September 12, 2019 • 33 mins
Find out how skipping stones works today with Daniel and Jorge
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Speaker 1 (00:08):
Hey, Daniel, what did it like to look at the
world through the eyes of a physicist? Oh, it's amazing.
It's red, there's green, and then there's blue. It's really
quite beautiful. I mean, like, are you always trying to
analyze and understand everything? Like it's the world just a
giant mess of puzzles and interesting phenomenon pretty much. It's
(00:29):
hard to turn that part of your brain off once
you've turned it on. I mean, don't you ever want
to just sit back, relax and just live in the
moment and experience things? Yeah? Every time I do that,
I think, how does that work? Living in a moment?
What do you relaxation? Anyway? I am handmade cartoonists and
(01:01):
the creator of PhD comics. Hi, I'm Daniel Whitson. I'm
a particle physicist and I'm incapable of fully relaxing. You
think that's a personal trade or is that just comes
with the job of being a physicist? Yeah? I can't
tell about the cause and effect there? Am I that
way because I became a physicist? Or did I become
a physicist because I am that way? Causation versus correlation?
(01:21):
That's right? But you know, sometimes you can do both.
Sometimes you can enjoy looking at the world and unraveling
its mysteries. You know. Uh, some people do puzzles for fun,
and for me, the universe is my puzzle. Well, hopefully
that's why people are listening in to this Welcome to
our podcast Daniel and Jorge Explain the Universe, a production
of My Heart Radio, in which we look at the
(01:42):
universe like a big puzzle and we try to take
it apart for you and hopefully entertain you along the
way with our terrible dad jokes and attain you and
also maybe ruin your vacations or your relaxation times, or
enhance your relaxation by showing you a whole other dimension
behind the simple plane of reality that everybody else experiences.
Do you feel like sometimes you are like a neo
(02:03):
in the matrix where you can see the code behind
everything around you. No, but I can totally do that
kick move. You can dodge bullets and look like Keanu Reeves. Hey,
I've never been shot. Okay, I can say that you
successfully dodge every bullet that's been fired at you, every
single bullet fired at anybody I successfully dodged. Oh wow,
(02:25):
that that's a lot of bullets of bullets. Yeah. So
today on the podcast will be tackling a phenomenon that
maybe hopefully a lot of people have done or tried
to do, or look at and think, hey, that's pretty cool. Yeah.
I think a lot more people have tried to do
it than successfully done it. Something you might do when
you're out for a relaxing stroll or maybe out on
vacation or just killing time or having some thoughts to yourself.
(02:49):
So to be on the program, we'll be talking about
physics of skipping stones. That's right. Why is that even possible?
Why is it that you can bounce a rock on water?
I mean it seems sort of crazy. If you had
never seen it, but somebody described it to you, you
would be pretty skeptical. Yeah, it's pretty cool. Um. And
(03:12):
this is you know, when you're walking by a lake
or a pond, or maybe just even like a fountain. Um,
you might have seen people or tried to do it yourself.
As you pick up a little stone, you throw it
and it skips along the surface of the water, sometimes
a lot. Yeah, what's your personal record? Jorge probably made
like ten skips and a stone. Maybe Ten's kind of
hard to count, right, isn't it hard to count? Especially
(03:35):
at the end there they get faster and faster. My
son likes to skip stones. I think his record is eight,
and mind is something more like five or six. Is
this a profession some people have, Like are their professional
stone skippers? There certainly are competitions where people fight voraciously
for the championship. I don't know if they get paid
for it, but they do pretty well. The current world
record for most number of skips is eight eight eight skips.
(03:59):
So you throw a rock at some water, a body
of water, and it skips eighty eight times. It's unbelievable, Like,
how do they even? They must have like a camera,
like the kind of used that really fancy tennis matches.
How far is that like when you throw it? Yeah,
it's actually interesting. In the US they have stone skipping
competitions and there they count the number of skips, whereas
(04:20):
if you go to sort of international world championships stone skipping,
they don't care about the number of skips. They care
about the distance and the furthest stone anybody's ever thrown
via skipping is a hundred and twenty one. It's longer
than a soccer field. It's like a seventh of a kilometer. Yeah,
it's huge, It's unbelievable. Um. So somebody has really figured
(04:43):
out how to do this thing. And you know, when
you're out there and you're skipping stones, you sort of
get a feeling for You're like, Okay, I need this
kind of stone. I gotta swing it this kind of way.
It's sort of like throwing a frisbee a little bit.
You sort of feel it out with your mind. This
is when it gets it in your wrist? Is it
in your you know, our movement? Yeah? About the water, yeah,
and about the perfect stone. And most people just sort
(05:05):
of figured out intuitively, right, they try this, they try that,
and eventually sort of get the hang of it and
they perfect it right, And that that's how most people
approach the world. Right. You learn how to walk, not
by like thinking about the physics of walking. You learn
how to ride a bike just by sort of trying
it out. Your rain gets trained to sort of learn
the physics intuitively. You don't like write down equations. Um,
(05:28):
but it is possible also to take this really fun
vacation activity and sort of ruin it by you know,
turning into an equation. No, it doesn't make you want
to appreciate it more, you know, wants you to understand
the physics behind it. Depends on what the answer is. Right.
If the answer is something oh hum, then you know,
maybe you've ruined the magic. But if the answer is
there's some crazy surprising physics in there, then yeah, then
(05:51):
you know, doing the physics has revealed something fascinating about
the universe. So we're in for some crazy surprising physics.
Turns out that the way most people think skipping stones
works is not the way it actually works. Well, I
think I've got the trick of it, but maybe we'll see.
We'll see what I'm doing corresponds to the physics of it.
All right, I'll give you a physics grade at the
end of it. All right, Well, it is kind of complicated,
(06:13):
and we were wondering how many people out there knew
what it takes, or how it works, or what's actually
happening when you skip a stone. So I walked around
the streets of Aspen, Colorado, a beautiful town in the
mountains with lots of stones and lots of little lakes
and ponds, so people could have plenty of chance to
practice their skipping and I asked, folks, do you know
(06:34):
why it's possible to skip a stone? Then most people respond, oh,
I don't skip stones. I just they're a gold bulls
skip stones, I get stoned. That's a different physics episode
probably somewhere and asking you can buy like a nine
dollar gold plated perfect skipping stone. So you went out
(06:56):
there and you ask people in the street if they
knew why is it possible to skip stones on a lake?
