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Why S-waves only travel in solids Video Lecture - Class 1
FAQs on Why S-waves only travel in solids
| 1. Why do S-waves only travel in solids? |  |
Ans. S-waves, also known as shear waves, can only travel through solid materials because they require the presence of shear stress to propagate. In solids, molecules are tightly packed and can vibrate in response to shear stress, allowing S-waves to travel. In contrast, liquids and gases lack the necessary shear strength to transmit these waves, resulting in the inability of S-waves to propagate through them.
| 2. Can S-waves travel through liquids? | |
Ans. No, S-waves cannot travel through liquids. Liquids do not possess the necessary shear strength required for the propagation of S-waves. Instead, liquids only support the transmission of P-waves, also known as primary waves, which propagate through compression and expansion of the material.
| 3. What is the difference between S-waves and P-waves? | |
Ans. The main difference between S-waves and P-waves lies in their mode of propagation. S-waves, or shear waves, propagate by causing particles to move perpendicular to the direction of wave travel. On the other hand, P-waves, or primary waves, propagate through compression and expansion of particles along the same direction as the wave travel. Additionally, S-waves are slower than P-waves and can only travel through solids, while P-waves can travel through solids, liquids, and gases.
| 4. Why are S-waves important in seismology? | |
Ans. S-waves play a crucial role in seismology as they provide valuable information about the nature of the Earth's interior. By analyzing the arrival times and amplitudes of S-waves recorded by seismic stations, scientists can determine the distance and location of seismic events, such as earthquakes. S-waves also help in identifying the composition and physical properties of different layers within the Earth's interior.
| 5. Are S-waves more destructive than P-waves in earthquakes? | |
Ans. In general, S-waves are considered to be more destructive than P-waves during earthquakes. This is because S-waves have a larger amplitude and move particles in a perpendicular motion, which can cause buildings and structures to sway and shake from side to side. On the other hand, P-waves, which move particles in a back-and-forth motion, generally cause less structural damage. However, the actual level of destruction caused by seismic waves depends on various factors, including the magnitude and depth of the earthquake, distance from the epicenter, and the local geological conditions.
Text Transcript from Video
In the last video I gave a
little bit of a hand wavy
explanation about why S-waves
don't travel in liquid or air.
What I want to do in this video
is give you a little bit more
intuitive understanding
of that, and really
go down to the molecular level.
So let's draw a solid.
And it has nice covalent
bonds, strong bonds
between the different molecules.
And the bonds are drawn
by these lines in between.
So if I were to
hit this solid, you
know I have this really
small hammer where I just
hit at a molecular
level, but if I
were to hit these molecules
hard enough so that they move
but not so hard enough
that it breaks the bonds,
then essentially what
it's going to look
like is this kind
of row of molecules
is going to move to the left.
So you're going to have
that row of molecules
moving to the left.
And then the row above it
won't fully move to the left
just yet, but it will
start to get pulled.
So let me just draw
all of the bonds.
I'm just drawing all
of the same bonds.
Because these are strong bonds
that we have in a solid--
Actually, they could
be ionic bonds as well.
Because they are strong bonds
that we have in this solid,
they'll essentially be pulled.
The top row will be
pulled in the direction
of the bottom row.
And so they'll start kind
of moving in that direction.
And then the bottom row will
essentially recoil back.
And then you fast
forward a little bit.
And so then the top row
will have moved to the left.
And now the bottom
row will start
to move back, especially
because, remember,
it's bonded to other
things down here.
It's bonded to more of
the solid down here.
So it would move back.
And you can see this
transverse wave,
you can see this
S-wave propagating.
Essentially right over here
the kind of peak of the S-wave
is here.
Now it has moved up.
Now, let's think about
the exact same situation
with the liquids.
In liquids you don't have these
strong ionic or covalent bonds
between the different molecules.
You just have these
weak kind of bonds,
usually formed due to polarity.
So in a liquid,
water's a good example,
you just have these kind
of weaker bonds formed
because water is
a polar molecule.
So the kind of
half-way polar sides
or the half-way positive
sides are somewhat
attracted to the
half-way negative sides.
So they kind of flow
past each other.
But if I were to hit these
water molecules right here
with my hammer,
what would happen?
Well, they're definitely going
to start moving to the left.
And actually, this one's going
to bump into that one, which
is going to bump
into that, which
is going to bump into that one.
They're going to
move to the left.
But these molecules aren't
going to move with them.
You could view it as
it's going to break
that very weak bond
due to polarity.
They're going to move
away from each other.
Let me draw these top
molecules in green.
