Video Transcript
Which of the following statements about S-waves is not correct? Number one: S-waves travel through solids, number two: S-waves are slower than P-waves, number three: S-waves are longitudinal, or number four: S-waves cannot travel through the Earth’s core.
Now, if we read the question carefully, it’s asking us which statement is not correct. That means that three out of the four statements are correct and we need to find the one that isn’t. So let’s go through each statement one by one and find the one that isn’t correct.
Number one: S-waves travel through solids: well, this statement must be correct because we can detect S-waves as well as P-waves at the surface of the Earth. The surface of the Earth of course — the crust — is a solid. Therefore, in order for us to be able to detect S-waves, they must be able to travel through solids. Therefore, statement number one is correct and it’s not the answer to our question.
Number two: S-waves are slower than P-waves. Well, we can ask the question, what do P and S actually stand for when we discuss P-waves and S-waves? The P in P-waves stands for primary and the S in S-waves stands for secondary. These refer to the order in which we detect them when there’s a seismic event. The P-waves, the primary waves, arrive at the detector first. The secondary waves or S-waves arrive later. The reason for this is because the secondary waves travel more slowly. Therefore, yes, S-waves are slower than P-waves. So this is a correct statement and it’s not the answer to our question either.
Number three then: S-waves are longitudinal. Well, this is something that we need to know. S-waves are actually not longitudinal. They are transverse waves. Therefore, statement number three looks like the answer to our question. However, let’s check that statement four is a correct statement so that we will know that number three is the answer to our question.
Number four: S-waves cannot travel through the Earth’s core. Well, there is a very specific reason for this statement being made. The Earth’s core is made up of two parts: the inner core and the outer core. The inner core is basically a solid ball of mainly iron, whereas the outer core is molten. It’s mainly iron and nickel as well, but it is molten. It’s a liquid. Now, one thing that we know about transverse waves which we’ve just said that S-waves are is that they cannot travel through liquids. And there’s a very specific reason for this. And it has to do with what transverse waves actually are.
Let’s start by drawing a diagram of particles — let’s say in a solid. At this point in time, they are at rest. However, a transverse wave suddenly comes along. Well, a transverse wave is basically when the particles move in the direction of the arrows — that is they move either up or down by a certain amount as the wave itself travels to the right. And we can see the shape of the wave. It is shown by the dotted orange line.
A few seconds later, we can see that the wave has moved to the right. We can show this by comparing the first peak that we see in the diagram. That peak has moved to the right as we can clearly see. However, the particles are only moving up and down, whilst the wave travels to the right. In other words, the particles are travelling transverse to the direction of travel of the wave. And that’s why this kind of wave is known as a transverse wave.
Now, we’ve said that these particles are particles in a solid. And this is the reason why this wave can actually propagate through this solid. It’s because the particles in a solid are strongly bonded to each other. This means that when one of the particles is moved — let’s say in this direction upwards — that bond pulls on the second particle as well to start moving too. And it’s for this reason that the wave can propagate through a solid. The displacement up or down of one particle changes the displacement of the particle next to it. And this is all because of the strong bonds between the particles.
Now, let’s compare what happens in a liquid. Let’s say we’ve got a wave coming along and all of these particles that we’ve drawn now are particles in a liquid. Well, the first particle gets displaced — upwards let’s say. But what happens to the second particle? Well, as we said earlier, it’s the bonds between particles in the solid that result in the displacement of the second particle. But we’ve got a liquid here now. The bonds are really, really weak.
Therefore, the displacement of the first particle is not enough to result in the displacement of the second. So that wave just stops propagating. It’s just this particle that gets displaced upwards and then nothing happens after that.
Therefore, transverse waves cannot travel through liquids. And hence, S-waves cannot travel through the Earth’s core, which means that there is indeed only one incorrect statement that we’ve been given. And so our final answer is that S-waves are longitudinal is the incorrect statement.
