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every waking second of our lives we hear
sound waves everywhere from the hum of a
fan to the clicking of your keyboard
sound waves permeate our lives but how
do they travel throughout the world
around us to answer this question we are
going to engage in a thought experiment
okay so let's say we have a bar of Steel
that reaches the 384,000 kilometers
distance between the surface of the
earth and the surface of the moon
overhead let's be real that's a very
long bar that could never feasibly exist
but for the sake of this thought
experiment let's say it does and that we
are going to suspend all of these
factors
let's pose this question if we were to
raise the Earth's side of the bar by one
meter what would happen to the farm moon
end of the bar would it move the meter
instantly would there be a couple
seconds delay possibly an hour delay or
maybe much longer delay
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as it turns out the moon end of the bar
would move 1 meter 18 hours in 24
minutes after we moved to the earth end
of the bar this is a massive delay and
it is due to the fact that when we push
on the bar on earth we are not pushing
on the entire 384,000 kilometers long
bar at once but rather we are just
pushing on this localised group of atoms
here
these atoms at the earth end of the bar
push on their adjacent atoms and then
the next group of atoms up the bar and
so on all the way until this pushing
reaches the moon so this pushing can
also be thought as a bouncing of atoms
and it does not travel up the bar
instantaneously but rather it takes time
for one group of atoms to push or bounce
into the next group of atoms
specifically in steel this rate of
bouncing at reacting to the adjacent
atoms travels up the bar at a rate of
five point eight kilometers per second
this speed is considered the speed of
sound in steel and it varies depending
on the material if the bar were made of
oak the distance between the Earth and
the moon would be covered in just under
27 hours that's a long time but
eventually the motion would get there
the point is that although materials
like a steel bar or wooden table look to
be a coherent single object and thus one
would expect the entirety of the object
to move instantly all at once
in reality forces and movements take
time to travel throughout the many atoms
in an object this traveling of movement
or propagation can be thought of as a
chain reaction of atoms pushing or
bouncing into one another the speed of
this propagation is called the speed of
sound and it is dependent on the
material or medium that the motion
passes through it is also dependent on
the temperature of that particular
medium now let's get back to the thought
experiment consider what will happen if
we move the steel bar differently on
earth
let's move the bar up and then after 10
minutes we move the bar back down what
will happen on the moon well similar to
our previous experiment after 18 hours
the bar will move up and then 10 minutes
after that it will move down now let's
say we move the bar faster specifically
up and down a hundred times a second for
a full minute and then stop
well again this shakey like motion will
start at the earth end of the bar make
its way up the bar at a rate of five
point eight km/s and after the 18 hour
delay the moon end of the bar will move
in the exact same shaking motion for one
minute
in essence forces and motions in a
material do not catch up to one another
even if the forces are in the opposite
direction but rather once a motion is
started it continues up the bar at
constant speed the propagation of this
motion is called a wave in this law
experiment the steel bar represents the
air around us in the chain reaction of
bouncing atoms up the steel bar
represents the propagation of a sound
wave
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there are five key concepts shown in
this thought experiment that will help
you better understand sound waves and
waves in general the first is that sound
waves are a balancing of atoms or
particles into one another second waves
can travel very far
however the atoms themselves only travel
a short distance third waves travel at
the speed of sound and these speeds are
material dependent forth sequential
waves in the same medium travel at the
same speed and thus one wave does not
catch up with the following wave and
finally fit the coup de Gras of concepts
a sound wave isn't really an object like
a steel bar a wooden table or atoms of
air but rather a sound wave is a
propagation of force or motion when we
hear something we are in essence hearing
the motion then an object makes or how
kind of that object is applying a force
to the air particles around it thereby
causing those air particles to bounce
around and that bouncing is picked up by
your ears and perceived as sound kind of
fascinating now I hope you enjoyed this
branch episode please be sure to comment
subscribe like and tell your friends and
family about what you learned today
this episode is about how sound waves
travel and branches of this episode are
what are sound waves what are the
different types of ways how do your ears
perceive sound what our frequency of
loudness if sound is the movement of air
particles then what is wind isn't that
also the movement of air particles and
what is the structure of the smartphone
speaker thanks for watching and until
next time
consider the conceptual simplicity yet
structural complexity of the world
around us
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