To think about it for a second and then listen
to what people had to say. Curface resistance tension should
be more friction gravity because it's hydroplinion, it's like tight tension.
Or if you' but if you put a rock on
the water, the supertension velocity as a flat surface, so
(07:20):
it's going to spin across the thought if it had
jagged edges, it was catching it pulled into the water,
gravity gravity would pull it down. Yeah, okay, I don't know.
All right, A lot of great answers here, a lot
of your favorite answers, right, gravity gravity, Yes, always works.
You can also say the Big Bang or physics, physics,
(07:45):
and that's usually how things work. Usually physics is the
way everything works. So yeah, that's a solid answer. I know,
we've got lots of people saying things like surface tension, right,
or hydroplaning or just jeez, I have no idea. Some
people had really never thought about it before. Or That's
one of my favorite moments is when I spark in
their minds this moment of curiosity, because then they always
(08:05):
turn around they ask me. They're like, well, what tell
me the answer? And now now you asked me, I
have to know. Yeah, a lot of answers like surface tensions,
the speed, the shape of the rock, what's what's going on? Yeah,
so based just on those answers, you might guess that
the answer is pretty complicated. There might be a lot
of things going on, right, some physics magic, some physics
not magic. And it turns out it is pretty complicated.
(08:27):
We should do magical physics. That can I say magical
physics physics ruining magic for hundreds of years, magic adjacent physics.
That's right. Well, the most popular answer that you heard
there is surface tension. And that makes some sense, right,
is it the surface tension? Turns out it's not surface tension. Right.
That's a tempting answer because you think, oh, the water
(08:47):
is sort of holding the surface together. There is some
resistance there and you may have even seen like bugs
skimming on the surface of water, or you can put
a paper clip down on water and it can sit
on the surface tension, right, So the a sense that
the water is doing something there to hold something up, right,
But it turns out service tension has nothing to do
with skipping stones. Maybe step us through a little bit.
(09:08):
What is surface tension? Yeah, service tension. It happens when
you have a liquid and that liquid is attracted to itself, right,
and so it wants to clump together. So for example,
if you put a drop of water on a table,
it doesn't just run totally flat, which is what you
would expect from a smoothly flowing liquid under gravity, right
gravity anything. It doesn't spread out, you know, out to
(09:30):
cover the whole table exactly. It stays together one one
molecule thick covering coding. But it stays as a droplet, right,
that's right. It stays as a droplet. And the reason
is that the molecules are attracted to each other. There's
some small force pulling the molecules to each other. And
we've talked on another episode of about how water is
a dielectric, and so it's neutral, right, the positive negative
(09:53):
charges balanced overall, but the positive negative charges aren't on
the same side of the water molecule, and so they
can line a in this way where the positive end
of one water molecule attracts the minus end of another one.
They can tug on each other a little bit. So
they like to stay together. They're like a little scared
crowd of children or something. They clump together in fear.
And it's like having a handful of magnets at a distance.
(10:16):
Maybe they are neutral to each other, but if you
get them closed, they'll they'll start to clump together. Yeah,
maybe a handful of magnets was a better analogy than
a clump of scared children. Probably where are they scared
because you're coming out them with a clump of of
of magnets Because I'm holding a rock right now. M Yeah,
so it clumps together. That's what surface tension is, is
(10:38):
the water clumping together um and so it um And
that happens also on the surface, right, not just inside,
but also on the surface. It's kind of like why
in space, if you see a blob of water, it
doesn't just spread out like a gas, it stays as
a blob of water. Yeah, exactly, because it's attracted to itself.
So that causes surface tension. Right, that's what surface tension.
And wait, what is it? What is it called surface tension?
(11:00):
Like what's happening at the surface. It's called surface tension
because at the surface, if you poke it, it's it
sort of holds together, like it creates this sort of
sheet at the surface that resists being torn apart or penetrated.
It's the same way that like the wall holds itself together. Right,
you have bonds between the molecules that are holding the
wall together. These bonds are much much weaker, which is
why water in this case is a fluid. Right, it's
(11:22):
just a It's just a weaker version of those same
bonds that hold all matter together, right, right, But there's
something special about the surface, isn't it? Like the isn't
it or maybe I'm wrong, is that the molecules on
the surface are sort of holding on a little bit
tighter to each other than the ones like inside of
the water could be you tell me you're the engineer.
They were completely lost. Now. But the key thing when
(11:46):
it comes to understanding skipping stones. Is you know, could
you use surface tension to skip a stone, because you can.
You can use it to float a bug, right, Like
some bugs stand on water because the water refus uses
to sort of be broken apart. That's right. Yeah, the
water holds itself together and creates a surface, right. Um,
(12:06):
But that only really works for very, very low weight
objects because the surface tension is not very strong. I mean,
you touch a drop of water and it clings to
you instead, right, It's very easy to break surface tension.
And if you just if it was just surface tension,
then you could be able to take a rock and
put it on water and it would float there, right.
But a rock that you could easily skip, if you
put it on the surface of the water, it would
(12:27):
just sink through. Some people might be wondering, is it
the surface tension that's creating some kind of trampoline on
the water that's maybe causing the stone to skip exactly.
It's kind of that was kind of the scenario we're
trying to debunk. Yeah, that's definitely not happening, because if
you put a stone on a trampoline, right, it doesn't
fall through the trampoline, right, So you know, the trampoline
(12:47):
is strong enough to bounce the stone back up. But
if you put a stone just lay it sort of
gently on the surface of the water, you know what's
gonna happen. It's not gonna sit there. It's gonna sink
unless your stone weighs the same as a paper clip, right,
which case you probably can't it. But anyway, but a
stone that you could skip won't sit gently on the water.
Um So service tension is not enough to provide sort
of trampoline effect there, even if it's going fast. I
(13:10):
could maybe imagine somebody thinking you throw a stone so
fast at a body of water that it actually kind
of bounces back from the hitting that service tension. It
does bounce back, but not because of the service tension, right.
And you can see this because you can skip stones
or skip objects on things without as much surface tension.
For example, you can even skip things off like the atmosphere.
