?v=hNUE1h3I_kw

Next take a plunge into the physics of diving
http://www.unc.edu/~lahol/ (for further readings)

Have fun.

[snowz]

tianzhu:

Thank you for your help.

http://farm3.static.flickr.com/2167/3530920904_2a982cebce_o.jpg\">

I think all the options for this MCQ are incorrect.Is it appropriate to classify as \"reproduce from roots\" ?

African Tulip, radish and carrot -----seeds
palm ------ suckers
yam ----- underground stem.

What's your view?

ginger, onion, yam - underground stems
begonia, bryophyllum - leaves
African Tulip - seeds
carrot, radish - roots (other examples are turnip, sweet potato and lotus roots)
palm - suckers
The answer is 3.

[tianzhu]

Living Together


Thank you for your help.

A population is a group of similar organisms that lives and reproduce together in the same place.
The only population I can identify is (B) --- caterpillar, butterfly, butterfly pupa
Could you help to explain your answer?

http://farm4.static.flickr.com/3565/3552205793_95266cf62b_o.jpg\">

[tianzhu]

Simple machines ---Gears


Thank you for your help.

Part (a) is straight forward.
Part (b) --- Without any doubts, Gear Y will turn more rounds as its diameter is smaller than Gear Z.But, how many more?Could we tell based on the information given in the question?

http://farm4.static.flickr.com/3333/3552205861_3037e9de49_o.jpg\">

[tianzhu]

Living Together/Food Web


Thank you for your help.

What's your take on part(b)?

http://farm4.static.flickr.com/3558/3553015310_8ae7f582f7_o.jpg\">

[tianzhu]

Heat


Thank you for your help.

For part(a) ---It's not a fair test.The areas of exposed surfaces to the surroundings as well as the area of contact between the ice and the materials affect the time taken for the ice to melt.What's your view?

For part(b) --- The temperature of the ice remains the same, while the temperature of material X decreases.Do you agree?

http://farm4.static.flickr.com/3395/3553015402_89930b555e_o.jpg\">