Q&A - PSLE Science

[Zack7]

YumYum:

Hi, one more Qn: http://i40.tinypic.com/2n6exdv.jpg\">


Answer key = 4, I dont understand the rationale. Can anyone pls explain?

thanks

first notice that there is 6 outlines in the left and 7 outlines in the right

this shows that even though the right puddle is smaller in surface area, it took an hour longer to evaporate.


next , notice that the area of evaporation on the left is much bigger relative to its original size than the right puddle. i.e, the contour lines are much spread out on the left, compared to the closely packed ones on the right.

this shows that per unit time (per hour in this case), the greater the area of the surface area, the greater the rate of evaporation.

[G Tan]

Zack7:

YumYum:

Hi, I need help with this Qn.


if bulb B doesnt light up, will A and C still light up? answer key = 4.

thanks http://i41.tinypic.com/zkiyow.jpg\">

hmm why is there a circle around bulb B? is it something that you drew in or ?

anyway, ignoring the circle around B, the answer should be 3, none of the bulbs will light up.

because if B is faulty, and the circuit is connected in series, there will be no complete loop.

bulb B does not act as an electric conductor, because current cannot flow through its faulty filament. hence electrical flow ends at B.

No... U r wrong... Look carefully at the connection!!

[G Tan]

struggling:

Hi everyone



Just trying a poll here : do you think it's necessary to send your kids for tuition in science to be able to score well?

After the meet the parents session with the teacher after CA1, the teacher told me that the section B in science is all about knowing the ' key phrases and key words' to use in the open ended answers.

Where to find such things? Just do all the practice from past papers, or got to go to tuition for the answers.?

I really want to avoid the tuition track if I can. Help please, those with experience! :imdrowning:


My comment is: It is because nowadays psle Science marking emphasizes alot on the use of science key words and specific explanation in the open-ended answers.

So, if a teacher has not been to PSLE marking before, he will not be able to guide his /her pupils well in answering...
Thanks in advance.

[Zack7]

G Tan:

Zack7:

[quote=\"YumYum\"]Hi, I need help with this Qn.


if bulb B doesnt light up, will A and C still light up? answer key = 4.

thanks http://i41.tinypic.com/zkiyow.jpg\">

hmm why is there a circle around bulb B? is it something that you drew in or ?

anyway, ignoring the circle around B, the answer should be 3, none of the bulbs will light up.

because if B is faulty, and the circuit is connected in series, there will be no complete loop.

bulb B does not act as an electric conductor, because current cannot flow through its faulty filament. hence electrical flow ends at B.

No... U r wrong... Look carefully at the connection!![/quote]ah... yes you are right

[YumYum]

thank you all

[helenLim2011]

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.

Hi

I think you got it wrong. The size of frictional force depends on the type of surface and the mass of the object - not the surface area.

[G Tan]

helenLim2011:

BOOMZ:

[quote=\"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.

Hi

I think you got it wrong. The size of frictional force depends on the type of surface and the mass of the object - not the surface area.[/quote]It do depends on the size of surface area...

[Zack7]

if we were assuming the simple model of classical friction, then friction does not depend on the contact surface area for 2 materials sliding over each other.


F_friction = u F_normal force

Pressure =Force /Area
PA = F

for a constant Force, i.e the weight of the object, increasing the area means a decrease in pressure.

thus, F does not change, and hence the normal force does not change.

if the normal force does not change, and ‘u’ is the coefficient of friction which is determined empirically, then F_friction does not depend on the area.


however, this is merely a model, simplified per say. in reality, surface areas do play a part in more complicated models. for example, surface areas play a significant role in the nano-scale regions.

but for the sake of a primary school physics question, i believe that friction being independent of surface area would suffice.

[G Tan]

Zack7:

if we were assuming the simple model of classical friction, then friction does not depend on the contact surface area for 2 materials sliding over each other.


F_friction = u F_normal force

Pressure =Force /Area
PA = F

for a constant Force, i.e the weight of the object, increasing the area means a decrease in pressure.

thus, F does not change, and hence the normal force does not change.

if the normal force does not change, and 'u' is the coefficient of friction which is determined empirically, then F_friction does not depend on the area.


however, this is merely a model, simplified per say. in reality, surface areas do play a part in more complicated models. for example, surface areas play a significant role in the nano-scale regions.

but for the sake of a primary school physics question, i believe that friction being independent of surface area would suffice.

In primary school context, surface area do affect amount of friction produced!!

[Zack7]

take a look at this video by MIT


http://www.youtube.com/watch?v=Oagnr0cMLIU

it shows that sliding friction is independent of mass and surface area.