1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    EXACTLY
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted