# 0.4 2d and 3d wavefronts  (Page 6/7)

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## Subsonic flight

Subsonic

Subsonic refers to speeds slower than the speed of sound.

When a source emits sound waves and is moving slower than the speed of sound you get the situation in this picture. Notice that the source moving means that the wavefronts, and therefore peaks in the wave, are actually closer together in the one direction and further apart in the other.

If you measure the waves on the side where the peaks are closer together you'll measure a different wavelength than on the other side of the source. This means that the noise from the source will sound different on the different sides. This is called the Doppler Effect .

Doppler Effect

when the wavelength and frequency measured by an observer are different to those emitted by the source due to movement of the source or observer.

## Supersonic flight

Supersonic

Supersonic refers to speeds faster than the speed of sound.

If a plane flies at exactly the speed of sound then the waves that it emits in the direction it is flying won't be able to get away from the plane. It also means that the next sound wave emitted will be exactly on top of the previous one, look at this picture to see what the wavefronts would look like:

Sometimes we use the speed of sound as a reference to describe the speed of the object (aircraft in our discussion).

Mach Number

The Mach Number is the ratio of the speed of an object to the speed of sound in the surrounding medium.

Mach number is tells you how many times faster than sound the aircraft is moving.

• Mach Number $<$ 1 : aircraft moving slower than the speed of sound
• Mach Number $=$ 1 : aircraft moving at the speed of sound
• Mach Number $>$ 1 : aircraft moving faster than the speed of sound

To work out the Mach Number divide the speed of the aircraft by the speed of sound.

$\mathrm{Mach}\phantom{\rule{4pt}{0ex}}\mathrm{Number}=\frac{{\mathrm{v}}_{\mathrm{aircraft}}}{{\mathrm{v}}_{\mathrm{sound}}}$

Remember: the units must be the same before you divide.

If the aircraft is moving faster than the speed of sound then the wavefronts look like this:

If the source moves faster than the speed of sound, a cone of wave fronts is created. This is called a Mach cone. From constructive interference, we know that two peaks that add together form a larger peak. In a Mach cone many, many peaks add together to form a very large peak. This is a sound wave so the large peak is a very, very loud sound wave. This sounds like a huge "boom" and we call the noise a sonic boom .

Sonic Boom

A sonic boom is the sound heard by an observer as a shockwave passes.

An aircraft flies at 1300 ${\mathrm{km}·\mathrm{h}}^{-1}$ and the speed of sound in air is $340\phantom{\rule{4pt}{0ex}}{\mathrm{m}·\mathrm{s}}^{-1}$ . What is the Mach Number of the aircraft?

1. We know we are dealing with Mach Number. We are given the speed of sound in air, $340\phantom{\rule{4pt}{0ex}}{\mathrm{m}·\mathrm{s}}^{-1}$ , and the speed of the aircraft, 1300 ${\mathrm{km}·\mathrm{h}}^{-1}$ . The speed of the aircraft is in different units to the speed of sound so we need to convert the units:

$\begin{array}{ccc}\hfill 1300{\mathrm{km}·\mathrm{h}}^{-1}& =& 1300{\mathrm{km}·\mathrm{h}}^{-1}\hfill \\ \hfill 1300{\mathrm{km}·\mathrm{h}}^{-1}& =& 1300×\frac{1000\mathrm{m}}{3600\mathrm{s}}\hfill \\ \hfill 1300{\mathrm{km}·\mathrm{h}}^{-1}& =& 361.1\phantom{\rule{4pt}{0ex}}{\mathrm{m}·\mathrm{s}}^{-1}\hfill \end{array}$
2. We know that there is a relationship between the Mach Number, the speed of sound and the speed of the aircraft:

$\mathrm{Mach}\phantom{\rule{4pt}{0ex}}\mathrm{Number}=\frac{{\mathrm{v}}_{\mathrm{aircraft}}}{{\mathrm{v}}_{\mathrm{sound}}}$

We can use this relationship to find the Mach Number.

3. $\begin{array}{ccc}\hfill \mathrm{Mach}\phantom{\rule{4pt}{0ex}}\mathrm{Number}& =& \frac{{v}_{aircraft}}{{v}_{sound}}\hfill \\ \hfill \mathrm{Mach}\phantom{\rule{4pt}{0ex}}\mathrm{Number}& =& \frac{361.1}{340}\hfill \\ \hfill \mathrm{Mach}\phantom{\rule{4pt}{0ex}}\mathrm{Number}& =& 1.06\hfill \end{array}$

The Mach Number is 1.06.

#### Questions & Answers

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
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it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
Do somebody tell me a best nano engineering book for beginners?
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
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Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
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CYNTHIA
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Harper
Do you know which machine is used to that process?
s.
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for screen printed electrodes ?
SUYASH
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Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
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Cied
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what's the easiest and fastest way to the synthesize AgNP?
China
Cied
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I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
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Porter
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Yasmin
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Cesar
I'm interested in nanotube
Uday
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
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Prasenjit
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Azam
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Prasenjit
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Damian
silver nanoparticles could handle the job?
Damian
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Azam
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Uday
I'm interested in Nanotube
Uday
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Prasenjit
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