(13:32):
You know, sometimes when spacecraft are trying to re enter
the atmosphere, there's an angle they have to enter it
where if they go too steep then they heat up
too much, but if they go too shallow, then they
skip off the atmosphere back into space. Oh you can
skip spaceships. Yeah, you can skip spaceships. I don't think
it's a good idea. I don't think anybody's ever done
it on purpose. I think it's a is there a
(13:53):
world record for that one or a competition? Yeah one,
And that guy bounced out into space and nobody's ever
seen him before. They said, they said the diploma, but
they didn't get it. You can also skip on on
surfaces that have no tension at all, Like you can
skip rocks on sand dunes, right, and sand doesn't have
any attraction to itself. There's no surface tension on sand. Dudes,
(14:14):
you don't get drops of sand, right, So you see
the skipping affected area in places with more or less
surface tension. And the bottom line, though, is the service
tension is just super weak and it can't contribute at all.
All right, So it's not surface tension that's helping you
or helping us skip stone. So there must be other things.
So let's get into it. But first let's take a
(14:35):
quick break. All right. We're talking about why is it
possible to skip stones on a lake, Like what's the
physics of it? And so we talked about how it's
(14:57):
not surface tension which is what a lot of people gets. Um,
And so maybe, uh, it might be something else. So
I would guess it maybe has something to do with
the spin that you put into the stone. Is that
kind of what's happening. It feels like a good answer, right,
because when you're skipping stones, you notice that if you
spin them really fast, they skip better. Right. We will
(15:17):
talk about it in a moment how the stone spinning
actually does help it skip, but it's not actually necessary.
I mean the spin what it does is it helps
make it fly straight, sort of like a frisbee, so
that it like it hits the water at a good
angle and in a good direction and all that stuff.
But it's not actually necessary. You can skip a stone
without spinning. It's possible. I always thought that it was
(15:38):
something to do with like conservation of angular momentum. You know,
like if you spin it, it wants to keep spinning
and that keeps it kind of level, which then kind
of makes it graze the surface of the water. Yeah,
I mean you're getting there. It's not actually necessary for spinning,
but you're right, it keeps it sort of flat, right, Um,
just the way a frisbee flies better when it's spinning
(15:58):
for the same reason as you say, angular momentum. It
doesn't like to tip over or it takes more of
a force to tip it over. Um. And it turns
out that you skip best when you hit the water
at just the right angle, and so spinning the stone
does help it hit at that right at that angle
that the good angle for for skipping. But it's not
actually necessary. You can skip a stone without spinning it.
(16:19):
It is possible. While you're blowing my mind here, you
can skip stones on sand and you can do it
without spinning. Yeah, yeah, exactly. Skipping turns out to be
quite the fascinating physics topic. Um. Alright, so then if
it's not surface tension or spinning, what's, um, what's happening
when we skip stones? Well, this is actually really interesting.
(16:40):
And it turns out there's a physicist about twenty years
ago whose son asked him, Hey, Dad, how is it
that you can skip stones? What's the physics of it?
And he was like, oh, I'm sure it's pretty simple,
And he started digging into it, and turns out it's
pretty complicated, and he spent months studying this and doing experiments,
and he finally came up with an equation, an equation
that predict how many skips you'll get from a stone
(17:03):
as a function of the angle of the stone, the
velocity of the stone, and the speed of skipping, the
weight of the stone, all this stuff, And so he
was able to isolate sort of what are the important
factors in getting a stone to skip? So wait, his
son asked him a question, like hey, Dad, and then
three years later he's like, here it is. Here's a
(17:24):
paper full of equations. That's what you wanted, right, I
just wanted yes or no. After that, the kid probably
totally forgot about it. He's like, oh, I don't care whatever, Um,
that's what it's like to have businesses for a parent.
The kid is still waiting by the lake with the stone,
and Dad, show me how you just gotta finish this equation.
(17:44):
Did he figure this out from like equations or was
this all experimental like I'm gonna change try different stone
sizes and different angles and different velocities, or a little
bit of book. It was a little bit of both.
He started out theoretically. He was like, can I understand
the forces involved and what we do all the time
in physics, which is critical? Is he simplified the problem.
He's like, Okay, let me assume the stone is perfectly flat.
(18:07):
Let me assume assume it's a perfect circle. Let me
assume it hits the water at this angle or whatever.
He made some assumptions to simplify the problem, and then
he was able to write down equations that he thought
would describe the forces involved and that let him make predictions.
He's like, okay, if these are the forces involved, right,
and he knows how the forces change is a function
of the angle you throw it in the speed, then
(18:29):
I can make predictions for how many times the stone
will skip. Then he went out and checked it and
he actually did some experiments to see if his equations
were valid. Yeah, exactly. I want to see what that
peer review process was like. You know, did somebody go
out there with a bunch of stones and his paper
and try this thing out? Um, I'm not quite sure,
but he published the paper. It's a guy from the
(18:50):
University of Lyon in France. And in his paper he
suggests that the critical variables are, of course, the flatness
of the stone, the viscas city of the fluid, right,
the viscosity of the liquid, which is not the same
thing as surface tension, the angle of the stone, and
its speed, So neither none of those things involved surface
(19:11):
tension or spinning. Yeah, and spinning again helps because it
helps you get the right angle right, and it's also
easier to throw throw a rock really fast if you're
spinning it sort of out the tip of your finger
as you throw it. So spinning is sort of like
a supplemental thing. It's not necessary, but it's helpful. I'm
waiting for the twist in the story where you say,
and then he became the world record horbholder. So in
(19:32):
the end, it's not really surface tension. It's much more
about like the viscosity of the water, right, because you know,
think about like meaning like the thickness of it, or
how goopy it is. Yeah, how goopy it is. Right,
water not only is it attracted to itself, which caused
the surface tension. It's a fairly fairly small force, but
it's also kind of viscous, right, It's like, you know,
(19:52):
there's a bit of goopiness to it, like honey is
goopier of course in water, right, And this comes from
how the molecules would rub each other, like the friction
the molecule to molecule friction, So it's a different physics, right.
Surface tension is about attraction of the molecules. This is
about how the molecules flow past each other. If something
is really viscous, like tar, or like a giant vat
(20:14):
of honey, like I would imagine if I throw us
kipping stone on top, which is kind of blurb kind
of you know, when it's skip or would it. I
don't know, that's a good question, I think. Um. I mean, obviously,
if it's really viscous, then it's a flat surface, then
it will skip, right, So there's probably some point duper
super duper but not sticky, yeah, exactly. And you know,
(20:37):
you can come up with examples of liquids that are
really viscous but don't have a lot of surface tension,
or the other way around. So there's a complicated relationship
between surface tension and viscosity. But in the end, what
you need is viscosity. And the reason is that when
the stone hits the water, you want the water to
not be able to sort of flow out of the
way fast enough, right, like you want the water to
(20:58):
push back, Yes, exactly, you need the water to push
back and so like when you jump off a diving
board and you land totally flat on the water, right,
If you've done that, you know that water can feel
like concrete, right, like belly flop, Like a belly flop, yeah,
a belly flop exactly. If you fall off a bridge,
for example, from high enough, you can die landing in water, right,
because water just can't get out of the way fast enough,
(21:20):
and so it pushes back on you just sort of
the way the ground does right when you land on it.
It doesn't get out of the way fast enough. Yeah,
Like if you're falling through air, Air is much much
less viscous than water, and it gets out of the way, right,
It just moves out of the way. I mean, there's
still some friction there, which is why you have a
terminal velocity, but it gets out of the way fast
enough for you to pass through it, right. But water
is much much more viscous, so it provides much much
(21:43):
more drag. And if you hit the water flat, then
you're gonna get a big force the other direction, um,
which is gonna make your belly smart or make your
rock skip. So that's the key, right, is that the
water pushes back up on the rock. You can't skip
rocks on clouds or air. You can skip rocks on air, actually,
I mean you can skip spaceships on air, but it
(22:03):
takes much much higher speed. Yeah, well this is a
perfect point to take a break. So it's about the
viscosity of the liquid. So the more viscous it is,
(22:25):
the easier it is to bounce it. Yes, you need
a flat object and a viscous liquid. And that's a
flat object, because if it's not flat, it's not flat.
Like I mean, the most extreme thing is you drop,
like you know, drop something really thin and sharp into
the water, it's just going to separate the water and
sink down to the bottom. Right, So you need to
drop something flat so that it into the water can't
(22:46):
move out of the way. You need to belly flop, Yes, exactly.
Skipping stones should be called belly flopping stones, don't. They
have funny names for this in other countries, like what
they call in England. Um, in England they call it
ducks and drakes, which makes absolutely no sense to me, Like,
are you shooting ducks? Like what's going on? Well, like,
if you're you're throwing stones at a leg to skip
(23:07):
to create those ripples and skips. That that's called ducks
and drakes. Ducks and drake's. Yeah, somebody out there in
England explain that to us. In France they have a
clever name for it. They call it rick o'che, which
you know makes some sense, right, your rock is ricocheting
off the surface of the water. What are some other
funny words? Well, these are words that don't even make
sense to me. Like in Ireland they call it stone scuffing,
the way you would scuff your shoes and get marks
(23:28):
on them, or something you got to say with an
Irish saxon. I can't do an Irish accent, not at all.
But every language has a word for it. And the
funniest one is that according to the Oxford English Dictionary, Right,
so according to the Brits, the way Americans say it
is they say it's called dapping, and that's not a
word I've ever heard. So, like, what are the Brits
(23:48):
doing over there telling us what we call skipping stones
and they're getting totally wrong. Well, it's all just ducks
and drakes to them, So they got no credibility over there?
Is that where the dap comes from? Where am I
thinking of the dead. I am not culturally relevant enough
to answer that question. But it's more than just the viscosity, right,
and more than just having silly name. Okay, so it's
(24:09):
more than just having um substance to the thing you're
trying to skip on, which which water does it has
enough fixed causity to skip stone? That's right? Because you
know when you jump off a diving board and belly flop,
you don't skip right, So there's a difference between belly
flopping and skipping. And then what else is going on here?
So another key is the angle. Right when you throw
(24:29):
the stone, you want it the front edge of the
stone to be higher than the back edge of the stone.
So you want the stone when it hits the water
for the trailing edge to hit first, all right, And
the reason is that you're pushing down on the water
and if you have that like a like a wheelie,
like you're you're tilted back. Yes, exactly, exactly, Though I
(24:51):
do not recommend trying to do this with your motorcycle
or your mountain biketycle, or do it and send us
a video. That would be awesome. I think that's called
geezs and gooses in England. I think you're probably right.
And um, what happens when you do that is then
you push down the water at an angle, and the
water creates kind of a ramp, right because you've you've
(25:14):
pushed it down to the back end is further down
than the front end, and so you have a ramp
that slopes up. Okay, and here's where the speed comes.
You've got to be moving fast. So you push down
on the water to create a ramp, and then that
ramp launches you back up into the air at the
same angle. You're not really bouncing then, it's more like
you're going into the water and then creating a ramp
(25:34):
for you to take off of. Yeah, well you're doing
both at the same time. One of my favorite things
about sort of projectile motion in physics is how you
can break things up into two dimensions that are totally independent. Right,
motion in one dimension is independent from motion in the
other dimension. Somebody in the question intersection said that the
reason that you can skip a rocket is because it's
moving really fast, um, in one direction, and so it
(25:56):
doesn't fall down, which is totally not true. Right, Like
you shoot the bullet, you know, it is moving really
really fast parallel to the surface of the earth. But
it's also at the same time falling and falling towards
the surface of the earth is not affected by your
motion parallel to the surface. And so you've got two
things going on here. One is you're bouncing perpendicular to
the surface of the water, right, but you're also moving parallel,
(26:19):
So you've got the belly flop in the perpendicular direction,
and you've got this little ramp action in the parallel direction. Well,
what happens if if you don't hit the water at
a good angle, like let's say your head, you can
hit the water kind of leaning forward instead of leaning back,
then you end up the front edge of your rock
will dig into the water and it'll slow it down
(26:39):
and it'll flip, and then you don't don't then you
don't get any more skips. You don't create this little ramp. Yeah,
because remember the key to getting a lot of skips
is doing the same thing over and over again. So
you want that when you leave the water, you're at
the same angle as when you hit the water, right,
so that you can hit the water again at that
same angle. So anything that flips you over start to spinning,
(27:00):
you're losing your energy and you're not gonna hit the
water at the right angle the next skip. Well, I
would have thought the spin was really important because without
the spin, wouldn't you lose that angle really quickly with
all the water slushing around. Yeah, spinning is important, and
nobody wins the stone Skipping World Championship without a really
good spin. But it's not technically necessary. But yeah, it's
really important to maintaining the angle and to maintaining your speed.
(27:24):
So there's a lot of different things going on when
just one simple rock hits the water, all right, and
then um so viscosity angle, and then what's the last
critical ingredient? Last critical ingredient is speed? Right? If you
are not moving fast enough, then you sink when you
hit the water because the ramps are dissolves, right, that
ramps is not doesn't stick around for very long. You
hit the water right, and you bounce back because the
(27:46):
water doesn't have a lot of time to move out
of the way. But um, you want to get off
of that ramp as soon as you can, because just
pass the ramp as a little divid right, and so
you want to get off of that ramp, and so
you need enough speed. You need to me moving faster
than the water is, of course moving faster horizontally or
does it help to go fast a little bit, you know,
(28:08):
into the water or do you get that anyways, you
need both. You need if you if you're not moving
fast enough perpendicular to the water, then you won't get
the bounce right. You need to be hitting the water
with enough speed that the water can't get out of
the way and pushes back at you. You also need
horizontal velocity so you can get off that ramp before
it dissolves. But if you're going to slow then you know,
(28:29):
you push the water into a ramp, but then you
you don't take off, You just kind of move exactly.
That's exactly the sound it makes. Why you must have
done these experiments. That's what it sounds like in every
slow motion video clip I've seen on YouTube. But every
time you bounce, you also do lose energy. It's impossible
not to because this friction and so you hit the
(28:52):
water and it slows you down a little bit. So
every bounce is in the best case scenario, like of
the neck of the previous ounce, And so that's why
you know, they get faster and faster. The time between
bounces it gets smaller and smaller because you're not going
as fast, you're not going as far up, so it
doesn't take as much time to come back down. So
that's pretty good. Like you only lose in each skip
(29:15):
or is that like a you know, is that is
that what the pros do? Or is that what happens
when anyone skip stones? Is that it's always now that's
the like top level performance, like eighty yeah, And that's
the key to getting a lot of skips is losing
the smallest amount of energy every skip, which means hitting
the water at just the right angle. And it's one
(29:37):
of these things where it's very sensitive. You're slightly off
of the optimal angle, you're gonna lose a lot more
energy in the first skip, and then you're gonna be
off on the next angle and the next angle, and
pretty soon you know you're sinking to the bottom of
the lake. That's pretty impressive. So that's how they get
to like a hundred yeah, exactly eight skips, which still
blows my mind. It feels like that would take like
(29:57):
an hour, you know, Like you throw the rock and
then you're just like and minutes later, still scount counting
skips or something. Well, towards the end, it's it's it's
not like skips skips skip. It's like, yeah, exactly, it's
like stuttering skips at the end, which is what tells
me what these really like massively competitive events. They must
have some sort of camera that's taking pictures to count
these skips. They can't have like a super spot or
(30:19):
something just using their eyeballs. You think it's at that
high tech. You think they have like corporate sponsors and
pro athletes, probably some Swiss watch companies you know, sponsoring it, etcetera. Yeah,
but the top guys have groupies and travel around and
fancy busses and all sorts of stuff. All right, So
then have you tried this? Since you're learning the secrets here,
have you tried and does it help? Did it help
(30:40):
you skip stones better? No? And that's the thing about physics.
It tells you how the universe works, It gives you inside,
but it doesn't help you actually navigate the situation. You know.
For example, we did that whole podcast episode about how
to ride a bike, right that just made me ride
bikes any better, especially because we learned that physics doesn't
know why why bicycles. Allan's right, but you can still
(31:01):
ride a bike. So there's this disconnect between understanding something
and being able to do it. Well, you know, it
did help me. I mean after learning all of this,
I did try it, and it does help. Like if
you know that you have to throw it at the
water kind of at a little bit of an angle
leaning bag that's a huge hill, like because you know
before you I would try to like throw it at
(31:23):
the water so it's perfectly parallel to the water. I
thought that was the secret, you know, but like once
you know you have to kind of lean it back
a little bit. It does help you skip better. All right,
well you go, I skips, but you know, physics has
improved your life for her for the first time, it
ruined my vacation, but it increase my standing in the
world rankings of It's amazing to me that the universe
(31:44):
is mathematical, that we can understand it at all, that
it seemed to follow these rules and we can write
down on paper manipulate with our minds um. It's amazing
that that works, and it's a joy to see what
it happens. You know that we can actually isolate the
critical elements and understand something. So the next time you
see something weird, the next time you see something you
don't understand, you know, think about the physics of it,
what's making that happen? Um, what's going on inside is
(32:07):
there's some microscopic explanation that can help you understand why
those ducks are chasing that funny bird or you know
why those clouds form in that weird way. And there's
physics all around us. And if you don't understand, just
send the question to Daniel and we'll answer it on
the podcast that's right before after I do a bunch
of experiments at Jorge's house skipping gold coins on my Hey,
(32:29):
can I borrow a few of those gold coins please?
I gotta do some experiments. Whoops, they sank to the
bottom of the lake. Sorry about that? All right? Well,
we hope you enjoyed Dad, And as Neo says, whoa,
thanks for tuning in. See you next time. Before you
(32:52):
still have a question after listening to all these explanations,
please drop us a line. We'd love to hear from you.
You can find us on Facebook, Twitter, and Instagram at
Daniel and Jorge that's one word, or email us at
feedback at Daniel and Jorge dot com. Thanks for listening
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio. For more podcast from
(33:14):
my heart Radio, visit the i heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. Yeah.
Hey, Daniel, what did it like to look at the
world through the eyes of a physicist? Oh, it's amazing.
It's red, there's green, and then there's blue. It's really
quite beautiful. I mean, like, are you always trying to
analyze and understand everything? Like it's the world just a
giant mess of puzzles and interesting phenomenon pretty much. It's
(00:29):
hard to turn that part of your brain off once
you've turned it on. I mean, don't you ever want
to just sit back, relax and just live in the
moment and experience things? Yeah? Every time I do that,
I think, how does that work? Living in a moment?
What do you relaxation? Anyway? I am handmade cartoonists and
(01:01):
the creator of PhD comics. Hi, I'm Daniel Whitson. I'm
a particle physicist and I'm incapable of fully relaxing. You
think that's a personal trade or is that just comes
with the job of being a physicist? Yeah? I can't
tell about the cause and effect there? Am I that
way because I became a physicist? Or did I become
a physicist because I am that way? Causation versus correlation?
(01:21):
That's right? But you know, sometimes you can do both.
Sometimes you can enjoy looking at the world and unraveling
its mysteries. You know. Uh, some people do puzzles for fun,
and for me, the universe is my puzzle. Well, hopefully
that's why people are listening in to this Welcome to
our podcast Daniel and Jorge Explain the Universe, a production
of My Heart Radio, in which we look at the
(01:42):
universe like a big puzzle and we try to take
it apart for you and hopefully entertain you along the
way with our terrible dad jokes and attain you and
also maybe ruin your vacations or your relaxation times, or
enhance your relaxation by showing you a whole other dimension
behind the simple plane of reality that everybody else experiences.
Do you feel like sometimes you are like a neo
(02:03):
in the matrix where you can see the code behind
everything around you. No, but I can totally do that
kick move. You can dodge bullets and look like Keanu Reeves. Hey,
I've never been shot. Okay, I can say that you
successfully dodge every bullet that's been fired at you, every
single bullet fired at anybody I successfully dodged. Oh wow,
(02:25):
that that's a lot of bullets of bullets. Yeah. So
today on the podcast will be tackling a phenomenon that
maybe hopefully a lot of people have done or tried
to do, or look at and think, hey, that's pretty cool. Yeah.
I think a lot more people have tried to do
it than successfully done it. Something you might do when
you're out for a relaxing stroll or maybe out on
vacation or just killing time or having some thoughts to yourself.
(02:49):
So to be on the program, we'll be talking about
physics of skipping stones. That's right. Why is that even possible?
Why is it that you can bounce a rock on water?
I mean it seems sort of crazy. If you had
never seen it, but somebody described it to you, you
would be pretty skeptical. Yeah, it's pretty cool. Um. And
(03:12):
this is you know, when you're walking by a lake
or a pond, or maybe just even like a fountain. Um,
you might have seen people or tried to do it yourself.
As you pick up a little stone, you throw it
and it skips along the surface of the water, sometimes
a lot. Yeah, what's your personal record? Jorge probably made
like ten skips and a stone. Maybe Ten's kind of
hard to count, right, isn't it hard to count? Especially
(03:35):
at the end there they get faster and faster. My
son likes to skip stones. I think his record is eight,
and mind is something more like five or six. Is
this a profession some people have, Like are their professional
stone skippers? There certainly are competitions where people fight voraciously
for the championship. I don't know if they get paid
for it, but they do pretty well. The current world
record for most number of skips is eight eight eight skips.
(03:59):
So you throw a rock at some water, a body
of water, and it skips eighty eight times. It's unbelievable, Like,
how do they even? They must have like a camera,
like the kind of used that really fancy tennis matches.
How far is that like when you throw it? Yeah,
it's actually interesting. In the US they have stone skipping
competitions and there they count the number of skips, whereas
(04:20):
if you go to sort of international world championships stone skipping,
they don't care about the number of skips. They care
about the distance and the furthest stone anybody's ever thrown
via skipping is a hundred and twenty one. It's longer
than a soccer field. It's like a seventh of a kilometer. Yeah,
it's huge, It's unbelievable. Um. So somebody has really figured
(04:43):
out how to do this thing. And you know, when
you're out there and you're skipping stones, you sort of
get a feeling for You're like, Okay, I need this
kind of stone. I gotta swing it this kind of way.
It's sort of like throwing a frisbee a little bit.
You sort of feel it out with your mind. This
is when it gets it in your wrist? Is it
in your you know, our movement? Yeah? About the water, yeah,
and about the perfect stone. And most people just sort
(05:05):
of figured out intuitively, right, they try this, they try that,
and eventually sort of get the hang of it and
they perfect it right, And that that's how most people
approach the world. Right. You learn how to walk, not
by like thinking about the physics of walking. You learn
how to ride a bike just by sort of trying
it out. Your rain gets trained to sort of learn
the physics intuitively. You don't like write down equations. Um,
(05:28):
but it is possible also to take this really fun
vacation activity and sort of ruin it by you know,
turning into an equation. No, it doesn't make you want
to appreciate it more, you know, wants you to understand
the physics behind it. Depends on what the answer is. Right.
If the answer is something oh hum, then you know,
maybe you've ruined the magic. But if the answer is
there's some crazy surprising physics in there, then yeah, then
(05:51):
you know, doing the physics has revealed something fascinating about
the universe. So we're in for some crazy surprising physics.
Turns out that the way most people think skipping stones
works is not the way it actually works. Well, I
think I've got the trick of it, but maybe we'll see.
We'll see what I'm doing corresponds to the physics of it.
All right, I'll give you a physics grade at the
end of it. All right, Well, it is kind of complicated,
(06:13):
and we were wondering how many people out there knew
what it takes, or how it works, or what's actually
happening when you skip a stone. So I walked around
the streets of Aspen, Colorado, a beautiful town in the
mountains with lots of stones and lots of little lakes
and ponds, so people could have plenty of chance to
practice their skipping and I asked, folks, do you know
(06:34):
why it's possible to skip a stone? Then most people respond, oh,
I don't skip stones. I just they're a gold bulls
skip stones, I get stoned. That's a different physics episode
probably somewhere and asking you can buy like a nine
dollar gold plated perfect skipping stone. So you went out
(06:56):
there and you ask people in the street if they
knew why is it possible to skip stones on a lake?
To think about it for a second and then listen
to what people had to say. Curface resistance tension should
be more friction gravity because it's hydroplinion, it's like tight tension.
Or if you' but if you put a rock on
the water, the supertension velocity as a flat surface, so
(07:20):
it's going to spin across the thought if it had
jagged edges, it was catching it pulled into the water,
gravity gravity would pull it down. Yeah, okay, I don't know.
