# Traveling Waves: Crash Course Physics #17

Here we have an ordinary piece of rope. It’s not one of those magician’s ropes that can mysteriously put itself back together once it’s been cut it in half. And it’s not particularly strong or durable. But you might say that it does have special powers, because it’ll demonstrate for us the physics of traveling waves. Ropes and strings are really good for this kind of thing, because when you move them back and forth the movement of your hand travels through the rope as a wave. By observing what happens to this rope when we try different things with it, we’ll be able to see how waves behave. Including, how those waves sometimes disappear
completely. How’s that for a magic trick? [Theme Music] This is a typical wave. And waves form whenever there’s a disturbance
of some kind. Often, when something about the physical world changes, the information about that disturbance gradually moves outward, away from the source, in every direction. And as the information travels, it makes a
wave shape. Think about all the disturbance you cause, for example, when you jump on a trampoline. When you hit the trampoline, the downward push that you create moves the material next to it down a little bit, too. And the same goes for the material next to
that, and so on. And while that information is traveling outward,
the spot where your feet first hit the trampoline is already recovering, moving upward again, because of the tension force in the trampoline. And that moves the area next to it upward, too. This up-and-down motion gradually ripples outward, covering more and more of the trampoline. And the ripples take the shape of a wave. Waves are made up of peaks, with crests — the bumps on top — and troughs — the bumps on the bottom. They have an amplitude, which is the distance from the peaks to the middle of the wave. They also have a wavelength, which is the distance between crests — a full cycle of the wave — and a frequency, which is how many of those cycles pass through a given point every second. Multiply the wavelength by the frequency,
and you get the wave’s speed — how fast it’s going. And the wave’s speed only depends on the medium it’s traveling through. That’s why the speed of sound — which is
a wave — doesn’t depend on the sound itself. It doesn’t matter how loud or quiet it is. It just depends on whether the sound is traveling through, say, air or water. Now, there are four main kinds of waves, and we can use our rope to show the difference between some of them: A pulse wave is what happens when you move the end of the rope back and forth just one time. One lonely crest travels through the rope
— that’s the pulse. Then there’s a continuous wave, which is what happens when you keep moving the rope back and forth. In that case, your hand is acting as an oscillator. Anything that causes an oscillation or vibration can create a continuous wave. Now, things that cause simple harmonic oscillation move in such a way that they create sinusoidal waves — meaning that if you plotted the waves on a graph, they’d look a lot like the graph of sin(x). But the waves we’ve mainly been talking
about so far are transverse waves — ones in which the oscillation is perpendicular to the direction that the wave is traveling in. When a wave travels along this rope, for example,the peaks are perpendicular to the rope’s length. The same thing was mostly true for the waves that you made on the trampoline: the waves were traveling along its surface horizontally, but the peaks were vertical. But there are also longitudinal waves, where the oscillations happen in the same direction as the wave is moving. In the case of a longitudinal wave, the back-and-forth motion is more of a compression-and-expansion. These are the kinds of waves that you get
by compressing and stretching a spring — and they’re also the kinds by which sound travels, which we’ll talk more about next time. But all waves — no matter what kind they
are — have something in common: They transport energy as they travel. At a microscopic level, waves occur when the movement of one particle affects the particle next to it. And to make that next particle start moving, there has to be an energy transfer. But how can you tell how much energy a wave has? Well, remember that an object in simple harmonic motion has a total energy of one-half, times the spring constant, times the amplitude of the motion squared. Which means for a wave caused by
