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Static Friction

  1. Jul 23, 2009 #1
    Ok so this is only my second thread (and post) on this site. My question is that if you push something that doesn't deform very much (like wood) or move because of static friction how is energy conserved. Only going to be in 8th grade so try to keep it simple please.

    :rofl: <-- just cause it's awesome.
  2. jcsd
  3. Jul 23, 2009 #2
    It's conserved because the surface you are sliding over will get hotter as the thing you are pushing loses speed. How "hot" something is (i.e. its temperature) is actually a measure of how fast its molecules/atoms are randomly futzing around. If something is hot its atoms are vibrating really fast (this is why we know so precisely the coldest possible temperature (what we call "absolute zero"). It is simply the point where all the atoms aren't moving at all). As you push something across a surface with friction you "leak" kinetic energy from the forward motion of the object into the random motion of the atoms of both the surface and the object. As a result you block or whatever slows down and things are hotter (to test this simply rub your hands together).
  4. Jul 23, 2009 #3
    I thought that static friction was something like me trying to push lets say a building but it doesn't move because of friction. My question would be where does the energy go? Into pushing the ground you brace your feet on?

    (Edit) So if its not moving how can it be sliding over anything?
  5. Jul 23, 2009 #4
    The energy escapes your body in the form of heat as you sweat. If nothing is moving (you or the building), that's the only way energy can be leaving.
  6. Jul 23, 2009 #5
  7. Jul 23, 2009 #6
    Thanks Cyrus. I understand now i think. Glad I'm not the only one confizzled with Maverick's comment. :)

    To Maverick, what I meant in my earlier comment was that wouldn't it be kinetic friction if it was moving? In fact you said I would "leak" KINETIC friction.

    Am i actually correcting someone older and much more intelligent?!! YES!
    Last edited: Jul 23, 2009
  8. Jul 23, 2009 #7
    No, you're leaking kinetic energy ([itex]\frac{1}{2}mv^2[/itex]). You gave it an initial position and, according to conservation of energy, once it has left your hands it should coast at a constant velocity but in the real world it will slow down. This is because its kinetic energy is "leaking"/is being transferred to the "thermal" energy (thermal energy, as I've already discussed is really just kinetic energy) of the floor and block. The result is your block coasts to a halt and things get warmer. There is no violation of conservation of energy, it's all accounted for.

    However, I misunderstood your question, I thought you were inquiring about something like a moving block coming to a stop when it passes over a rough surface. If you are pushing against a brick wall I wouldn't really say it is friction working against you. When you are pushing against a wall and you do not pass through the wall it is said that the wall is applying a NORMAL force (not normal in the "ordinary" sense but normal in terms of something perpendicular to a surface, like the normal of a plane).

    Ultimately, as i'm sure you're starting to realize, what we call a "frictional" forces and what we call "normal" forces is really arbitrary. Similarly, in things like grade 8 they love to talk about "chemical" energy, "thermal" energy, etc. In reality there is only kinetic energy (which is related to how fast it is moving) and potential energy (which is how much moving it has the POTENTIAL to do). Now, of course, these concepts are far more complicated then that but I don't think that's a terrible way to think about it. Also, solid objects (and liquids and gases and plasmas) are made of atoms that are stuck into some rigid (in the case of solids) structure by electronmagnetic attractions. The result of this is that a solid resists being deformed (like trying to push two magnets with the same pole together). Ultimately this is the origin of the "normal" force we see at the level of people and brick walls and such but really it's the result of these trillions of atoms that like to be kinda close to each other but don't like to be TOO close to each other.
  9. Jul 23, 2009 #8


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    Actually I very much liked Maverick's posts.

