Q&A - PSLE Science
-
atutor2001:
Just some clarifications.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.
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. -
Zack7:
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.
.....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.
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. -
atutor2001:
let me clear your misunderstanding
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.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.
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.
friction force does depend on surface area. that is reality. but in physics, science, we assume independence for the normal day regime
if you are going to say friction force depends on surface area, then can you give an equation that says so? can you give the relationship between surface area vs friciton force?
if you can, then case closed. but i can tell you that is not possible at the current level of knowledge. that is why we can only obtain such 'constants' (coefficients) through experiment.
now why do we assume independence? the fact that surface area plays a very minuscule role in friction gives the green light to assuming independence in the normal day regime.
now let me ask you this question
F= ma
are you going to add in the quantum Schrodinger equation that is the analog to this motion equation?
the obvious answer is no. why? because we assume the quantum effects are minuscule in the classical, or normal day regime.
but hey, reality says it is dependent on quantum effects. this is what you are saying. sure you are right, but is it useful to know that quantum effects affect the classical regime? no. because we know its effects are small... just like in the case of friction, the surface area plays a very small non-linear role. that is why we have assumptions in theories because it works and it simplifies our lives.
and this is why we say classical regimes are mostly independent of quantum effects. -
Zack7:
I have not come across any equation linking frictional force to the contact area. The only link to the contact area is through the coefficient of friction. The coefficient of friction is not a constant. It depends on the pressure of the contact area which is a function of the contact area. Therefore, frictional force is a function of the contact area.
let me clear your misunderstanding
friction force does depend on surface area. that is reality. but in physics, science, we assume independence for the normal day regime
if you are going to say friction force depends on surface area, then can you give an equation that says so? can you give the relationship between surface area vs friciton force?
if you can, then case closed. but i can tell you that is not possible at the current level of knowledge. that is why we can only obtain such 'constants' (coefficients) through experiment.
now why do we assume independence? the fact that surface area plays a very minuscule role in friction gives the green light to assuming independence in the normal day regime.
now let me ask you this question
F= ma
are you going to add in the quantum Schrodinger equation that is the analog to this motion equation?
the obvious answer is no. why? because we assume the quantum effects are minuscule in the classical, or normal day regime.
but hey, reality says it is dependent on quantum effects. this is what you are saying. sure you are right, but is it useful to know that quantum effects affect the classical regime? no. because we know its effects are small... just like in the case of friction, the surface area plays a very small non-linear role. that is why we have assumptions in theories because it works and it simplifies our lives.
and this is why we say classical regimes are mostly independent of quantum effects.
Quantum mechanics has always been my nightmare. However, I also have not come across any academician attempting to find frictional force using quantum mechanics.
I have attached this link on static friction. http://www.utwente.nl/ctw/tr/Research/Publications/PhDTheses/thesis_Deladi.pdf
It is long and dry but I have extracted the main points of interests below:
Due to the difficulties encountered in practice, friction has been explored since ancient times. The first recorded studies on friction are dated in fifteenth century and belong to Leonardo da Vinci (1452-1519). His observations became two hundred of years later two of the well-known laws of sliding (dynamic) friction introduced by Guillaume Amontons (1663-1705), namely:
1. Friction force is directly proportional to the applied load.
2. Friction force is independent of the apparent area of contact.
Leonardo da Vinci introduced also the concept of coefficient of friction (μ) as the ratio of the friction force Ff to normal load N:
μ = Ff/N
More detailed experimental studies on friction were conducted by Charles-Augustin Coulomb (1736-1806) who completed the laws of friction with the third
law:
3. Dynamic friction force is independent of the sliding velocity
These empirical laws have been proved to be valid under certain conditions for many material couples. However, these laws are not valid for all material couples.
For instance, the coefficient of friction between polymers sliding against themselves or against metals or ceramics decreases by increasing the normal load (i.e. contact pressure), which is in contradiction with the first law.
The fact that the 3 laws are not applicable to all material pairs necessitates us to move back to the very basic i.e. frictional force depends on the coefficient of friction. However, this coefficient depends on the contact pressure i.e. the contact area. Therefore, frictional force depends on the contact area. -
atutor2001:
I have not come across any equation linking frictional force to the contact area. The only link to the contact area is through the coefficient of friction. The coefficient of friction is not a constant. It depends on the pressure of the contact area which is a function of the contact area. Therefore, frictional force is a function of the contact area.Zack7:
let me clear your misunderstanding
friction force does depend on surface area. that is reality. but in physics, science, we assume independence for the normal day regime
if you are going to say friction force depends on surface area, then can you give an equation that says so? can you give the relationship between surface area vs friciton force?
if you can, then case closed. but i can tell you that is not possible at the current level of knowledge. that is why we can only obtain such 'constants' (coefficients) through experiment.
now why do we assume independence? the fact that surface area plays a very minuscule role in friction gives the green light to assuming independence in the normal day regime.
now let me ask you this question
F= ma
are you going to add in the quantum Schrodinger equation that is the analog to this motion equation?
the obvious answer is no. why? because we assume the quantum effects are minuscule in the classical, or normal day regime.
but hey, reality says it is dependent on quantum effects. this is what you are saying. sure you are right, but is it useful to know that quantum effects affect the classical regime? no. because we know its effects are small... just like in the case of friction, the surface area plays a very small non-linear role. that is why we have assumptions in theories because it works and it simplifies our lives.
and this is why we say classical regimes are mostly independent of quantum effects.
Quantum mechanics has always been my nightmare. However, I also have not come across any academician attempting to find frictional force using quantum mechanics.
I have attached this link on static friction. http://www.utwente.nl/ctw/tr/Research/Publications/PhDTheses/thesis_Deladi.pdf
It is long and dry but I have extracted the main points of interests below:
Due to the difficulties encountered in practice, friction has been explored since ancient times. The first recorded studies on friction are dated in fifteenth century and belong to Leonardo da Vinci (1452-1519). His observations became two hundred of years later two of the well-known laws of sliding (dynamic) friction introduced by Guillaume Amontons (1663-1705), namely:
1. Friction force is directly proportional to the applied load.
2. Friction force is independent of the apparent area of contact.
Leonardo da Vinci introduced also the concept of coefficient of friction (μ) as the ratio of the friction force Ff to normal load N:
μ = Ff/N
More detailed experimental studies on friction were conducted by Charles-Augustin Coulomb (1736-1806) who completed the laws of friction with the third
law:
3. Dynamic friction force is independent of the sliding velocity
These empirical laws have been proved to be valid under certain conditions for many material couples. However, these laws are not valid for all material couples.
For instance, the coefficient of friction between polymers sliding against themselves or against metals or ceramics decreases by increasing the normal load (i.e. contact pressure), which is in contradiction with the first law.
The fact that the 3 laws are not applicable to all material pairs necessitates us to move back to the very basic i.e. frictional force depends on the coefficient of friction. However, this coefficient depends on the contact pressure i.e. the contact area. Therefore, frictional force depends on the contact area.
no, by your logic, the fact that the 3 laws are not applicable to all material pairs means you cannot even talk about the coefficient of friction.
in fact, since it doesn't apply to all material pairs, this model should be scrapped. -
Zack7:
The words in colour were not my logic. They were extracted from the research paper whereby the link I have attached.atutor2001:
I have not come across any equation linking frictional force to the contact area. The only link to the contact area is through the coefficient of friction. The coefficient of friction is not a constant. It depends on the pressure of the contact area which is a function of the contact area. Therefore, frictional force is a function of the contact area.
Quantum mechanics has always been my nightmare. However, I also have not come across any academician attempting to find frictional force using quantum mechanics.
I have attached this link on static friction. http://www.utwente.nl/ctw/tr/Research/Publications/PhDTheses/thesis_Deladi.pdf
It is long and dry but I have extracted the main points of interests below:
Due to the difficulties encountered in practice, friction has been explored since ancient times. The first recorded studies on friction are dated in fifteenth century and belong to Leonardo da Vinci (1452-1519). His observations became two hundred of years later two of the well-known laws of sliding (dynamic) friction introduced by Guillaume Amontons (1663-1705), namely:
1. Friction force is directly proportional to the applied load.
2. Friction force is independent of the apparent area of contact.
Leonardo da Vinci introduced also the concept of coefficient of friction (μ) as the ratio of the friction force Ff to normal load N:
μ = Ff/N
More detailed experimental studies on friction were conducted by Charles-Augustin Coulomb (1736-1806) who completed the laws of friction with the third
law:
3. Dynamic friction force is independent of the sliding velocity
These empirical laws have been proved to be valid under certain conditions for many material couples. However, these laws are not valid for all material couples.
For instance, the coefficient of friction between polymers sliding against themselves or against metals or ceramics decreases by increasing the normal load (i.e. contact pressure), which is in contradiction with the first law.
