Video: Properties of P-Waves

Which of the following statements about P-waves is not correct? [A] P-waves travel through liquids. [B] P-waves travel through solids. [C] P-waves are slower than S-waves. [D] P-waves are longitudinal.

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Video Transcript

Which of the following statements about P-waves is not correct? A) P-waves travel through liquids. B) P-waves travel through solids. C) P-waves are slower than S-waves. D) P-waves are longitudinal.

Now, this final statement, the fact that P-waves are longitudinal, is something that we should recall about P-waves. They are actually longitudinal waves. But, in this question, we’re looking for a statement about P-waves that is not correct. And, therefore, because this statement is correct regarding P-waves, that cannot be the answer to our question. So immediately, just by recalling the nature of P-waves, we’ve eliminated one of the four options. Now, let’s look in a little bit more detail at the consequences of the fact that P-waves are longitudinal waves. Let’s start by imagining that we’ve got a boundary between some solid object and some liquid object.

Let’s imagine, first of all, that these pink dots represent the atoms making up the solid object. They are very nicely ordered because they’re the atoms making up a solid object. And let’s also imagine that these black dots represent the atoms making up a liquid object. Now, in a liquid, atoms are a lot more free to flow around each other and have a lot more unrestricted movement. But for simplicity’s sake, we’ve drawn them to be equally as ordered as the particles in the solid here. But anyway, let’s now imagine that a longitudinal wave comes along in this direction. And let’s also remember that a longitudinal wave is the kind of wave that propagates because the medium through which the wave is moving will oscillate back and forth in the direction parallel to which the wave itself is moving.

In other words, the particles in the solid — which can slightly jiggle backwards and forwards because although they can’t move freely, they can at least move a little bit — will allow a longitudinal wave to pass through the solid. Because as the particles jiggle backwards and forwards, the wave moves from left to right. And, actually, the same is true for liquids. Because even though these liquid particles are free to move in whatever direction they want and they’re not restricted as to where they can go. The fact of the matter is that these particles within the liquid can still move back and forth as the wave moves, in this case, from left to right. And one particle moving forward will bump into the next particle, causing that want to move forward and causing the first one to lose energy. And then lots of collisions between lots of particles in the liquid will allow a longitudinal wave to pass through the liquid.

And so, as we’ve seen, longitudinal waves can pass through both solids and liquids. Therefore, looking at statements A and B — which say that P-waves, which are longitudinal, travel through liquids and through solids — well, these statements are correct statements about P-waves. Whereas we’re looking for a statement that is not correct. And, hence, we can eliminate these options as well, which only leaves us with option C. This one says that P-waves are slower than S-waves. However, we should recall that P-waves are called P-waves because the P actually stands for primary. And this refers to the fact that when seismic waves are released, the first kind of wave to be detected anywhere is the P-wave, the primary wave. And then sometime later, the secondary or S-waves come along.

And this means that P-waves are actually faster than S-waves, which means that we’ve found the incorrect statement about P-waves. That incorrect statement is that P-waves are slower than S-waves. Because they, actually, are faster than S-waves.

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