The Doppler Effect - By MITK12Videos
Transcript
| 00:08 | imagine you're standing by the side of the road when | |
| 00:10 | a fast sports cars drive by man makes you sound | |
| 00:17 | kind of like that . So the sound is higher | |
| 00:19 | as it's coming towards you and it's lower , it's | |
| 00:21 | moving away from . So there's some interesting interaction between | |
| 00:24 | sound and motion , which we call the Doppler effect | |
| 00:27 | . The first thing we need to understand is sound | |
| 00:30 | . Now , sound is actually just a bunch of | |
| 00:32 | pulses being sent through the air and you can feel | |
| 00:34 | that really easily . Just go like this . So | |
| 00:41 | make that funny noise , josh is sending air through | |
| 00:43 | his lips which are then bouncing off each other at | |
| 00:46 | some rate . If you look closely you can see | |
| 00:48 | it , you can also hear the sound of he's | |
| 00:49 | making now . If you squeeze your lips together tighter | |
| 00:53 | , then they bounce against each other faster like this | |
| 00:57 | . Mhm . And as they're bouncing faster , A | |
| 01:00 | faster rate of vibration means that the pitch you here | |
| 01:03 | is higher . So how does motion come into this | |
| 01:05 | ? Well , imagine that your friend is standing over | |
| 01:08 | here throwing snowballs at you one by one now , | |
| 01:11 | which would you rather have your friend just standing there | |
| 01:13 | throwing snowballs at you or running towards you throwing snowballs | |
| 01:17 | ? You it at the same rate . So even | |
| 01:19 | if he's throwing the snowballs at the same rate , | |
| 01:21 | you receive more of them . If he's coming towards | |
| 01:23 | you and if your friends running away from you throwing | |
| 01:26 | the balls , you're even happier because you'll get hit | |
| 01:28 | less and less often . So this same phenomenon happens | |
| 01:32 | with sound . People had discovered this theoretically , but | |
| 01:36 | there's no really easy way to test this because back | |
| 01:38 | in the 18 hundreds , there weren't any fast cars | |
| 01:41 | to just drive value and you didn't , you didn't | |
| 01:43 | have this happened to us every day , but in | |
| 01:46 | 18 40 it never happened until railroads were being built | |
| 01:50 | . There was a new train line from neutral to | |
| 01:52 | Amsterdam . And so this guy buys ballot , decided | |
| 01:56 | to test this idea about sound by putting a bunch | |
| 01:59 | of trumpet players on a train . So what that | |
| 02:02 | does is that they play a certain pitch while they're | |
| 02:05 | riding on the train . That's guaranteed to be the | |
| 02:07 | same pitch the whole train ride . And you can | |
| 02:09 | put somebody on the ground and listen as the train | |
| 02:11 | goes by and hear the pitch shift . Now we | |
| 02:14 | still have trumpets Like this one . Gosh , plays | |
| 02:17 | the Trumpet . But instead of using a train , | |
| 02:21 | we're gonna use a car first , we'll ride with | |
| 02:25 | the trumpet to see how it sounds . Yeah , | |
| 02:52 | now we'll listen to the trumpet as it drives by | |
| 02:56 | . Mm . Mhm . Mhm . Yeah . Yeah | |
| 03:10 | . Mhm . Yeah . Finally let's hear the constant | |
| 03:16 | pitch emitted by the trumpet together with the Doppler shift | |
| 03:20 | heard from the roadside . Uh huh . So , | |
| 03:40 | one interesting thing to notice happens from our experimental setup | |
| 03:43 | . So here's the road the car was driving by | |
| 03:48 | here and we were standing very close to the road | |
| 03:50 | right there . So what we heard is a pitch | |
| 03:52 | shift that happened very fast like this . Yeah , | |
| 03:57 | so the pictures only changing in this area right here | |
| 04:00 | . Now , if you're standing a little farther away | |
| 04:02 | from the road , we would hear the pitch change | |
| 04:04 | having much more gradually like this , it would start | |
| 04:09 | and end at the same pitches , but the change | |
| 04:11 | would happen over a much longer period of time . | |
| 04:13 | The other really interesting thing about the Doppler effect is | |
| 04:16 | that it works for light to and in fact that | |
| 04:19 | was doctors original motivation for studying this , this effect | |
| 04:22 | and with light instead of the pitch changing , the | |
| 04:26 | color changes . And the Doppler effect is called the | |
| 04:29 | red shift often . And the red shift is the | |
| 04:33 | reason we know that the universe is expanding . Check | |
| 04:36 | that out |
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The Doppler Effect is a free educational video by MITK12Videos.
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