Mr.p: Good morning. Let's learn about waves together, shall we?
BBB: YES! Okay. Sure.
[intro]
Mr.p: When you think of "waves",
the most common visual which probably comes to mind
is waves moving across a body of water like a pond.
While a water wave is most definitely an example of a wave, there are many more.
Sound waves are how you are hearing me,
visible light is an electromagnetic wave and is how you see me,
radio waves are also an electromagnetic wave and are likely how your electronic device is receiving this video.
seismic waves are waves of energy which travel through the Earth,
and waves on a string are elastic potential energy stored in a string. Bobby: Mr. p? Mr. P: Yes, Bobby?
Bobby: Electromagnetic waves are different than the
other ones you mentioned, right?
Mr. p: That's right Bobby, visible light and radio waves are both examples of electromagnetic waves
which are not mechanical waves and do not require a medium to travel through.
We will discuss electromagnetic waves in detail in later lessons.
This lesson is about mechanical waves of which
sound waves, seismic waves, and waves on a string are all examples....
Alright, so a mechanical wave is a disturbance of a medium
which travels through the medium, transferring energy from one place to another.
Please realize waves transfer energy from one location to another, however,
they do not move matter from one location to another.
Wave motion is the motion of the disturbance of the medium, not the motion of the medium itself....
We will use waves on a spring to show the properties of mechanical waves.
We will start with a single wave pulse traveling through the spring.
Okay, yeah. So that happens way too fast to be able to see it
Which is why I have slowed it down 32 times slower than real speed.
Again, this a single wave pulse moving through a medium.
Bobby, what is the medium through which the wave pulse is moving?
Bobby: The wave pulse is in the spring, so the medium the wave pulse travels through
must be the spring, right?
Mr.p: Bobby, that is correct. The medium for this wave pulse is the spring.
Remind me class, after the wave pulse goes by did the medium move from one place to another?
BBB: No.
Mr. p: Then class, what actually moves through the medium?
Billy and Bo: Energy.
Bobby: But it looks like the spring moves, so that does not make sense to me.
Mr. p: Okay Bobby. Let's follow the motion of the piece of tape
which is on the spring and therefore is a part of the medium.
As the wave pulse travels along the spring, the tape does move, however,
the overall displacement of the piece of tape is pretty darn close to zero,
because the medium does not change locations. Bobby, does that help?
Bobby: Yeah. I can see the overall displacement of the spring is almost zero. Thanks.
Billy: Right The energy contained in the disturbance of the medium travels along the spring.
In other words, the wave is a pulse of energy traveling through the medium.
Bo: That make sense. And I bet the larger the amplitude of the wave, the more energy contained in the wave.
Bobby: We know amplitude is the maximum displacement of the wave from equilibrium position.
Does that mean we can treat the equilibrium position
as the position of the medium before and after the wave passes by a point,
or where the spring is before and after the wave is there?
Mr.p: Y'all are correct.
A larger amplitude means more energy
and where the spring starts and ends is the equilibrium position.
Bobby: And I bet a larger amplitude wave has more energy
because your hand has to do more work to give it that amplitude, right?
Mr.p: Yes. Very nice Bobby.
Okay, what we have shown so far is a single wave pulse.
We can also have something called a periodic wave.
A wave pulse is a single disturbance of a medium,
whereas a periodic wave is a connected series of wave pulses.
A periodic wave is also sometimes called a continuous wave.
Billy: Okay. A wave pulse is a single wave,
and a bunch of wave pulses connected to one another is a continuous or periodic wave. Okay.
Mr. p: Now I do want to talk about two different types of mechanical waves.
The wave we have been looking at this whole time is called a transverse wave.
A transverse wave is where the direction of the wave propagation
is perpendicular to the direction of the disturbance of the medium.
As you can see, the direction of wave propagation in this example is horizontal
and the direction of the disturbance of the disturbance of the medium is vertical.
Therefore, the direction of wave propagation is perpendicular
to the direction of the disturbance of the medium. Hence, transverse wave.
Billy: And, according to the Cambridge dictionary, transverse means "in a position or direction
that is at an angle of 90 degrees to something else".
That must be why it is called a transverse wave.
Mr. p: Billy, that is correct.
Examples of transverse waves are waves on the string of guitar,
ripples on the surface of a pond,
seismic s-waves, which are waves in the Earth much like water waves,
and even "The Wave" as performed by a crowd at a sports game.
Now the other type of wave is a longitudinal wave.
A longitudinal wave is where the direction of wave propagation is parallel
to the direction of the disturbance of the medium.
Bobby: You just said the same thing as for a transverse wave
only you replaced perpendicular with parallel, right?
Mr. p: Bobby, that is correct. And in order to create a longitudinal wave on this spring,
I grab the spring, compress it a bit, and let go.
BBB: Uuuh? I don't see the longitudinal wave. Where is it?
Mr.p: Right it is difficult to see. So let's zoom in a little bit.
BBB: I still don't see it. Yeah. Uh. Yeah
Mr. p: So, it is difficult to see longitudinal waves on this spring,
so I am going to change to demonstrating longitudinal waves on a slinky...
I reach down again and compress a portion of the slinky, and let go,
and you can see a wave pulse of compressed slinky travels horizontally
and the direction of wave propagation is also horizontal.
In other words, this is a longitudinal wave because the direction of wave propagation is parallel
to the the direction of the disturbance of the medium.
And when we zoom in a bit, you can see that compression,
or higher density section of the spring move from left to right in the spring.
Bo: Uh, rather than compressing the spring and then releasing it.
Could you push it in and then pull it back to create a full wave pulse instead of just a half wave pulse?
Mr. p: Good point Bo. I could do that. This is what full longitudinal wave pulse looks like.
I've added a piece of tape,
so you can see that the sling does come back to its original position
making the overall displacement of this wave equal to zero, or at least pretty close to zero.
Again this is called a longitudinal wave.
Billy: Right. Because according to the Cambridge dictionary longitudinal means "lengthwise"
or "in the direction of the longest side", which means parallel.
Bobby: Yeah.
Bo. Of course.
Mr. p: Correct Billy.. Examples of longitudinal waves
are the sound waves you use to hear me and seismic p-waves.
P is for primary or pressure wave.
Seismic p-waves are waves of high and low compression
moving through the Earth much like sound waves do.
To review. We learned that mechanical waves are disturbances of a medium
which travel through the medium transferring energy from one place to another....
We can have a single wave pulse or periodic waves which are continuous waves pulses..
And there are two different types of mechanical waves; transverse and longitudinal.
The difference has to do with how the disturbance of the medium
moves relative to the direction of wave propagation.
In a transverse wave, the two are perpendicular to one another.
In a longitudinal wave, the two are parallel to on another....
Thank you very much for learning with me today, I enjoyed learning with you.