They're essentially just
going to flow past each other.
They're going to
flow past each other.
And this guy might have had also
weak bonds with stuff below it,
too.
I should draw it
as dotted lines.
But because of the
impact here, these guys
are just going to flow.
They're actually going to
compress in this direction.
You're going to have a
P-wave, a compression wave,
go in this direction, where
this one bumps into that one,
and then goes back, and then
this one bumps into that one
and goes back, and then this
one bumps into that one.
But the bonds aren't
strong enough,
and it's even more
the case with air,
but the bonds aren't strong
enough for these blue guys
to take these green
guys for a ride.
And the bonds are
also not strong enough
for the adjacent
molecules to kind of help
these blue guys to retract
to their original position.
So when I talked about the
elasticity in the last video
that's what I was talking about.
The bonds aren't strong enough
to cause the things that
have deformed to kind of
move back to where they were,
and also the bonds
aren't strong enough
to allow the things
that are deformed
to pull other things with it.
And so that's why, in general,
S-waves only travel in solid,
and they won't travel
in liquid or air.
little bit of a hand wavy
explanation about why S-waves
don't travel in liquid or air.
What I want to do in this video
is give you a little bit more
intuitive understanding
of that, and really
go down to the molecular level.
So let's draw a solid.
And it has nice covalent
bonds, strong bonds
between the different molecules.
And the bonds are drawn
by these lines in between.
So if I were to
hit this solid, you
know I have this really
small hammer where I just
hit at a molecular
level, but if I
were to hit these molecules
hard enough so that they move
but not so hard enough
that it breaks the bonds,
then essentially what
it's going to look
like is this kind
of row of molecules
is going to move to the left.
So you're going to have
that row of molecules
moving to the left.
And then the row above it
won't fully move to the left
just yet, but it will
start to get pulled.
So let me just draw
all of the bonds.
I'm just drawing all
of the same bonds.
Because these are strong bonds
that we have in a solid--
Actually, they could
be ionic bonds as well.
Because they are strong bonds
that we have in this solid,
they'll essentially be pulled.
The top row will be
pulled in the direction
of the bottom row.
And so they'll start kind
of moving in that direction.
And then the bottom row will
essentially recoil back.
And then you fast
forward a little bit.
And so then the top row
will have moved to the left.
And now the bottom
row will start
to move back, especially
because, remember,
it's bonded to other
things down here.
It's bonded to more of
the solid down here.
So it would move back.
And you can see this
transverse wave,
you can see this
S-wave propagating.
Essentially right over here
the kind of peak of the S-wave
is here.
Now it has moved up.
Now, let's think about
the exact same situation
with the liquids.
In liquids you don't have these
strong ionic or covalent bonds
between the different molecules.
You just have these
weak kind of bonds,
usually formed due to polarity.
So in a liquid,
water's a good example,
you just have these kind
of weaker bonds formed
because water is
a polar molecule.
So the kind of
half-way polar sides
or the half-way positive
sides are somewhat
attracted to the
half-way negative sides.
So they kind of flow
past each other.
But if I were to hit these
water molecules right here
with my hammer,
what would happen?
Well, they're definitely going
to start moving to the left.
And actually, this one's going
to bump into that one, which
is going to bump
into that, which
is going to bump into that one.
They're going to
move to the left.
But these molecules aren't
going to move with them.
You could view it as
it's going to break
that very weak bond
due to polarity.
They're going to move
away from each other.
Let me draw these top
molecules in green.
They're essentially just
going to flow past each other.
They're going to
flow past each other.
And this guy might have had also
weak bonds with stuff below it,
too.
I should draw it
as dotted lines.
But because of the
impact here, these guys
are just going to flow.
They're actually going to
compress in this direction.
You're going to have a
P-wave, a compression wave,
go in this direction, where
this one bumps into that one,
and then goes back, and then
this one bumps into that one
and goes back, and then this
one bumps into that one.
But the bonds aren't
strong enough,
and it's even more
the case with air,
but the bonds aren't strong
enough for these blue guys
to take these green
guys for a ride.
And the bonds are
also not strong enough
for the adjacent
molecules to kind of help
these blue guys to retract
to their original position.
So when I talked about the
elasticity in the last video
that's what I was talking about.
The bonds aren't strong enough
to cause the things that
have deformed to kind of
move back to where they were,
and also the bonds
aren't strong enough
to allow the things
that are deformed
to pull other things with it.
And so that's why, in general,
S-waves only travel in solid,
and they won't travel
in liquid or air.
About this Video
Apr 20, 2026 Last updated
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