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What the IB thinks of friction

  1. Nov 9, 2006 #1

    Chi Meson

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    I teach an AP/IB physics class in a high school. On the last IB ("International Baccalaureate") examination, there was a question about friction that went like this:

    [an object was described to be at rest on an incline] "Show that the object must start to slip when the angle of incline is 45 degrees or less."

    The IB is convinced that a coefficient of friction cannot be greater than 1.0 . I and many other physics teachers across the globe scratched our collective heads at this. What are we supposed to do with all the tables and textbooks that describe coefficients of static friction which are greater than 1.0? Rubber on pavement, for example (1.2--1.4) .

    The report issued by the IB organization says "teachers are advised to tell students that when the coefficient of friction appears to be greater that 1.0, factors other than friction must be considered."

    I am at this moment looking at a rubber stopper at rest on a sheet of 150 grit sandpaper (stapled to a board) at an angle of nearly 50 degrees. I need to know, what is not "friction" about that?
     
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  3. Nov 9, 2006 #2

    ZapperZ

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    I'd say show your example to that organization, and ask them to tell you what ARE the "other factors" that they had in mind.

    That is utterly ridiculous. Who are the clowns who make up that organization anyway?

    Zz.
     
  4. Nov 9, 2006 #3

    Chi Meson

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    Interstingly, the rubber stopper sticks better to 180 grit sandpaper, to which it holds up to 60 degrees, indicating a coefficient of 1.7. Furthermore, the stopper slides with constant speed down a smooth piece of masonite while at 52 degrees, indicating a coefficient of KINETIC friction of 1.3.

    COrrect me if I am wrong, but I have been teaching that friction is the name given to the cumulative effect of all dissipative forces that arise from two surfacesin contact. The forces can include: normal forces of micro size bumps among the surface, cohesiona dnadhesion effects, electrostatic attraction, and vacuum/atmospheric pressure effects, surface tension, and heck even Van der Waals if the surfaces are smooth enough.

    There is no "law of friction." Any of the general rules of friction (does not depend on surface area or speed of motion) have exceptions.

    In answer to Zz's query, I don't know who or what is the IB authority on Physics.
     
  5. Nov 9, 2006 #4

    ZapperZ

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    I can try looking up some references on the origin/mechanism that creates friction between surfaces when I get back to the office, but I highly doubt that there's any First Principle derivation for it. So if this is true, then friction is purely a phenomenological effect, and you simply measure it to get its value. Thus, there is no a priori physical limit to the value of its coefficient - at least, within reason.

    Zz.
     
  6. Nov 9, 2006 #5

    Gokul43201

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    1. Although I "feel" I too was somehow given the impression that the coefficient can never exceed 1 (sometime during my highschool education, I imagine), I've never subsequently seen an explanation for this. If you do find any attempts, please link them in this thread.

    2. I think it's okay to talk of other forces than friction that prevent slipping between a pair of surfaces. The way I like to distinguish between what I'd call friction and what I wouldn't is using the relationship between the force and the normal reaction. If the force scales roughly like the normal reaction (ie: the coeff. is independent of mass), I call it friction. When it doesn't (eg: a block with sticky tape on the bottom), I say there's other forces involved, and I label them "adhesion".

    That's just the nomenclature I like to follow. While it provides for a consistent meaning for the coefficient, it doesn't impose an upper limit on its value.

    3. Have you tried the rubber stopper on sandpaper experiment with different masses of rubber?
     
  7. Nov 9, 2006 #6

    berkeman

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    You can also use race car cornering as an example. To get the > 1g lateral accelerations that are common with good tires, the mu has to be >1, right?
     
  8. Nov 9, 2006 #7
    This is wrong, end of discussion. The angle of static friction, or angle of repose is simply the phi=arctan(mu-static).

    What is true is that the angle is 45 degrees for the variation of friction force vs applied load. This is obvious, becuase there is no motion and the friction force = applied load up until kinetic motion occurs.
     
  9. Nov 9, 2006 #8

    Gokul43201

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    It's not at all obvious to me what you are saying here.
     
  10. Nov 9, 2006 #9
    Draw a graph. Label the x axis the applied load P. Label the Y axis the friction force F.

    Because it is static, F=P for all values of P until kinetic friction.