    Oh and I'm sure the slang and bad terminology was slightly emphasized only because of:

    I'm also fairly unfamiliar with the concepts of energy, so if possible, could we expand this topic to why it takes considerable force to begin moving a block, but not as much force to keep it moving? I would guess it had something to do with the difference between static and kinetic friction. Any explanations please?
    Last edited: Jul 23, 2009
  10. Jul 23, 2009 #9

    I'm also fairly unfamiliar with the concepts of energy, so if possible, could we expand this topic to why it takes considerable force to begin moving a block, but not as much force to keep it moving? I would guess it had something to do with the difference between static and kinetic friction. Any explanations please?[/QUOTE]

    The reason the coefficient of static friction is higher than the coefficient of kinetic friction (i.e. the reason it takes a larger force to get a body to start moving then it takes to keep it moving) for most materials is essentially due to chemical bonding. Often in older textbooks you'll hear some explanation about jagged surface and having time to "mesh" surfaces once stopped and such but this explanation is really not very correct. This link seems to do a decent job of talking about things better: http://amasci.com/miscon/miscon4.html#fric
  11. Jul 23, 2009 #10


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    This doesn't exactly explain why the static friction is higher than the kinetic friction.
    "It is due to the chemical bonding"
    Why is this chemical bonding stronger when the objects aren't moving?

    But I know this dwells outside the physics side of things, but could someone with knowledge on the subject tell me if learning more about the specific properties of this chemical bonding is reasonably simple to understand, and maybe an article?
  12. Jul 23, 2009 #11


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    As worded, your question states that the object doesn't move because of static friction. From a physics standpoint, you aren't adding any energy to the block because you haven't changed it's velocity. However your muscles consume chemical energy even whey they aren't moving, so this situation is bit more complicated.

    If the block is sliding, then friction converts kinetic energy into thermal energy.

    For some surfaces, such as teflon on teflon, the static and dynamic coefficients of friction are about the same, and this would reduce jerk if used as the resistance for a control mechanism. However modern precise control mechansims use "slideways", which are typcially inverted v shaped rails with holes in them that oil is force through. The sliders are v-shaped and ride on the rails, and since fluid resistance in motion increases with speed, the result is a smooth stick-slip friction free control mechanism.
  13. Jul 23, 2009 #12
    Modern books still use the explanation of http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/simscape/ref/trans_friction1a.gif".

    If you want more depth, read:

    [1] Statics 10th ed, R.C. Hibbeler page 379 -383

    This book will also refer you to:

    [2] J. Krim, Scientific American, October 1996.

    for factors such as temperature, density, cleanliness, and atomic/molecular attraction between contacting surfaces.
    Last edited by a moderator: Apr 24, 2017
  14. Jul 23, 2009 #13
    Its not chemical bonding, there is an attraction between the two surfaces due to some funny physics force that holds molecules near each other (physicists can fill in this with the correct term, as im an engineer I dont care why they stick together, just that they do).

    Surfaces are rough, they have asperities. When these come into contact they adhere to each other whilst static more have time to adhere and require more shear force (assumed main mechanism of breaking the bonds) to seperate when then they are moving becuase less can stick together.

    So although smooth surfaces tend to have lowe coefficients of friciton, this is true only to a point. Once a surface gets very smooth (were talking microns and no contaminents, so a vacuum environment) incredibly high coefficients of friction are measures (can be up to 10 in like - like materials). Like to unlike materials dont tend to bond as well for some reason.

    EDIT: It seems I did actually learn something from tribology.
  15. Jul 23, 2009 #14


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    Staff: Mentor

    Thread re-opened. Lets stay on topic.
  16. Jul 23, 2009 #15

    I didn't mean covalent bonding or something I just called Van Der Waals and such "chemical bonding" because that seemed the quickest way to get an intuitive enough picture at an 8th grade level, and yes the source of these forces is EM, i'm well aware of that. Now who's being pedantic ;)
    Last edited by a moderator: May 4, 2017
  17. Jul 23, 2009 #16
    I think you can simply tell him its "Van Der Waals forces", the weakest of all chemical bonds that are formed and broken during the process of motion and are one key reason (of several) that result in friction.

    You don't need a verbose paragraph of poor verbiage. I don't mean this as an attack on you, but you're almost talking down to him.

    I also didn't mean "covalent bonding, or something". I said coulombic friction. You're throwing around words here left and right without weighing in on what they mean.

    Edit: I replaced the word "Coulombic Forces" with "Columbic Friction" in my previous post. I also changed the link to something a little better.

    By all rights, you should not tell him he's leaking energy. This is an extremely poor thing to teach him. Sorry if this is off topic Mentors, but this is a very important point. If you want, you can move this to https://www.physicsforums.com/forumdisplay.php?f=19".

    Just try to be techincally precise in what you say.

    To szekely: You don't leak energy. Please ignore that.
    Last edited by a moderator: Apr 24, 2017
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