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P17-18 Bow Waves

hi folks this time we're going to talk

about bow waves now a bow wave occurs

when the speed of a wave source is

actually going faster than the speed of

the wave it's producing through a medium

and when and this occurs kind of a

v-shaped wave is produced within that

medium you've seen this if you have ever

gone in a rowboat or a powerboat or it's

even produced by by a little duck that's

swimming through a pond and you get a

v-shaped wave that is produced by this

motion so let's talk about how about bow

wave gets produced so let's begin by

saying I have a little bug and my little

bug is just hopping up and down inside

of the water and every time he hops he

produces a little ripple of waves that

are going to go outward at the velocity

of the wave in all directions around him

now let's say that my little bug starts

swimming in that direction now if he is

hopping and swimming it sounds like a

very busy little bug as he if he is

hopping up and down and swimming forward

what happens is between every hop he is

moving a little bit faster in the

forward direction then the waves are

moving behind him and so the forward

waves are going closer and closer and

closer together and because of his

additional motion the waves behind him

are further and further and further

apart

so the velocity of the source is a

little bit lower slower than the

velocity of the wave now if my bug as

he's hopping up and down to make waves

and he starts just getting a little

faster if the velocity of the wave

source my little happen bug exactly

equals the velocity of the wave in the

material then it's going to have this

nice thick position right in the front

of that bug and that is going to produce

a big compression wave right in front of

him if he goes faster if his velocity

keeps getting bigger so that the bugs

velocity is actually larger

and the velocity of the wave he's going

to hop make a wave go fast

hop make a wave go fast hop make a wave

and because his velocity is so much

faster than the velocity of the wave in

the material white produces behind him

is a bow wave and it is a large v-shaped

wave that's going to go behind him now

you might say to yourself Mary this is

fabulous but why do I care about bugs

hopping in water well this becomes a lot

more interesting if you transform my bug

into an airplane and if you transform my

bug into an airplane you now have an

airplane that is just sitting there

rumbling on the tarmac - one that is

moving through the air - one that is

going exactly the speed of sound and now

you have an airplane that is going

faster than the speed of sound what it

does is it produces this huge massive

bow wave behind it that is a

three-dimensional conic cone shaped wave

that it is going to drag behind it

through its entire flight and that big

massive conic wave that it's going to

drag behind it through its entire flight

well ladies and gentlemen we have just

created a sonic boom and that occurs

when the velocity of the source is

faster than the velocity of the wave in

its medium so this sonic boom is a

three-dimensional wave it is conical it

is a big cone that this airplane is

going to drag behind it many of us have

probably had the experience that we hear

a sound wave from a jet and we look

straight up because that's where we hear

the sound coming from but the jet

because it's traveling five hundred

miles an hour has moved way over a long

distance away by time you know you and I

did it around - looking straight out

because it's moving so fast but if it's

producing a sonic boom this big conical

compression wave is being dropped behind

it

throughout its entire journey a lot of

people think that a sound wave constant

yeah

sonic boom is only produced when the

plane goes sonic supersonic it's not

true this the sonic boom is produced for

the entire trip so if an airplane is

traveling across the a hole in United

States it's going to be dragging a sonic

boom behind it across the entire United

States and these compression waves can

rattle windows on houses and annoy cows

and fields and annoy people living in

the quiet hills of Wisconsin and that's

why sonic booms can be a challenge

now airplanes are not the only things

that produce sonic booms if you steer a

pistol or a rifle crack when there is a

gunshot

that is pretty cuz it has produced a

little sonic boom it's creating a bow

way this is a pretty picture of all of

the compression waves that you're

getting in this firearm a couple things

that are really nifty is you can see

this big compression wave out in front

of the bullet this is the bullet

projectile going forward here is that

really really pretty bow wave in front

of it this is the other thing that's

really pretty from a physics standpoint

the explosion was caused by the gasses

and the gun and you see gases expelled

forward and gases expelled backwards

that is a combination of Newton's third

law and a combination of conservation of

momentum that you have things moving in

two directions if you ever hear the

crack of a whip the tip of a whip is

actually going faster than the speed of

sound and so that correct sound yet you

get at the end of a whip is actually

going faster than the speed of a sound

speed of sound these are fabulous images

of military aircraft breaking the sound

barrier and those huge compression waves

right over the lifting surface of the

Jets is causing that conical you can see

that cone shape here's another picture

from below

that I think is so pretty actually

showing you that conical picture of it's

actually pushing the air molecules

together compressing them so much that

you're taking gaseous air excuse me

gaseous water vapor compressing it down

into liquid water vapor and you can see

the liquid water vapor and it's making a

little cloud right on that lifting

surface very very pretty now the speed

of sound and this whole sonic boom

situation a lot of people don't realize

how it impacts your normal day traveling

if you have ever taken a commercial

airline flight commercial airline

flights across the continental United

States travel from five to six hundred

miles per hour it's pretty average

there's some variety there it's based on

altitude winds Jetstream and things like

that but if you're gonna try and fly

let's say you're gonna try and fly Los

Angeles to New York that is about 2,500

air miles between the two flying at an

average speed of 500 miles an hour

that's a five-hour flight in my lifetime

and I've been long are alive a long time

that flight has never gotten faster and

technology and so many things have

gotten better and better and better well

how come we can't fly faster from New

York to LA the limiting factor is the

speed of sound because if we had

aircraft that flew faster than the speed

of sound it would produce these big

sonic booms that would then be trailed

all the way across the United States

and would cause massive amounts of noise

pollution and it would be very

distracting and disturbing to humanity

now there have been some attempts to

improve the technology back in the 70s

through the 2000s the British and the

French created the Concorde aircraft and

the Concorde aircraft was a commercial

airliner and it could only fly over

oceans what it would do primary flights

were between France and New York and

they had some other flights but they

were all over the water they would take

off they would

supersonic over the ocean they would go

subsonic and then land at New York or

Washington DC or places like that when

on a return trip they would take off go

supersonic over the ocean and then go

subsonic before landing they would go

Mach two on average now Mach means

approximately the speed of sound when

you're talking about traveling through

air so this is about twice as fast

typical flight times la - excuse me New

York to London is about seven hours if

you were gonna do that on the Concorde

it was about half the time

tickets were about three to four times

more expensive to get that incredibly

fast speed but people who are big

executives were worth excuse me they

found the time worth the price so it

worked why don't we do that anymore it

was very expensive to operate these

aircraft and so they eventually got old

they were decommissioned and but that's

not the end of the story right now there

are a variety of different companies

NASA itself is actually working on

creating low boom technology there's

some private corporations doing this and

instead of having all of the sound

compression wave broken just on that

leading lifting edge of that aircraft

they are trying to create either create

these compression waves where have these

compression waves spread out around the

entire lifting body of the aircraft or

produce quiet sonic booms so who knows

in your lifetime my lifetime maybe

you're going to be able to fly from the

Midwest to Disneyworld down in Florida

in an hour which would be kind of a hoot

you could go and come in a day and it

wouldn't be a big deal but so the

technology is being worked on and right

now flights are limited by the speed of

sound all right ladies and gents that

will do we'll see you another time

by