When the source and the wave move at the same velocity

Physics

Waves and optics

Introduction to Waves
Amplitude, Period, Frequency and Wavelength of Periodic Waves
Introduction to the Doppler Effect
Doppler effect formula when source is moving away
When the source and the wave move at the same velocity
Mach Numbers
Specular and Diffuse Reflection
Specular and Diffuse Reflection 2
Refraction and Snell's Law
Refraction in Water
Snell's Law Example 1
Snell's Law Example 2
Total Internal Reflection
Virtual Image
Parabolic Mirrors and Real Images
Parabolic Mirrors 2
Convex Parabolic Mirrors
Convex Lenses
Convex Lens Examples
Doppler effect formula for observed frequency
Concave Lenses
Object Image and Focal Distance Relationship (Proof of Formula)
Object Image Height and Distance Relationship

When the source and the wave move at the same velocity When the source and the wave move at the same velocity

Back

When the source and the wave move at the same velocity

⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.

In the last several videos, we assumed that the velocity of
the source, of the object that's emitting the wave,
we've assumed that that's less than the velocity of the wave.
And we saw what happens with the Doppler
effect and all of that.
But what I want to do in this video is not make this
assumption.
In particular, let's see what happens, at least, at first
for our formulas, and then get a conceptual understanding.
Let's see what happens when the velocity of the source is
equal to the velocity of the wave.
The first thing we might try to do is just apply this new
assumption into the formulas that we had in the last video.
And those formulas were here.
These are the observed period and frequency for an observer
that's in the direction of the object.
And if we make this assumption, that the velocity
of the sound-- and the velocity of the source-- we're
not necessarily dealing with soundwaves, although that
might be an easy visualization for you.
That tends to be how I visualize it.
But what happens to these formulas when the velocity of
the source is equal to the velocity of the wave?
If these two quantities are equal up here, you have
something, subtract the same thing from it.
This numerator right here becomes 0.
So it'll turn this whole thing into being 0.
So the period, or the observed period, will be 0, which means
you don't have to wait any time at all
between successive crests.
The entire waveform just gets infinitely bunched together.
So it's kind of like one impulse.
And if we look at the frequency, we can either look
directly at the formula, and you'll see, you have something
divided by 0 right here.
So you can say this is 1 over 0, or you could just say that
the frequency is 1 over the period, and you get this thing
that's undefined.
But if you want to think about, what does the frequency
approach as the velocity of the source approaches the
velocity of the wave, if this thing is only a little bit
less that thing, this is going to be a very, very, very
small, very, very small positive number.
So when you divide these quantities by that very, very,
very small positive number, you're
going to approach infinity.
So the frequency is undefined at the speed, at the velocity
of the wave, but it's going to approach infinity.
It will approach infinity as the source approaches the
velocity of the wave. Not necessarily a soundwave.
I keep using soundwaves, because that's how I tend to
visualize things.
And we'll talk in future videos specifically about
soundwaves.
And we'll touch on it a little bit in this video.
So what is this telling us?
Does this make any sense?
And if you think about it, at least to me, it starts to make
sense just what you saw in the last couple of videos.
The last couple of videos, when something was moving
slower than the speed of sound, you had,
OK, I'm here now.
And I'm about to release the next crest. If I go one period
ago, maybe I was right there.
And the crest that I had released at that time period
maybe has traveled this far, just like that.
If you go a period before that, I would
have been over there.
And crest that I released then would have traveled that far.
We saw this in the last two videos.
And if you go the period before that, I
would have been there.
And the crest that I would have released would
have been that far.
This was the whole reason why the Doppler effect happens.
Because the observer sitting right here-- let me do this in
a separate color-- the observer sitting right here is
going to experience these crests more frequently than an
observer sitting out here.
Because the wavelength gets compressed, because every time
this guy releases a new crest, or a new cycle,
he has moved forward.
He's moved forward in the direction of this
motion right here.
So let's think about what happens when he is exactly
moving at the speed of the wave. So let's say that the
source is here now.
This is right where he is.
