1
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2
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You've probably heard that the
top speed of some type of

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aircraft is Mach 2.

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Or maybe you've watched some
type of science fiction movie,

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or some type of fighter movie,
where they say, hey we are

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going to travel at Mach 1.8.

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And you had a sense, because
they're using it in the same

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context that you would use
as speed, that this is

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some type of speed.

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That these numbers represent
some type of speed.

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And if you assume that,
you were right.

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And the speeds they represent
are actually a multiple, or

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depending on how you want
to view it, multiples or

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fractions of the speed of
sound in that medium.

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So if someone tells you that
the Mach number is 2, what

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that tells you is that they are
traveling two times the

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speed of sound, in that medium,
for those conditions.

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And I make those last two
qualifiers on it because the

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speed of sound is not
always the same.

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It has a different speed
depending on whether it's

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traveling in air, or water, or
even depending on what makes

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up the air.

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And even if it is only traveling
in a certain type of

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air, or a certain makeup, a
mix of gases, it'll also

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change depending on
the temperature.

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So if you're at sea level, at
roughly about 20 degrees

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Celsius, the speed of sound is
300, so sea level-- do a

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different color, maybe blue for
sea level-- so if you're

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at sea level, the speed
of sound is about

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340 meters per second.

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This is at 20 degrees Celsius,
so it's a nice comfortable

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temperature at sea level, which
is roughly about 760

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miles per hour.

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Now if the temperature were to
drop, so if temperature goes

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down, so does the
speed of sound.

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And likewise, if the temperature
goes up, the speed

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of sound goes up.

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So when someone tells you that
something is traveling at Mach

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2, they're saying its traveling
at two times the

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speed of sound for
that medium.

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Usually they're talking
about something

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traveling through air.

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And for the speed of sound at
the temperature that they're

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going through.

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So if they're at some super-high
altitude, and

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normally to travel at these
types of speeds, you have to

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be at a fairly high altitude
where the air is less dense.

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You're also traveling in an
environment where the

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temperature is significantly
lower than 20 degrees Celsius.

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So you might say, hey, does
that mean if someone's

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traveling at Mach 2, does that
mean that they're definitely

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traveling at 680 meters
per second, or at

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1520 miles per hour?

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And of course, whenever someone
gives an air speed,

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it's relative to the air.

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And the answer is no.

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They're probably going to be
traveling a little bit slower

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than that, because at the
altitude that they're

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traveling the speed
of sound is lower.

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Now there are ground speed
records where they are

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traveling in kind of the Mach
range where their velocities

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are high enough to say
it in terms of Mach.

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And for those types of
situations, they are traveling

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at multiples of 760 miles per
hour, 340 meters per second.

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Now you're probably saying, hey,
who is this person right

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here that I've cut and pasted
from Wikipedia?

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And this Ernst Mach.

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And this is who the Mach
number is named after.

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Ernst Mach, and he was this
Austrian physicist and

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philosopher.

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He did a ton of research
on shockwaves and

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soundwaves, and whatnot.

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So the Mach number is
named after him.

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