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How do things move?

  1. Oct 12, 2013 #1
    Maybe a silly question, but I nevertheless interested in the answer.

    A steel rod oriented north south sits on a table. Consider two molecules, N and S, at the the north and sound end of rod respectively.

    A force in the northerly direction is applied on point S of the rod and the rod moves 1 inch north.

    My question is basically, does point S move before point N or does point N move before point S, or do they actually move simultaneously to the limit of our observational powers?

    What I'm wondering is, at the molecular level, if you imagine the steel molecules to be incompressible spheres (like glass marbles in column) in a lattice, each sphere tangent to its neighbor, then in order for a sphere at the at the south end to move in the northerly direction, its neighbor to the north must move before it can move into the space the neighbor formerly occupied. So that suggests the idea that the force is transmitted down the rod to the end of rod at the north end, and the first movement occurs at point N moves an infinitessmal distance northward (pushing air out of the way) which then makes room for the sphere at Point a's neighbor to the south to move into the space it formerly occupied and so on. So on the molecular level, while the force is applied at point S and travels North, the movement starts at the north end and "travels" to the south end.

    Alternatively I guess, the idea that molecules are better pictures as "compressible elastic" sphere, which deform but exert pressure when deformed (like a ballon), so the steel rod moves in the same way a column of balloons in a tube might move.

    The mechanism I can't quite understand is the one in which the two points N and S move at precisely the same moment, and how to understand how that could happen? How does the the molecule at point N "know" to move when a pressure is a applied at point "S"?
  2. jcsd
  3. Oct 12, 2013 #2


    Staff: Mentor

    The point S moves before the point N. Before the point N can react to the force applied at S the disturbance must propagate along the length of the rod. It does so at the speed of sound in the material.

    The resulting delay is well within our observational powers. Here is a similar experiment by one of our forum members with modest lab equipment:

    Note, that he shows some further refinements on the next page.
  4. Oct 12, 2013 #3
    Thanks! But still wondering then, if point S moves north, before the molecule immediately to the north of it has moved, that suggests either the two molecules are occupying the same space at the same time. Is that what is going on? Or maybe more like the molecules are forced closer to each other than they would like to be? Like magnets repelling each other?
  5. Oct 12, 2013 #4
    I've never actually studied this but I would think that the molecules inside the material get forced closer together, then the Coulomb forces inside the material push them apart.
  6. Oct 13, 2013 #5


    User Avatar

    Staff: Mentor

    The latter - they're forced closer to each other than they want to be.

    You can think of the molecules of an object as if they're connected by springs, and the stiffer these springs, the more rigid the object. But no matter how stiff they are, there's always some tiny amount of give.
  7. Oct 13, 2013 #6


    Staff: Mentor

    Yes, they are forced closer than their equilibrium spacing. That is why it is often called a compression wave.
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