Q&A - PSLE Science

[Sun_2010]

ngl2010:

Oldschool:

[quote=\"ngl2010\"]Hi. Can someone help to explain why frictional force is not dependent on the size of surface area?


TIA

Hi,

Interesting that this is brought up in the primary section.

For primary level (or even to high school), it is sufficient/reasonable to consider that frictional force is not dependent on the size of the surface area. This (ambiguity) lies with the model used by early physicists in their investigation of friction where the early model limits that it is independent of area of contact. As such, based on the classic empirical model, frictional force is independent of surface area which is valid in many circumstances.

However, the phenomena – friction, is far more complex. In reality, the classic model is insufficient to explain many more examples.

Regards

Hi Oldschool. Thank you for the explanation. My son's teacher only stated that the frictional force is not dependent on the size of surface area. It was just my curiosity on why it is so because if my son plays on slide, he will slide faster if contact area is small. But maybe it is just my impression because I have never really timed it.[/quote]Good observation ngl2010.

This is something that seems against the commonsense. Actually, an easier to understand formula, though it is not a part of the primary science syllaus, is that the frictional force is proportional to the Normal force.
The normal force is dependent on the weight which is independent of the area of contact. The normal force will be lesser than the weight on an incline plane, as it is equal to a component of the weight.

As to your query, for the slide, my personal feel is it is faster when you lean back than with the back upright. And I think that has to do more with CG shifting, than friction. Gotta experiment this vacation
Anyone care to explain?

[Oldschool]

Sun_2010:

Good observation ngl2010.

This is something that seems against the commonsense. Actually, an easier to understand formula, though it is not a part of the primary science syllaus, is that the frictional force is proportional to the Normal force.
The normal force is dependent on the weight which is independent of the area of contact. The normal force will be lesser than the weight on an incline plane, as it is equal to a component of the weight.

As to your query, for the slide, my personal feel is it is faster when you lean back than with the back upright. And I think that has to do more with CG shifting, than friction. Gotta experiment this vacation
Anyone care to explain?

Hi,

Caution: Please exercise caution when experimenting.

Torque will be at played when CG is introduced. And you do not really want to end up as a 'human pendulum' i.e. end up going down, head first.

Regards

[dyh]

Sun_2010:

ngl2010:

[quote=\"Oldschool\"]
Hi,

Interesting that this is brought up in the primary section.

For primary level (or even to high school), it is sufficient/reasonable to consider that frictional force is not dependent on the size of the surface area. This (ambiguity) lies with the model used by early physicists in their investigation of friction where the early model limits that it is independent of area of contact. As such, based on the classic empirical model, frictional force is independent of surface area which is valid in many circumstances.

However, the phenomena – friction, is far more complex. In reality, the classic model is insufficient to explain many more examples.

Regards

Hi Oldschool. Thank you for the explanation. My son's teacher only stated that the frictional force is not dependent on the size of surface area. It was just my curiosity on why it is so because if my son plays on slide, he will slide faster if contact area is small. But maybe it is just my impression because I have never really timed it.

Good observation ngl2010.

This is something that seems against the commonsense. Actually, an easier to understand formula, though it is not a part of the primary science syllaus, is that the frictional force is proportional to the Normal force.
The normal force is dependent on the weight which is independent of the area of contact. The normal force will be lesser than the weight on an incline plane, as it is equal to a component of the weight.

As to your query, for the slide, my personal feel is it is faster when you lean back than with the back upright. And I think that has to do more with CG shifting, than friction. Gotta experiment this vacation
Anyone care to explain?[/quote]My thought is this. Assume we have 2 blocks with same mass but different base areas lying on a level surface. How much force is required to push the blocks sideways? The amount of force should be the same if the frictional force is equal for both blocks. Reason is because the bigger base block exerts less pressure on the surface. But the bigger base compensates for this lower pressure and hence both blocks require same amount of force to move. What do you think?

[NYPS]

Can sby :please: give me the meanings for:


Changed variable
Independent variable
Controlled variable

[Oldschool]

NYPS:

Can sby :please: give me the meanings for:


Changed variable
Independent variable
Controlled variable

Hi,

Generally,

Independent variables –> A variables that is changed or tested.
Changed variables -> A variable that is affected by the Independent variables. Usually, it is also the variable that is measured.
Controlled variables -> Variables that are kept constant so as to achieve a fair test.

For example, the following test,
http://i43.tinypic.com/rmqph1.jpg\">
(taken from post @ http://www.kiasuparents.com/kiasu/forum/viewtopic.php?p=960101#p960101 )

Independent variables -> The different materials A to D.
Changed variables -> Temperature of the material under illumination
Controlled variables -> Intensity of light, the distance, surface and surface area of the material, etc.

Regards

[GDragon]

Oldschool:

NYPS:

Can sby :please: give me the meanings for:


Changed variable
Independent variable
Controlled variable

Hi,

Generally,

Independent variables –> A variables that is changed or tested.
Changed variables -> A variable that is affected by the Independent variables. Usually, it is also the variable that is measured.
Controlled variables -> Variables that are kept constant so as to achieve a fair test.

For example, the following test,
http://i43.tinypic.com/rmqph1.jpg\">
(taken from post @ http://www.kiasuparents.com/kiasu/forum/viewtopic.php?p=960101#p960101 )

Independent variables -> The different materials A to D.
Changed variables -> Temperature of the material under illumination
Controlled variables -> Intensity of light, the distance, surface and surface area of the material, etc.

Regards

:thankyou: :udaman:

[kiasuparentsirock]

How do you do this?


A farmer living alongside a river had a farm with cows gazing on the field. Further down the river was an industrial area. On the left of the river, the land was fertile with well-grown trees and leafy grasses. The land on the opposite side of the river was barren and brown with hardly any vegetation.

Give two reasons why the land on the left of the river was more fertile than the land on the right of the river.

[ngl2010]

kiasuparentsirock:

How do you do this?


A farmer living alongside a river had a farm with cows gazing on the field. Further down the river was an industrial area. On the left of the river, the land was fertile with well-grown trees and leafy grasses. The land on the opposite side of the river was barren and brown with hardly any vegetation.

Give two reasons why the land on the left of the river was more fertile than the land on the right of the river.

I like your Avatar. Sorry :offtopic:

[kiasuparentsirock]

ngl2010:

kiasuparentsirock:

How do you do this?


A farmer living alongside a river had a farm with cows gazing on the field. Further down the river was an industrial area. On the left of the river, the land was fertile with well-grown trees and leafy grasses. The land on the opposite side of the river was barren and brown with hardly any vegetation.

Give two reasons why the land on the left of the river was more fertile than the land on the right of the river.

I like your Avatar. Sorry :offtopic:

[kiasuparentsirock]

How do you do this?


Marianne placed 2 glass jars, X and Y, in the sun. She put a thermometer in each jar so that she could read the temperature of the air in the glass jars. Then, she covered Y with a lid.

The lid in Y caused the temperature in both jars to be different. Explain How.

In what way does the above set up similar the greenhouse effect?