All right, A lot of great answers here, a lot
of your favorite answers, right, gravity gravity, Yes, always works.
You can also say the Big Bang or physics, physics,
(07:45):
and that's usually how things work. Usually physics is the
way everything works. So yeah, that's a solid answer. I know,
we've got lots of people saying things like surface tension, right,
or hydroplaning or just jeez, I have no idea. Some
people had really never thought about it before. Or That's
one of my favorite moments is when I spark in
their minds this moment of curiosity, because then they always
(08:05):
turn around they ask me. They're like, well, what tell
me the answer? And now now you asked me, I
have to know. Yeah, a lot of answers like surface tensions,
the speed, the shape of the rock, what's what's going on? Yeah,
so based just on those answers, you might guess that
the answer is pretty complicated. There might be a lot
of things going on, right, some physics magic, some physics
not magic. And it turns out it is pretty complicated.
(08:27):
We should do magical physics. That can I say magical
physics physics ruining magic for hundreds of years, magic adjacent physics.
That's right. Well, the most popular answer that you heard
there is surface tension. And that makes some sense, right,
is it the surface tension? Turns out it's not surface tension. Right.
That's a tempting answer because you think, oh, the water
(08:47):
is sort of holding the surface together. There is some
resistance there and you may have even seen like bugs
skimming on the surface of water, or you can put
a paper clip down on water and it can sit
on the surface tension, right, So the a sense that
the water is doing something there to hold something up, right,
But it turns out service tension has nothing to do
with skipping stones. Maybe step us through a little bit.
(09:08):
What is surface tension? Yeah, service tension. It happens when
you have a liquid and that liquid is attracted to itself, right,
and so it wants to clump together. So for example,
if you put a drop of water on a table,
it doesn't just run totally flat, which is what you
would expect from a smoothly flowing liquid under gravity, right
gravity anything. It doesn't spread out, you know, out to
(09:30):
cover the whole table exactly. It stays together one one
molecule thick covering coding. But it stays as a droplet, right,
that's right. It stays as a droplet. And the reason
is that the molecules are attracted to each other. There's
some small force pulling the molecules to each other. And
we've talked on another episode of about how water is
a dielectric, and so it's neutral, right, the positive negative
(09:53):
charges balanced overall, but the positive negative charges aren't on
the same side of the water molecule, and so they
can line a in this way where the positive end
of one water molecule attracts the minus end of another one.
They can tug on each other a little bit. So
they like to stay together. They're like a little scared
crowd of children or something. They clump together in fear.
And it's like having a handful of magnets at a distance.
(10:16):
Maybe they are neutral to each other, but if you
get them closed, they'll they'll start to clump together. Yeah,
maybe a handful of magnets was a better analogy than
a clump of scared children. Probably where are they scared
because you're coming out them with a clump of of
of magnets Because I'm holding a rock right now. M Yeah,
so it clumps together. That's what surface tension is, is
(10:38):
the water clumping together um and so it um And
that happens also on the surface, right, not just inside,
but also on the surface. It's kind of like why
in space, if you see a blob of water, it
doesn't just spread out like a gas, it stays as
a blob of water. Yeah, exactly, because it's attracted to itself.
So that causes surface tension. Right, that's what surface tension.
And wait, what is it? What is it called surface tension?
(11:00):
Like what's happening at the surface. It's called surface tension
because at the surface, if you poke it, it's it
sort of holds together, like it creates this sort of
sheet at the surface that resists being torn apart or penetrated.
It's the same way that like the wall holds itself together. Right,
you have bonds between the molecules that are holding the
wall together. These bonds are much much weaker, which is
why water in this case is a fluid. Right, it's
(11:22):
just a It's just a weaker version of those same
bonds that hold all matter together, right, right, But there's
something special about the surface, isn't it? Like the isn't
it or maybe I'm wrong, is that the molecules on
the surface are sort of holding on a little bit
tighter to each other than the ones like inside of
the water could be you tell me you're the engineer.
They were completely lost. Now. But the key thing when
(11:46):
it comes to understanding skipping stones. Is you know, could
you use surface tension to skip a stone, because you can.
You can use it to float a bug, right, Like
some bugs stand on water because the water refus uses
to sort of be broken apart. That's right. Yeah, the
water holds itself together and creates a surface, right. Um,
(12:06):
But that only really works for very, very low weight
objects because the surface tension is not very strong. I mean,
you touch a drop of water and it clings to
you instead, right, It's very easy to break surface tension.
And if you just if it was just surface tension,
then you could be able to take a rock and
put it on water and it would float there, right.
But a rock that you could easily skip, if you
put it on the surface of the water, it would
(12:27):
just sink through. Some people might be wondering, is it
the surface tension that's creating some kind of trampoline on
the water that's maybe causing the stone to skip exactly.
It's kind of that was kind of the scenario we're
trying to debunk. Yeah, that's definitely not happening, because if
you put a stone on a trampoline, right, it doesn't
fall through the trampoline, right, So you know, the trampoline
(12:47):
is strong enough to bounce the stone back up. But
if you put a stone just lay it sort of
gently on the surface of the water, you know what's
gonna happen. It's not gonna sit there. It's gonna sink
unless your stone weighs the same as a paper clip, right,
which case you probably can't it. But anyway, but a
stone that you could skip won't sit gently on the water.
Um So service tension is not enough to provide sort
of trampoline effect there, even if it's going fast. I
(13:10):
could maybe imagine somebody thinking you throw a stone so
fast at a body of water that it actually kind
of bounces back from the hitting that service tension. It
does bounce back, but not because of the service tension, right.
And you can see this because you can skip stones
or skip objects on things without as much surface tension.
For example, you can even skip things off like the atmosphere.
(13:32):
You know, sometimes when spacecraft are trying to re enter
the atmosphere, there's an angle they have to enter it
where if they go too steep then they heat up
too much, but if they go too shallow, then they
skip off the atmosphere back into space. Oh you can
skip spaceships. Yeah, you can skip spaceships. I don't think
it's a good idea. I don't think anybody's ever done
it on purpose. I think it's a is there a
(13:53):
world record for that one or a competition? Yeah one,
And that guy bounced out into space and nobody's ever
seen him before. They said, they said the diploma, but
they didn't get it. You can also skip on on
surfaces that have no tension at all, Like you can
skip rocks on sand dunes, right, and sand doesn't have
any attraction to itself. There's no surface tension on sand. Dudes,
(14:14):
you don't get drops of sand, right, So you see
the skipping affected area in places with more or less
surface tension. And the bottom line, though, is the service
tension is just super weak and it can't contribute at all.
