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

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

## 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?

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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?

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?

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.

Hm... I'm not sure. I couldn't find a rope or jump rope. I'm finding it hard to visualise

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..?

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?

Nice! The wave moves forward but not the point

EXACTLY
just be sure you can tell which way the point DOES move.

The point moves up and down at the same place