    Therefore, the slope is 45%. But that is the slope of the friction force vs applied load.

    That slope is NOT the slope of the physical system.
     
  11. Nov 9, 2006 #10

    Chi Meson

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    You're just saying that this slope is 1.0, right?

    Edit:

    right
     
  12. Nov 9, 2006 #11

    Chi Meson

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    I like the way you said that.
     
  13. Nov 9, 2006 #12

    Gokul43201

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    Okay. That's clearer.
     
  14. Nov 9, 2006 #13
    Hmm."Must" is a heavy term.Good glue make miracles.:rolleyes:
     
  15. Nov 10, 2006 #14
    I think it is illogical for a friction coefficient to exceed 1 because that would mean the friction would be equal or greater than the normal force. (friction= (coef.)(Normal)) So imagine the microscopic look at the edge of the object and the slope... how could all the normal force be destined to friction, that is, with what shape would the microscopic peeks cause this? I don't know if I'm explaining myself, let me now. Those are just my thoughts.
    I'm just 19 but I too have wondered that ever since I was at highschool and just saw coef. that were smaller than 1 .
    I was an IB student btw.
     
  16. Nov 11, 2006 #15

    Chi Meson

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    The normal force does not "become" the frictinal force. The coefficient is merely defined as the ratio of these two dinstinct forces. Normal forces are by definition the force from the surface that points in a direction perpendicular to the surface. It arises entirely due to the electrostatic repulsion of the electrons that surround the exterior of all objects.

    The frictional force(s) are those surface forces that act parallel to the surface at the point of contact. As I mentioned, there is not a single cause of this "parallel componant of surface forces." The bumpiness of the surfaces is one aspect, forces of electrostatic attraction (adhesion and cohesion, hydrogen bonding, Van der Waals, all names for the same underlying phenomenon) account for most of the other sources of the frictional force. Here is an engineers table decribing some standard coefficients.
    http://www.engineershandbook.com/Tables/frictioncoefficients.htm

    Note that rubber against some "solids" can have a coefficient as high as 4.0 . ZapperZ described it as "phenomenological effect," which means we describe friction in terms of "what it does" rather that "what it is."

    And we describe the coefficient as the ratio of the frictional force over the normal force. There is no "Law" to friction. If we define the coefficient in such a way, and if I observe rubber stopper at rest on a piece of masonite at an angle of 60 degrees, the coefficient of static friction is 1.7 for this arrangement.
     
  17. Nov 12, 2006 #16

    Gokul43201

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    I don't see how you can know this, unless you assume that this "static frictional force" is proportional to the normal reaction or you want to define the coefficient as a function of the weight of the stopper. With many soft, polymeric materials (like rubber), there's either a non-zero value of friction in the limit of zero normal force and/or a non-linearity in the friction vs. normal force that becomes pronounced above certain values.

    For related discussion:
    http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000113000024011293000001&idtype=cvips&prog=normal [Broken]

    Also see: Physics of Sliding Surfaces, edited by B. N. J. Persson and E. Tossati ~Kluwer, Dordrecht, 1996
     
    Last edited by a moderator: May 2, 2017
  18. Nov 13, 2006 #17

    Chi Meson

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    This is exactly the assumption made by elementary analysis of friction.
    That's interesting news. It again shows that friction is a very sticky subject (heh heh) when going into deeper analysis.

    Thanks for the link. More ammo for my letter to the IBO.
    Edit: Hey, they want $23 for that article! Any other links to similar data?
     
    Last edited: Nov 13, 2006
  19. Nov 14, 2006 #18
    Chi Meson, where are you from?

    (just wondering cause I was an IB student...)
     
  20. Nov 14, 2006 #19

    vanesch

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    Yes, that's also what came immediately to mind: what about glue ?
    I'm totally with Zapper here, there's no reason to have the STATIC friction coefficient limited to 1.

    The only requirement you have on friction, is that, overall, it can not *produce* work but can only at most "consume" it (that is, there has to be entropy production, and not an entropy sink), but that applies only to the dynamical case, because the static friction force doesn't do any work, and even then, one has to be careful of how the reference frames are set up. This essentially comes down to stating that a friction force must have a negative projection onto the relative velocity of the two objects.
     
