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Describe the motion of the rock particle at point B during t

  1. May 24, 2015 #1
    1. The problem statement, all variables and given/known data
    6 Earthquakes produce transverse and longitudinal seismic waves that travel through rock. Figure 8 shows the displacement of the particles of rock at a given instant, for different positions along a transverse wave. Figure 8: jg76fp.png
    (b) Describe the motion of the rock particle at point B during the passage of the next complete cycle. (2 marks)

    2. Relevant equations

    3. The attempt at a solution
    So I thought that point B of the next cycle is the point at equilibrium after point C. So then I figured that the particle would move down (from point B or equilibrium) to maximum negative displacement, and then up to equilibrium. And that would be the end of the cycle (which starts at point C?). The answer however is this: (oscillates from equilibrium to maximum positive displacement, back to equilibrium, then to max negative displacement) and back to equilibrium /starting position /rest position.

    So, I guess one difference is that they explained the motion during the whole cycle, but why? Aren't they asking for point B?
  2. jcsd
  3. May 24, 2015 #2
    Notice that the wave itself is moving in the forward direction. At a later moment, that entire wave will move forward (to the right). What will happen to point B then?
  4. May 24, 2015 #3
    Oh... so point B will also move forward..? But still, isn't point B always going to be at that point (equilibrium after max positive) on the wave?
  5. May 24, 2015 #4
    Careful - I think I can see where your problem is.

    Consider I have a jump rope. I'll mark a spot along it. In fact if you have a rope nearby you should do this!

    Now lay the rope on the floor and have it stretched out. take one end and give it a quick a up and down jerk (you are making the wave!) and that wave will travel down the length of the rope. Now ask yourself - did the rope move along with the wave? Did the marked piece of of rope travel forward with the wave when the wave reached it? t you find out.

    This is a very important thing to know about waves so let me know what you find out.
  6. May 24, 2015 #5
    Hm... I'm not sure. I couldn't find a rope or jump rope. I'm finding it hard to visualise
  7. May 24, 2015 #6
    But if I think about it, I guess it means that at some instant, the equilibrium point before A will be at point B, since the wave is moving forward..?
  8. May 24, 2015 #7
    thankfully you don't need a rope - use youtube!

    take a look. put your finger on a point along that rope. does it move forward? the wave clearly does. But what about a point on the rope? in other words, which way does the medium move?
  9. May 24, 2015 #8
    Nice! The wave moves forward but not the point
  10. May 24, 2015 #9
    just be sure you can tell which way the point DOES move.
  11. May 24, 2015 #10
    The point moves up and down at the same place
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