And he's right about to release a new crest. So where
was he at one period ago?
So let's say he was here one period ago.
So one period ago, if he's right going to release crest
or cycle right now, one period ago, he
released another cycle.
And where has that cycle gone?
Well, we're assuming that the wave is traveling at the same
velocity as this guy.
But it's going radially outward.
So whatever he released then, it will have traveled at the
same velocity as himself.
So it will have gotten this far.
He released it one period ago, and that's where he was one
period ago.
Over the course of the next period, he traveled there and
so did the wave. The wave also traveled there.
Now, where was this character two periods ago?
When I talk about the period, I'm talking about the actual
period of the wave. Every period, or how long does it
take between similar points in the cycle?
And I like to think of them as the crests in the cycle.
So two periods ago, he was here.
And he released a-- you can imagine, a pulse, or crest.
And where will that be now?
Well that will have traveled as far as he did.
He traveled that far, and so will the
pulse that he released.
It will have traveled-- actually I have to make it a
little bit more symmetric.
It will have traveled that far.
And if you go three periods ago, I think you get the idea.
If you go three periods ago, he was here.
And he released a pulse then, or crest, or a cycle of the
wave. And where will that be now?
Well, it will have traveled as fast as he's gotten.
So it will have gotten this far.
Of course, it's traveling that velocity in every direction,
radially outward.
Now think about the situation for the observer.
Think, in particular, about the observer who's sitting
right here.
Let's say he's just out of the way so that this thing doesn't
run into him and kill him, and what whatever else.
But he's just out of the way, just enough to experience the
sound, but not directly collide with this object
that's emitting the-- well, I shouldn't say sound-- emitting
the wave. I want to be general right here.
We're not assuming that this is a soundwave.
What's he going to experience?
Well, he's not going to-- well, if we assume this is a
soundwave, he's not going to hear anything until the thing
passes right there.
And right when the thing passes, it has all of the
sound that it generated coming with it at exactly
that moment in time.
Instead of hearing things periodically, all of the wave
fronts smack this guy all at once.
And perceptually, instead of hearing a pitch, because
you're hearing something periodic, you're just going to
hear a big thump, because all of that sound energy is
getting to you at the same time.
You're just going to hear a thump.
Because it's no longer really a frequency.
All of the energy is coming to you at the same time.
And when you are talking about sound, specifically, and
especially, when you are trans-sonic, which means
you're around the speed of sound, or parts of you are
above or below the speed of sound, and you move into
supersonic speeds, that's what people relate
to the sonic boom.
And we're going to talk a little bit more about that in
the next video and mach numbers, all that, because I
just find all of that fascinating.
But I think this is intuitive.
Because when you look at this, everything is just reaching
you at exactly the same time.
And this was the case of soundwaves, but it would be
true of any type of waves.

Questions
Tips & Feedback

Be specific, and indicate a time in the video:

At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?

Have something that's not a question about this content?

Post a tip or feedback
General discussion about the site
Report a technical problem with the site
Request a video or feature

This discussion area is not meant for answering homework questions.

Formatting tips

Cancel or

( total)

Share a tip

When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...

Suggest a fix

At 2:33, Sal says "double bonds" but should say "single bonds."

Have something that's not a tip or feedback about this content?

Ask a question
General discussion about the site
Report a technical problem with the site
Request a video or feature

This discussion area is not meant for answering homework questions.

Formatting tips

Cancel or

Discuss the site

For general discussions about Khan Academy, visit our Reddit discussion page.

Flag inappropriate posts

Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.

abuse

disrespectful or offensive
an advertisement

not helpful

low quality
not about the video topic
soliciting votes or seeking badges
a homework question
a duplicate answer
repeatedly making the same post

wrong category

a tip or feedback in Questions
a question in Tips & Feedback
an answer that should be its own question

about the site

a question about Khan Academy
(Visit our FAQ)
a post about badges
a technical problem with the site
(Report a problem)
a request for videos or features