simple harmonic motion, every particle in the wave will also have that same total energy of (half k A squared). All of this together tells us that a wave’s
energy is proportional to its amplitude, squared. In other words, if you double the wave’s
amplitude, you get four times the energy. Triple the amplitude, you get nine times the energy. So why is the relationship between amplitude and energy transport so important? Well, the intensity of a wave is related to
the energy it transports. More specifically, its intensity is equal to its power, divided by the area it’s spread over and power is energy over time. So, changing the amplitude of a wave can change its energy — and therefore its intensity — by the square of the change in amplitude. And this relationship is extremely important for things like figuring out how much damage can be caused by the shockwaves from an earthquake. But waves also get weaker as they spread out, because they’re distributed over more area. A spherical wave, for example — one that
ripples outward in all directions — will be spread over the surface area of a sphere that gets bigger and bigger, the farther the wave travels. The surface area of a sphere is equal
to (4) times (pi) times (its radius squared). So, as a spherical wave moves farther from its source, its intensity will decrease by the square of the distance from it. Two meters away from the source, and the intensity of the wave will be 4 times less than if you were 1 meter away. Three meters away, and it’ll be 9 times
less. That’s why being just a little bit farther away from the source of an earthquake can sometimes make a huge difference. Now, let’s go back to the waves we were
making with the rope. Suppose you attach one end of the rope to a ring that’s free to move up and down on a rod. Then, with your hand, you send a pulse — in the form of a crest, rippling along it. When the pulse gets to the end of the rope, the rope slides along the rod. But then, it slides back to where it was. That motion — the sliding back — reflects
the wave back along the rope, again as a crest. But something totally different happens, if you attach the end of the rope so it’s fixed, and can’t move. Now, if you send a pulse along the rope, it
will still be reflected — but this time, as a trough. The wave was inverted. That’s because, when the pulse reached the fixed end of the rope, it was trying to slide the end of the rope upward. But it couldn’t, because the end of the
rope was fixed. So, instead, the rope got yanked downward. And the momentum from that downward movement carried the rope below the fixed end, inverting the wave. Now sometimes, multiple waves can combine. For example: Say you send two identical pulses — both crests — along a rope, one from each end. When the two pulses overlap, they’ll combine to make one crest, with a higher amplitude than the original ones. That’s called constructive interference
— the waves build on each other. Now, let’s say you do the same thing again. This time, both waves have the same amplitude,
but one’s a crest and the other is a trough. And when they overlap, the rope will be flat. It looks like the waves just disappeared! That’s called destructive interference,
when waves cancel each other out. Constructive and destructive interference
happen with all kinds of waves — pulse or continuous, transverse or longitudinal. And sometimes, we can use the effects to our advantage. Noise-canceling headphones, for example, work by analyzing the noise around you and generating a sound wave that destructively interferes
with the sound waves from that noise, canceling it out. There’s a lot more to talk about when it comes to the physics of sound, but we’ll save that for next time. Today, you learned about traveling waves, and how their frequency, wavelength, and speed are all connected. We also talked about different types of waves,
including pulse, continuous, transverse, and longitudinal waves, and how they all transport
energy. Finally, we discussed reflection and interference. Crash Course Physics is produced in association
with PBS Digital Studios. You can head over to their channel to check
out amazing shows like Physics Girl, Shank’s FX, and PBS Space Time. This episode of Crash Course was filmed in
the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and
our equally amazing graphics team is Thought Cafe

