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B Friction force

  1. Jun 9, 2016 #1
    at the beggining of basic mechanics, we learned about friction, moreover we distinguish between static and dynamic friction. there is something quite disturbing: when an object on an inclined rod is static we can say that the static friction is holding it in place. now, whe we push the object up the rod, somtimes instead of stoping in place, the object slips down the rod, in the same incline that it was static without the movement up. if we assume that we live in sequential world, then in some point the object was static again.
    so how can we explain the difference between the two?
    sorry for my bad grammer..
  2. jcsd
  3. Jun 9, 2016 #2


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    If I understand properly, what you're saying is that if we give the object a shove upwards, there is a moment when it is motionless at the highest point it reaches. And we already know that static friction is sufficient to hold the object in place when it isn't moving. So why doesn't the object slip back down from that highest point? why doesn't static friction hold it there, just as it held the object in place before we started it upwards?
  4. Jun 9, 2016 #3
    dynamic friction is less than static friction. Once it is moving it will continue to move with dynamic friction determining its motion.
  5. Jun 9, 2016 #4


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    You mean something like a ring on a rod, which can remain static at a certain incline. But can also slide up and down again, without being suck at the highest point?

    I think there is some oscillatory dynamics going on, when something dangles on a rod. So the ring is never fully static at the highest point, because it swings or spins slightly.
  6. Jun 9, 2016 #5
  7. Jun 10, 2016 #6


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  8. Jun 10, 2016 #7
  9. Jun 10, 2016 #8
    i mean something like a log on a wooden rod.
  10. Jun 10, 2016 #9
    maybe, but i suspect that the explanation is in the atoms level..
  11. Jun 10, 2016 #10


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    I still have no clear picture of the experiment. But wood is a complex material.
  12. Jun 10, 2016 #11

    this video shows the phenomenon, watch closely from 20:00 and notice that.
    anyway, was anyone troubled about that?
  13. Jun 10, 2016 #12


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    If the incline angle is such that the static friction can barely hold the log in place, then you get such behavior: Sometimes it gets stuck, sometimes not. Small irregularities determine the outcome.
  14. Jun 10, 2016 #13
    Not so obvious. :smile:
    There is no rod or log in your video. At least not around the minute 20.
    You have a block sliding on a wooden incline. You can see the reason for the initial confusion, I hope.
  15. Jun 10, 2016 #14
    you mean a rod is restricted to only a specific object? i think of it as a shape, like a circle can be a variety of objects.
  16. Jun 10, 2016 #15
    Contact surfaces on both the ring and rod interlock physically (topographically) and to an extent chemically. The ring may "bind" at one point on the rod as the surfaces may key together, but due to irregularities in the rod it may not "bind" at another point on the rod. If the ring has been slid up the rod, then the contact surface of the ring will be deformed (topographically and thermally), the rod surface will also be deformed where the ring was slid over it.
  17. Jun 10, 2016 #16
    You can think any way you want as long as you make clear what you think. :smile:
    But making yourself understood relies on using the common definitions of the words and the usual meaning people will assume (given that no re-definition of the word is explicit in the text):

  18. Jun 10, 2016 #17
    This was an excellent question as posed by eliad and as clarified by Nugatory. It all comes down to ideal cases vs. reality. So we are assuming that the static friction is just enough to keep the object from sliding at the particular angle of interest i.e. if the friction is infinitesimally less then the object would slide. Of course in the real world things aren't perfectly uniform. There will be a variation of the coefficient of friction along the surface, just due to material variation and variation in how the surface was sanded/polished etc. The very act of moving the object along the surface will change both ever so slightly. So ideally when the object reaches its highest point and its velocity is zero it should stay there; in reality it could stay or start to slide.
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