  21. Nov 17, 2006 #20

    Gokul43201

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    So you do not require that the frictional force be proportional to the normal reaction between the surfaces? How then, do you define the coefficient?
     
  22. Nov 17, 2006 #21

    Gokul43201

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    I was able to access it for free. I'll take another look.

    And are you sure this elementary analysis applies to your rubber stopper on masonite?
     
    Last edited: Nov 17, 2006
  23. Nov 18, 2006 #22

    PerennialII

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    Resonates pretty well with my experiences, every 'more detailed' investigation of surface phenomena studying & involving friction eventually "diverges" when friction couples with a number of phenomena and can't find a definition which would stand (or well, it becomes a complex model itself).
     
  24. Nov 19, 2006 #23

    Chi Meson

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    My rubber stopper adheres (ha ha) exactly to this elementary definition. There is no glue nor any form of "stickiness" on either of the surfaces (so adhesion cannot be blamed). Both surfaces are very, very smooth, so the effect can not be blamed on the soft rubber hanging onto a ledge of sorts or "grabbing on to pointy bits" (Can't come up with a better line for that). It is a straight forward case of micro-sized bumps and the "nature" of the two surfaces in contact. When on an incline the ratio of parallel frictional force to perpendicular normal force equals the tangent of the angle of incline. This again is exactly the basic, elementary analysis of the frictional force.

    THe IBO "authorities" explicitly said that "all objects will slide when the angle of incline is 45 degree or less," and came back with the further statment that coefficients of friction are never greater than one.

    Both of these statements are not true. If they define "friction" such that only certain contributors to the overal frictional effect are considered, then they are all alone.
     
    Last edited: Nov 19, 2006
  25. Nov 19, 2006 #24

    vanesch

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    Well, first of all, a static frictional force is a binding force which links 2 potentially free parameters. It can be replaced by a holonomic binding as long as the "friction force" which is the replacement of this holonomic binding by a force remains within certain boundaries. So a static friction force is a holonomic binding with a limit on its validity ; this limit makes it actually a non-holonomic binding (a binding which cannot be expressed by a relationship between the generalized coordinates and time alone but involves also the generalized velocities).

    So in a way, you can replace, in a Lagrangian formulation, the static "friction" aspect by a holonomic binding (using a lagrange parameter), from the lagrange parameter, you can calculate the equivalent binding "force" F_B and then you can find out whether this binding force F_B is within its allowed limits (for instance as a function of the normal force F_N). If this is so, then the problem has been correctly solved involving the binding. If not, then the static force f_B has been overcome and must be replaced by another binding (a kinematic friction force, for instance, or nothing), in other words, the binding itself has been released.
    Nothing stops you, for a given configuration, to calculate the ratio of F_B_max over F_N, and call that the static friction coefficient, but there's no reason that this should be a constant, for several situations. In some dry friction situations, this is applicable, but not always.
    Glue is such an example: F_B_max = f(F_N), but it has a non-zero offset (even without F_N, or worse, with negative F_N, there is a non-zero F_B_max as long as the glue doesn't break). A big F_N on a glued surface will however increase the F_B_max.
    So a glued surface does everything a static friction force should do: introduce a potential holonomic binding between generalized coordinates, as long as the binding force is within certain limits, as a function of the normal force. Given the non-zero offset, for zero F_N, we have an infinite ratio of F_B_max and F_N, and hence an infinite coefficient of friction.
     
  26. Nov 19, 2006 #25

    arildno

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    First of all, whether the friction force is strictly proportional to the magnitude of the normal force,
    i.e, [tex]\frac{\partial\frac{F}{N}}{\partial{N}}=0[/tex])
    , should be resolvable by experiment, for example by measuring the frictional force for different loads.
    And from what I know, the N-independence of [itex]\mu[/itex] is at least as strongly verified as the area-independence of [itex]\mu[/itex]

    I don't see why one should bother to try to make a quasi-theoretical formulation out of an evidently macroscopic phenomenon like static/kinetic friction. A waste of time, if you ask me..
     
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