The fact that the 3 laws are not applicable to all material pairs necessitates us to move back to the very basic i.e. frictional force depends on the coefficient of friction. However, this coefficient depends on the contact pressure i.e. the contact area. Therefore, frictional force depends on the contact area.
no, by your logic, the fact that the 3 laws are not applicable to all material pairs means you cannot even talk about the coefficient of friction.
in fact, since it doesn't apply to all material pairs, this model should be scrapped.
It is common knowledge that coefficient for friction is relatively independent only for hard material. When it comes to soft material like rubber or even our own hand, applying a constant coefficient of friction is irrelevant. The equation F= coeff x normal is still true but the the coeff will change depending on the conditions and one of them is the contact area.
You don't seem to be really interested in friction. -
atutor2001:
The words in colour were not my logic. They were extracted from the research paper whereby the link I have attached.Zack7:
[quote=\"atutor2001\"]
I have not come across any equation linking frictional force to the contact area. The only link to the contact area is through the coefficient of friction. The coefficient of friction is not a constant. It depends on the pressure of the contact area which is a function of the contact area. Therefore, frictional force is a function of the contact area.
Quantum mechanics has always been my nightmare. However, I also have not come across any academician attempting to find frictional force using quantum mechanics.
I have attached this link on static friction. http://www.utwente.nl/ctw/tr/Research/Publications/PhDTheses/thesis_Deladi.pdf
It is long and dry but I have extracted the main points of interests below:
Due to the difficulties encountered in practice, friction has been explored since ancient times. The first recorded studies on friction are dated in fifteenth century and belong to Leonardo da Vinci (1452-1519). His observations became two hundred of years later two of the well-known laws of sliding (dynamic) friction introduced by Guillaume Amontons (1663-1705), namely:
1. Friction force is directly proportional to the applied load.
2. Friction force is independent of the apparent area of contact.
Leonardo da Vinci introduced also the concept of coefficient of friction (μ) as the ratio of the friction force Ff to normal load N:
μ = Ff/N
More detailed experimental studies on friction were conducted by Charles-Augustin Coulomb (1736-1806) who completed the laws of friction with the third
law:
3. Dynamic friction force is independent of the sliding velocity
These empirical laws have been proved to be valid under certain conditions for many material couples. However, these laws are not valid for all material couples.
For instance, the coefficient of friction between polymers sliding against themselves or against metals or ceramics decreases by increasing the normal load (i.e. contact pressure), which is in contradiction with the first law.
The fact that the 3 laws are not applicable to all material pairs necessitates us to move back to the very basic i.e. frictional force depends on the coefficient of friction. However, this coefficient depends on the contact pressure i.e. the contact area. Therefore, frictional force depends on the contact area.
no, by your logic, the fact that the 3 laws are not applicable to all material pairs means you cannot even talk about the coefficient of friction.
in fact, since it doesn't apply to all material pairs, this model should be scrapped.
It is common knowledge that coefficient for friction is relatively independent only for hard material. When it comes to soft material like rubber or even our own hand, applying a constant coefficient of friction is irrelevant. The equation F= coeff x normal is still true but the the coeff will change depending on the conditions and one of them is the contact area.
You don't seem to be really interested in friction.[/quote]
take a look
http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#ass
when you use that equation, you have these inherent assumptions.
now, are you going to tell primary school kids research paper material stuff?
friction is taught to be independent of surface area all the way up till university.
and you seem to like to pepper your arguments with personal attacks? you cannot argue properly? -
Zack7:
My apology if you viewed my last statement as a personal attack. It was made because you wrote that those were my logic showing that you may not have read the article.
take a look
http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#ass
when you use that equation, you have these inherent assumptions.
now, are you going to tell primary school kids research paper material stuff?
friction is taught to be independent of surface area all the way up till university.
and you seem to like to pepper your arguments with personal attacks? you cannot argue properly?
Any way, thanks for the site you have attached. I have read that many times before and was questioning the validity especially when they cited the example of tire width and tire pressure as evidence that friction is independent of surface area. To my limited knowledge, friction of a rotating tire is a whole new ball game. Have you heard of internal friction? I tried understanding it and gave up. So it is not a very good example.
Our daily live experience will tell us that it is very much more difficult to move a large rubber mat (without sucker) compared to a small one. It is something practical that kids must understand; that the difficulty is due to the increase in frictional force due to the increase in contact area and not due the increase in weight. With this knowledge they can then understand why it is easier to grip a bottle with the whole hand than with just 2 fingers - because the area of contact with whole hand is much bigger and the frictional force will be greater.
Friction in higher study is usually only a small sub-topic under mechanics. It is covered as another type of force and is never dealt in detail or totality. Therefore, it is confined to the use of the formula F = uN which is true for all situations. However, kids were not taught that the u will change if the contact area changes. For hard surfaces, the change may be insignificant for general calculation i.e. non-precision work. However, for soft surfaces, the change can be drastic. I don't recall any coverage in our school or uni syllabus on the factors affecting the coeff \"u\". That could be the reason for the misconception for people who limit themselves to this little exposure to think that friction is independent of contact area. -
atutor2001:
My apology if you viewed my last statement as a personal attack. It was made because you wrote that those were my logic showing that you may not have read the article.Zack7:
take a look
http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#ass
when you use that equation, you have these inherent assumptions.
now, are you going to tell primary school kids research paper material stuff?
friction is taught to be independent of surface area all the way up till university.
and you seem to like to pepper your arguments with personal attacks? you cannot argue properly?
Any way, thanks for the site you have attached. I have read that many times before and was questioning the validity especially when they cited the example of tire width and tire pressure as evidence that friction is independent of surface area. To my limited knowledge, friction of a rotating tire is a whole new ball game. Have you heard of internal friction? I tried understanding it and gave up. So it is not a very good example.
Our daily live experience will tell us that it is very much more difficult to move a large rubber mat (without sucker) compared to a small one. It is something practical that kids must understand; that the difficulty is due to the increase in frictional force due to the increase in contact area and not due the increase in weight. With this knowledge they can then understand why it is easier to grip a bottle with the whole hand than with just 2 fingers - because the area of contact with whole hand is much bigger and the frictional force will be greater.
Friction in higher study is usually only a small sub-topic under mechanics. It is covered as another type of force and is never dealt in detail or totality. Therefore, it is confined to the use of the formula F = uN which is true for all situations. However, kids were not taught that the u will change if the contact area changes. For hard surfaces, the change may be insignificant for general calculation i.e. non-precision work. However, for soft surfaces, the change can be drastic. I don't recall any coverage in our school or uni syllabus on the factors affecting the coeff \"u\". That could be the reason for the misconception for people who limit themselves to this little exposure to think that friction is independent of contact area.
no. the fact that the independence assumption holds true for most materials shows that it is good.
i go back to F=ma
imagine you push a jelly
we know that in real life, it will not conform to F=ma. you have to take into account the viscosity , tensile strength, stress deformity etc...
so are you going to say F is dependent on variables more than m and a and etc...?
no. you say F is ma with a host of assumptions. like constant mass. etc... and other variables like those named above not playing significant roles.
and of course, just like in friction, F=ma is not a good gauge for jelly as opposed to a hard object.
and not everything that science describes is intuitive. it doesn't mean in your daily life contact friction appearing to depend on surface area is a justification that it has dependence. -
Zack7:
I think F=ma may be quite at good gauge for the acceleration of a jelly if the acceleration is not too big to cause distortion to the surface of the jelly.
no. the fact that the independence assumption holds true for most materials shows that it is good.
i go back to F=ma
imagine you push a jelly
we know that in real life, it will not conform to F=ma. you have to take into account the viscosity , tensile strength, stress deformity etc...
so are you going to say F is dependent on variables more than m and a and etc...?
no. you say F is ma with a host of assumptions. like constant mass. etc... and other variables like those named above not playing significant roles.
and of course, just like in friction, F=ma is not a good gauge for jelly as opposed to a hard object.
and not everything that science describes is intuitive. it doesn't mean in your daily life contact friction appearing to depend on surface area is a justification that it has dependence.
When instability of the surface occurs, scientists are aware of its effect and formulas have long been developed to take care of that, way back to 1950.
Therefore, F=ma is applicable only if there is no distortion to the surface of the object. In sec sch, they will limit their knowledge to F=ma for solid. But in university, I am sure they will be made aware that F=ma is totally not applicable for liquid or gas. There are many established equations for them to understand and apply.
Back to frictional force where F=uN, although the change in the coefficient \"u\" with respect to the contact area is negligible for most materials, we cannot ignore that \"u\" is actually dependent on the contact areas because many of these materials where the \"u\" changed drastically with the area are common materials that the kids are exposed to in their daily life.
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