Q&A - PSLE Science

[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.

[atutor2001]

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.

Just some clarifications.

The video demonstrated that friction coefficient is independent of mass and not that friction is independent of mass. Friction is dependent on mass because :

Frictional Force = friction Coefficient x Normal Force
= friction Coefficient x (mass x g)
= mass x [friction Coefficient x g]


The equation Friction Force = friction coeff x normal force, is an empirical formula. It is not derived from any theory. It is based on the assumption that the \"friction coefficient\" is independent of the \"pressure\" at the surfaces in contact with each other. In reality, the \"friction coefficient\" is not independent of the \"pressure\". When the \"pressure\" is small, the coefficient is usually greater than when the \"pressure\" is smaller.

Therefore, if 2 objects of the same mass and material are resting on the same surface, the object with a bigger surface area in contact will be pressing down with a smaller pressure and the frictional force will be greater. Try this out. Place an aluminium foil flat on a plank. Take another aluminium foil of exactly the same size and fold it as many times as possible to form a small cuboid and place it on the same plank. When the plank is tilted, the first one to move will definitely be the foil that has been folded.

[Zack7]

atutor2001:

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.

Just some clarifications.

The video demonstrated that friction coefficient is independent of mass and not that friction is independent of mass. Friction is dependent on mass because :

Frictional Force = friction Coefficient x Normal Force
= friction Coefficient x (mass x g)
= mass x [friction Coefficient x g]



The equation Friction Force = friction coeff x normal force, is an empirical formula. It is not derived from any theory. It is based on the assumption that the \"friction coefficient\" is independent of the \"pressure\" at the surfaces in contact with each other. In reality, the \"friction coefficient\" is not independent of the \"pressure\". When the \"pressure\" is small, the coefficient is usually greater than when the \"pressure\" is smaller.

Therefore, if 2 objects of the same mass and material are resting on the same surface, the object with a bigger surface area in contact will be pressing down with a smaller pressure and the frictional force will be greater. Try this out. Place an aluminium foil flat on a plank. Take another aluminium foil of exactly the same size and fold it as many times as possible to form a small cuboid and place it on the same plank. When the plank is tilted, the first one to move will definitely be the foil that has been folded.

Yes friction force depends on mass.

But no, it does not depend on surface area.

The classical modeling of friction involves the assumption that friction force is independent of surface area.

Which means the empirical equation that you use has the above assumption.

In fact from the video, if you notice, when the prof released both of the blocks with different surface areas, both of them dropped at approximately the same angle even though the surface area differed by more than 2times. This tells you that surface area plays a very small part and the assumption of independence is good in this regime.

Of course there are always exceptions, then perhaps there r higher level of theories to describe. But for a primary school syllabus, if we were to follow the standard model of friction, then no, surface area is taken to be independent.

[atutor2001]

Zack7:

.....But no, it does not depend on surface area.

The classical modeling of friction involves the assumption that friction force is independent of surface area.

Which means the empirical equation that you use has the above assumption.

In fact from the video, if you notice, when the prof released both of the blocks with different surface areas, both of them dropped at approximately the same angle even though the surface area differed by more than 2times. This tells you that surface area plays a very small part and the assumption of independence is good in this regime.

Of course there are always exceptions, then perhaps there r higher level of theories to describe. But for a primary school syllabus, if we were to follow the standard model of friction, then no, surface area is taken to be independent.

Maybe it is useful to understand why the empirical formula was developed. Empirical formulas are developed when the actual formula is too complex and no physicist is able to come up with the complete basis for actual relationship between frictional force, mass and type of surface yet. Such formula is only an \"approximation\". It is intended mainly as an aid for engineers to make a good-enough estimate on frictional force - something they need for their daily work. However, engineers do know that in reality, frictional force does depend on the size of the surface areas that are in contact. Therefore, probably the Primary school syllabus is geared to teach our young the real situation and thus teaching the students that frictional force does depend on the surface area that is in contact.

ps
This is a link to a table on static and kinetic coefficient
http://blog.mechguru.com/machine-design/typical-coefficient-of-friction-values-for-common-materials/

You can find at the end of the table, the following qualification :

\"....The friction coefficient value changes not only with material but also with the position and orientation of the participation materials.

Hope this can help clear your doubts on whether does frictional force depend on the surface area in contact.