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

  • Thread starter physicsSOS
  • Start date
  • #1

Homework Statement


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:
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(b) Describe the motion of the rock particle at point B during the passage of the next complete cycle. (2 marks)

Homework Equations




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?
 

Answers and Replies

  • #2
66
15
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?
 
  • #3
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?
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?
 
  • #4
66
15
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.
 
  • #5
Hm... I'm not sure. I couldn't find a rope or jump rope. I'm finding it hard to visualise
 
  • #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..?
 
  • #7
66
15
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?
 
  • #8
Nice! The wave moves forward but not the point
 
  • #9
66
15
EXACTLY
just be sure you can tell which way the point DOES move.
 
  • #10
The point moves up and down at the same place
 

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