A bump in "Friction Curve" which separates static and kinetic state of an object is not a fangle in the field of basic physics. Here below are some sayings:
A: "That bump is caused by extra force to speed up the object (F=ma)." This means if we use very, very and very slow speed to drag the object, the bump would disappear.
B: "A is only partially correct. The contact surfaces have their own morphologies (many micro-peaks and valleys sticking with each other). Before moving the object, you need extra force to overcome this mechanical resistance caused by morphology." This seems to say the smoother contacting surfaces are, the more insignificant the bump would be. For ideal smooth surfaces, there won't be the bump.
Z: "Well. I agree with A&B in some sense. However, the surface interaction, which is actually molecular/atoms interaction, could also not be ignored. At static state, the interaction, mainly attraction, is stronger than the kinetic state. And we can also use this bump to measure atomic/molecular interaction force under standard measurement so as to rule out the influences of the rate of force and surficial morphology." This is to say even under very slow dragging force and super smooth surfaces, the bump would also appear.
What's your opinion?
Thank you, David. Yes, the wringing effect could well rule A's assumption out though the exact mechanism is still unknown. Rather than air pressure or surface tension, I think molecular attraction are playing a more important role as surface goes much smoother.
Actually, I'm considering if an experiment to test molecular interaction force could be designed from this simple friction experiment when the weight of factors (molecular interaction, surface condition, air pressure, acceleration etc) to that bump are clearly known.
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