## 100 thoughts on “Traveling Waves: Crash Course Physics #17”

1. Nil Cifci says:

the other guy explains it better

2. Mohit Bhatt says:

i wish the boy had explained coz i cant understand her accent ;(

You rock

4. Juan Esteban Otero says:

Dani se comio a maria

5. Stephen Daniels says:

I wish she didn't talk so fast

6. avantika SEHGAL says:

you are beautiful….

7. SJM Music says:

i have no idea where i'd be without crash course

8. Sofia says:

I love you I was so stressed about not knowing the content but you're so clear

9. Ahmed Mutaz says:

10. Roger Bourgeois says:

It's a scientific FACT that for people to learn, and better absorb information, PAUSES are necessary. For a science based show, trying to help people learn, why do you delete all the natural pauses between information chunks during editing? Style over substance? Thumbs down.

11. A Simple Cephalon says:

Huh, a Crash Course Physics video I can actually wrap my brain around.

SAY YOUR PRAYERS THE WORLD IS ENDING PEOPLE!

12. Sleta 09 says:

13. Zayneb Al-Bedaiwi says:

Nice job! This video is so good!!!

14. Sebastian says:

++ for making and sharing this crash course
– for talking through this without a break (I can press the pause button to take time to reflect but it does make the listening experience any nicer)

15. FLETCH says:

( ͡° ͜ʖ ͡°)

16. Suko says:

I did not understand a thing… I'm an idiot

17. Melani bindang says:

why is the teacher so fineee

Good luck on the ap test

19. gmanf27 says:

She’s cute and all but does everyone on tv need to have an accent to get hired

20. Aluvhagiwi Mutshinyali says:

this video surely makes physics way easier, special thanks to the crash course team

21. Bonnie Liu says:

THANK YOU CRASH COURSE!!! Helped so much with APs and classes 🙂 Keep up the awesomeness!

22. Mohanned Abdulaziz says:

some things were complex but this is soooo good

23. Philosjutsu says:

This chick is a dude, right?

24. Daku Hikari Rengo says:

😃 here's a magic trick , what did I come here for?

😆 not the waves

👀 best episode of crash course ever

let's just keep putting you on episodes

😎 we can LEARN all day 🤣

25. Noor Alisalem says:

fav channel for studying <3!

26. Kevin Phillips says:

I'm coming to name a geometry dash level! XD

27. manpreet grewal says:

Everyone go watch freesciencelessons he explains better truss me man

28. Sarah Hobbes says:

🔥💞👏🏻

29. Sir Venlige says:

i love this channel

30. Karina Matos says:

This is perfectly explained.

31. Donnie XL says:

way to go, you took something inherently interesting and made it absolutely boring.

32. space boy says:

all of these comments are the same lmao

33. Deepak Sah says:

Its better understood when the speed is lowered to 0.75x …✌

34. aNoOp pRaSaNnAn says:

You are sooo good….i had become an addict to physics bcze of YOU😍

35. chris lomas says:

My RC Pulse Jet engines run on these Physics.Just Gasoline powered Organ Pipes.

36. enes akar says:

So many foreign words and fast talking, I can't understand well. I reckon you to talk at a decent speed, even if English is your main language physics are not like reading a book you need to understand the mechanics rather than just reading

37. Anam Ali says:

Thank you so much Crash Course! It all makes a lot more sense now!

i love you crashcourse

39. أحمد says:

Thanks from all of my heart to someone who put arabic sub 😍

40. Ivan Mijatović says:

Odlican video jako mi se svidja😍😍

41. Shrii Varshini says:

Pls make more videos like this!!!

42. Bach Nguyen says:

Boring

43. FeleciaTheDon says:

44. Karma says:

What about Reflection…Refraction, Diffraction and Polarisation?

45. Mike Carlson says:

So if a single pulse traveling left along a rope as a trough hits a fixed end and comes back to the right as a crest, could we say the energy it has before hitting the end is equivalent to the energy it has after since the crest is opposite the trough and the direction is opposite before and after, so energy is completely conserved?

46. Malick Sy says:

My physic teacher is also indian my maths too

47. Grayson Beverly says:

y is she indian/–/;

48. Christian And Kaya says:

Fantastic video.

49. Mᴀɴɪ ᴋᴀɴᴛᴀɴ says:

You are as sexy as you'r voice

50. Touka Kirishima says:

hahha so fast yet im interested

51. Tikam Singh negi says:

Good speech mam

52. Alma Kallstenius says:

Thank you for saving my physics exam result!

53. Marcus Bzduch says:

Why is it necessary to edit out all the pauses??? It makes the information feel like one long run-on sentence. I get editing for time but it makes the presentation quite annoying.

54. karthik D says:

Now i understood constructive and destructive interference with more clarity

55. יקיר חדד says:

Absolutely funtastic explanations!! You're great!

56. John-Paul Mitchell says:

This is a huge improvement in the speaker's voice. She's speaking more slowly and anunciating. The auto-generated captions only had six mistakes.

57. ANDREW GROTH says:

Awesome!

58. clorox bleach says:

She is hot

59. EHclipse says:

Using this to study for my waves test today

I have to say, this really explains what my teacher told me in 3 hours, in less than 10 minutes

61. Hessah Sultan says:

My teacher always plays your videos

62. Yousra Ahmed says:

She is talking really fast that I can not gather the information 😩

63. Brock Rohl says:

boiiiiiiiiiiiiiii

64. Natalie Trejo Garcia says:

what is a oscillator

65. Natalie Trejo Garcia says:

all of your ifmtion is helping me for the sencete app ans test

66. Jesus Ibarra Mendoza says:

really really helped lol-_-

67. Evelyn L. says:

I had to change the speed to .75x

68. Scott Prueter says:

Wow, no one has mentioned how Hot the host is? Wow, hot. And informative of course .

69. Kevessi says:

Who is here, because they have physic test next day

70. willy billy says:

nice

71. willy billy says:

jk
no jk to th e jk before

72. Prasath Abishek says:

why my mind gives me memory of priya anjali rai while looking at this girl 🙁

73. Trainer Popplio says:

You condensed a topic in 7 minutes in what took my school about 90 minutes

74. shanae alexandra says:

Apparently this is the easiest unit in my class and my brain is mush

75. wailinburnin says:

Everything about this young woman is attractive, she makes waves for anyone for whom she has attention. I can imagine children crowding around her, old people truly sad to see her leave a room, friends and boyfriends captivated when she places her attention on them. Or maybe it was just the fine work of the sound guy, the lighting guy, the writer, and the editor but I can understand where the concept of goddesses comes from from this video. The reciting of the formula for the surface area of a sphere, I am going to scroll back and play it now repeatedly to bask in the euphoria. Thank you Crash Course.

76. #.Aman. Khatkar says:

Physics is great.
That’s it.

77. Prove Logic says:

Is my physics teacher the only one trying to pipe?

78. The Student Official says:

Lol 3 hours binge watching this might worth more than 3 years of my physics class

79. CalAbunga says:

this video was so good i came in my pants

80. Johnny Fong says:

ni me gusto bb

81. Jagroop Singh says:

She makes me want to kermit sudoku.

82. Jennae Manning says:

As they say : u learn something everyday

83. bman 94 says:

I watched this using noise canceling headphones.. made me trip at the end

84. GET TRIGGERED says:

your face reminds me of a pornstar

Thank you, You helped me a lot!!😊❤️

86. Production Mash says:

I kept scrolling and I couldn’t find the comment I was looking for. So I’m just gonna straight-up say it, she is very pretty.

87. chula says:

Indian are great

88. Denny fazli says:

I LOVE YOU BECAUSE I ALWAYS FOUND NORMAL VIDEOS SO SLOW AND I LITERALLY GOT ANGRY WATCHING THEM BUT THIS IS PERFECT

89. Zaza Ma says:

she is great.!

90. Mido Mostafa says:

she doesn't repeat herself, focused, I hope other guys in @CrashCourse do the same. I always put astronomy videos' playback speed at 1.75

91. Kelly Biddick says:

This can cause harmonic vibration

92. Michael Nevius says:

lol im in class rn taking notes

guys, I have my physics final tomorrow! wish me luck cuz I'm gonna NEED it!

94. G30f says:

I need her as my science teacher

95. Bianca Atienzo says:

Wow thank you.

96. Lyrove UwU says:

I lost it at 3:50

97. Stelissa says:

Very good visually but the dialogue is way too fast and many assumptions are made about the learners' existing knowledge. I'd have broken down the content into more 'digestible' pieces and used a simpler vocabulary. These videos are good for 'final exam' revision but not so good for 'first encounters'.

98. dhiraj choudhary says:

She's talking faster than the speed of light

99. jasper jonkers says:

isn't a continuous wave the same as a transverse wave

100. Bill Geddes says:

M.I.A?