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Kindly rectify my notion of sliding friction

  1. Jul 24, 2014 #1
    The following is my take on friction. I wish to verify if my hypothesis is correct.

    When trying to visualize sliding friction i have the following notion. I am swayed to consider friction as a force that arises primarily due the the interaction between the molecules(or atoms) of the surfaces in contact. Thus when an object is in contact with the other, it shall make bonds (weak). When we try to pull an object resting on other (ex . A box on a flat table) , we need to break the existing bonds between the two in order for the motion to begin. Thus until and unless we have applied a force strong enough to break those bonds, the motion does not occur (this explains the concept of static friction). This is then followed by motion of an object. The resistive force we now observe, called the kinetic friction is due to the same reason however since there isn't enough time for the bonds to form (although there is still some attraction) we experience lesser resistance. And that is why kinetic friction is less than static friction. Moreover since this phenomenon involves breaking and temporary formation of bonds heat will be released as observed in the physical realm ! Also the bonds formed will be stronger when the weight of the object is more, implying the friction in this case does depend on normal reaction as indicated by the formula f=μN !

    However when reading Feynman lectures on physics vol 1, i found Feynman using the concept of vibrations to describe the phenomenon. How the motion deforms the atoms of the substance setting them into vibration thus producing heat. He further says 'Many people believe that the friction to be overcome to get something started (static friction) exceeds the force required to keep it sliding (sliding friction), but with dry metals it is very hard to show any difference'. This is in contrast to my considerations.

    My apologies for being so descriptive. Any help is appreciated.
    P.s : I am a graduate in physics. :/
  2. jcsd
  3. Jul 24, 2014 #2
    Hi. I'm sure there are much more qualified people here who can give better answers, but I would like to comment:

    I think that thinking about some weak chemical bonds between particles of the two solids in contact is not accurate. The picture I have in my mind is of two rough surfaces, each containing countless little bumps and pits. Gravity pushes the upper surface down onto the lower surface, and therefore the bumps of the top surface fall into pits on the bottom surface and vice versa. When you push the upper surface laterally, you are literally trying to push matter through matter - and the natural repulsion between electronic clouds tries to stop the motion from starting. Once the motion starts, the two surfaces have relative momentum wrt each other, which makes it easier for molecules to move through molecules. In my mind this is consistent with the picture Feynman uses, as moving molecules through molecules thermally excites both.

    *Actually, the idea of molecules moving through molecules is not entirely correct either, as what happens is that molecules rearrange themselves to allow motion to occur. Bonds between molecules of the SAME surface are broken, and the surface of the solid is re-arranged in general.
    Last edited: Jul 24, 2014
  4. Jul 26, 2014 #3
    I am pretty sure that this is not the right approach to friction. As feynman mentions, the following approach requires no dissipation of energy in the form of heat, which is otherwise observed !

    Could you please elaborate on this. I an unable to understand the concept of breaking of the SAME bonds !

    P.s : I would be grateful, if you could provide some reason as to why the hypothesis of weak bonds doesn't fit in. When i say weak bonds i am talking of the forces of adhesion !

    Thank you ! :)
  5. Jul 27, 2014 #4
    Hmmm... I have not read Feynman's book, but I really should. As a general rule, everything Feynman writes is correct, unless corrected by Feynman himself!
    It is the "SAME surface", and not the "SAME bonds" :)

    What I meant is: say we have two rough surfaces, each with a certain configuration of molecules. Assume they are in contact in the presence of gravity. Then my hand wavy explanation would be: because the two surfaces are rough, their features become mingled up, and when we try to set the two surfaces in relative motion, a mechanical force arises which resists the relative motion of certain features with respect to each other. If we keep increasing the force that we apply, we will eventually reach a point where the force is large enough to break some of the inter-moleular bonds that define some of the features of the two surfaces. For example, a peak of surafece A falls into a valley of of surface B, we apply force but nothing happens since the peak is stuck, and only when a large enough force is applied does the peak break free from its surface and motion begins.

    Originally I had two objections, which after some thought I withdraw:

    1) Weak bonds do not explain the dependance on how smooth a surface is (via coeff. of friction).
    Clearly untrue. The rougher the surfaces in contact, the larger the effective area of contact. Hence the larger the total adhesive force is.

    2) Weak bonds do not explain dependance on gravitational force.

    Does the gravitational force pushing one surface onto another allows more adhesive bonds to form? I don't know but it sounds possible.

    I won't go as far as agreeing with your explanation of friction in terms of adhesive bonds. It seems incorrect to me, because it is so different in nature than the repulsive force between molecules (due to electron clouds repelling each other). Here is an example which is very silly to show the difference: assume we have two surfaces. Surface A is shaped like a bowl. Surface B is shaped like dome that fits perfectly into the bowl of surface A. The two surfaces are perfectly smooth, so that they would be no friction forces between the two had they been flat. Of course if we try to set the two surfaces in motion we wouldn't be able to without breaking (one of) them.

    P.S.: I will read Feynman's chapter on friction, and return to you, hopefully with a better understanding :)
  6. Jul 27, 2014 #5
    The molecules at the "friction interface" interact with electromagnetic forces. i dont think we can say that they form bonds for sure not chemical bonds, but they interact in a similar way since all the types of chemical bonds are due to the electromagnetic force.

    As wikipedia says :"The complexity of these interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory"

    So its very complex what exactly is happening at the friction interface. Anyway maybe reading the article at wikipedia will help http://en.wikipedia.org/wiki/Friction.

    PS I find your mini theory of this "weak bonding" quite good. Just because it cant explain the difference between static and kinetic friction in dry metals wont put too much worry, maybe the molecules of dry metals create those weak bonds quite fast regardless of the relative motion at the friction interface.
  7. Jul 27, 2014 #6
    Also, all mechanical forces are of course electromagnetic in nature, and are typically very complicated to describe from first principles. For example, hitting a wall with a hammer is an extremely complicated electromagnetic interaction.

    Friction in my mind has a part which cannot be accounted for by adhesive bonding (picture two visibly jagged surfaces in relative motion). Call it a mechanical part. I'm sure that there is an enormous amount of literature on different models of friction - and its a matter of us picking up a good reference!
  8. Jul 27, 2014 #7
    One more thing: it is not clear to me how the adhesive bonding picture explains the fact that no static friction is encountered if an object is lifted upward.
  9. Jul 27, 2014 #8
    Thats exactly what i had in my mind also, but i guess this "weak adhesive bonding" has a required condition : An external net normal force to keep the molecules "close enough" so the electromagnetic interaction take place. When we lift the object we cancel the net normal force that keeps the molecules close enough.
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