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A little help if you aren't busy

  1. Nov 21, 2009 #1
    So I'm relatively new to the world of physics (Junior in high school) and I'm having a little trouble sometimes with piecing together some things. These are NOT homework questions, I have read the rules on that thank you, these are some conceptual questions more than anything. I'd love to better understand these things in part because I don't understand them now and in part because I'd like to learn more intricate parts of physics when I'm able to. Anyways....

    When you have a ball fixed hanging from a string, with another string hanging below that and you tug sharply on the lower string it is able to remove the string and not the ball however when you tug slowly on the string and gradually add tension you can sever the connection of the upper string and it's fixture. I don't quite understand why exactly. It makes sense in that I've seen it done, I just don't understand what is acting differently. When you tug quickly, you're accelerating the string downward and it should have a quick application of a large force? and when you tug slowly it somehow spreads more through the whole system? I don't quite get something here.... (I know this is very basic but I'm not quite sure what is going on)

    When something on a surface with friction has a force applied to it, the static friciton keeps it from moving with greater resistance than the kinetic friction once it begins moving, is this because there is some sort of bonding or interaction between the surface and the object that has trouble occurring when it is sliding? I've had rolling friction described to me recently as the rolling friction being like static friction on the edge of the ball that produces torque on the ball and throws it to the side....which I'm guessing I misheard...what is going on there?

    Lastly, this has been troubling a friend of mine as well....when solids touch, what keeps the lattices of structure of one from going through that of another? Is it the electric repulsion of the individual atoms? What keeps a rock from going through a table?

    I know these are probably very simplistic question for the majority of you but any and all help would be greatly appreciated!

    Thank you for your time! (If anything needs clarifying because I did not describe what I was asking well enough, let me know)
     
  2. jcsd
  3. Nov 21, 2009 #2

    HallsofIvy

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    The string and ball are at least slightly "elastic", not "rigid". That means that when you pull on the string, which then pulls on the ball, it takes a fraction of a second for the "pull" to be transmitted up the string, then through the ball, to the upper string. If you pull very quickly, you will reach the breaking force of the lower string before that pull has been "transmitted" to the upper string. If you pull slowly, however, before you reach the breaking force of the lower string, the upper string will feel that force and the weight of the ball which, together, will be greater than the breaking strength of the upper string.

    Remember that the surfaces involved are not "perfectly smooth"- there are tiny ridges. It is these ridges pressing against one another that form the "bonding or interaction". Once it has started sliding, those ridges are basically "jumping over" each other.

    Yes, in terms of individual atoms, it is the repulsion of the outer electrons that keeps solids from moving through one another. Each atom is electrically neutral but the positive nucleus is surrounded by negative electrons.

     
  4. Nov 21, 2009 #3
    Thank you very much! This helped me clarify somethings quite a bit.
     
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