All right, So it's not surface tension that's helping you
or helping us skip stone. So there must be other things.
So let's get into it. But first let's take a
(14:35):
quick break. All right. We're talking about why is it
possible to skip stones on a lake, Like what's the
physics of it? And so we talked about how it's
(14:57):
not surface tension which is what a lot of people gets. Um,
And so maybe, uh, it might be something else. So
I would guess it maybe has something to do with
the spin that you put into the stone. Is that
kind of what's happening. It feels like a good answer, right,
because when you're skipping stones, you notice that if you
spin them really fast, they skip better. Right. We will
(15:17):
talk about it in a moment how the stone spinning
actually does help it skip, but it's not actually necessary.
I mean the spin what it does is it helps
make it fly straight, sort of like a frisbee, so
that it like it hits the water at a good
angle and in a good direction and all that stuff.
But it's not actually necessary. You can skip a stone
without spinning. It's possible. I always thought that it was
(15:38):
something to do with like conservation of angular momentum. You know,
like if you spin it, it wants to keep spinning
and that keeps it kind of level, which then kind
of makes it graze the surface of the water. Yeah,
I mean you're getting there. It's not actually necessary for spinning,
but you're right, it keeps it sort of flat, right, Um,
just the way a frisbee flies better when it's spinning
(15:58):
for the same reason as you say, angular momentum. It
doesn't like to tip over or it takes more of
a force to tip it over. Um. And it turns
out that you skip best when you hit the water
at just the right angle, and so spinning the stone
does help it hit at that right at that angle
that the good angle for for skipping. But it's not
actually necessary. You can skip a stone without spinning it.
(16:19):
It is possible. While you're blowing my mind here, you
can skip stones on sand and you can do it
without spinning. Yeah, yeah, exactly. Skipping turns out to be
quite the fascinating physics topic. Um. Alright, so then if
it's not surface tension or spinning, what's, um, what's happening
when we skip stones? Well, this is actually really interesting.
(16:40):
And it turns out there's a physicist about twenty years
ago whose son asked him, Hey, Dad, how is it
that you can skip stones? What's the physics of it?
And he was like, oh, I'm sure it's pretty simple,
And he started digging into it, and turns out it's
pretty complicated, and he spent months studying this and doing experiments,
and he finally came up with an equation, an equation
that predict how many skips you'll get from a stone
(17:03):
as a function of the angle of the stone, the
velocity of the stone, and the speed of skipping, the
weight of the stone, all this stuff, And so he
was able to isolate sort of what are the important
factors in getting a stone to skip? So wait, his
son asked him a question, like hey, Dad, and then
three years later he's like, here it is. Here's a
(17:24):
paper full of equations. That's what you wanted, right, I
just wanted yes or no. After that, the kid probably
totally forgot about it. He's like, oh, I don't care whatever, Um,
that's what it's like to have businesses for a parent.
The kid is still waiting by the lake with the stone,
and Dad, show me how you just gotta finish this equation.
(17:44):
Did he figure this out from like equations or was
this all experimental like I'm gonna change try different stone
sizes and different angles and different velocities, or a little
bit of book. It was a little bit of both.
He started out theoretically. He was like, can I understand
the forces involved and what we do all the time
in physics, which is critical? Is he simplified the problem.
He's like, Okay, let me assume the stone is perfectly flat.
(18:07):
Let me assume assume it's a perfect circle. Let me
assume it hits the water at this angle or whatever.
He made some assumptions to simplify the problem, and then
he was able to write down equations that he thought
would describe the forces involved and that let him make predictions.
He's like, okay, if these are the forces involved, right,
and he knows how the forces change is a function
of the angle you throw it in the speed, then
(18:29):
I can make predictions for how many times the stone
will skip. Then he went out and checked it and
he actually did some experiments to see if his equations
were valid. Yeah, exactly. I want to see what that
peer review process was like. You know, did somebody go
out there with a bunch of stones and his paper
and try this thing out? Um, I'm not quite sure,
but he published the paper. It's a guy from the
(18:50):
University of Lyon in France. And in his paper he
suggests that the critical variables are, of course, the flatness
of the stone, the viscas city of the fluid, right,
the viscosity of the liquid, which is not the same
thing as surface tension, the angle of the stone, and
its speed, So neither none of those things involved surface
(19:11):
tension or spinning. Yeah, and spinning again helps because it
helps you get the right angle right, and it's also
easier to throw throw a rock really fast if you're
spinning it sort of out the tip of your finger
as you throw it. So spinning is sort of like
a supplemental thing. It's not necessary, but it's helpful. I'm
waiting for the twist in the story where you say,
and then he became the world record horbholder. So in
(19:32):
the end, it's not really surface tension. It's much more
about like the viscosity of the water, right, because you know,
think about like meaning like the thickness of it, or
how goopy it is. Yeah, how goopy it is. Right,
water not only is it attracted to itself, which caused
the surface tension. It's a fairly fairly small force, but
it's also kind of viscous, right, It's like, you know,
(19:52):
there's a bit of goopiness to it, like honey is
goopier of course in water, right, And this comes from
how the molecules would rub each other, like the friction
the molecule to molecule friction, So it's a different physics, right.
Surface tension is about attraction of the molecules. This is
about how the molecules flow past each other. If something
is really viscous, like tar, or like a giant vat
(20:14):
of honey, like I would imagine if I throw us
kipping stone on top, which is kind of blurb kind
of you know, when it's skip or would it. I
don't know, that's a good question, I think. Um. I mean, obviously,
if it's really viscous, then it's a flat surface, then
it will skip, right, So there's probably some point duper
super duper but not sticky, yeah, exactly. And you know,
(20:37):
you can come up with examples of liquids that are
really viscous but don't have a lot of surface tension,
or the other way around. So there's a complicated relationship
between surface tension and viscosity. But in the end, what
you need is viscosity. And the reason is that when
the stone hits the water, you want the water to
not be able to sort of flow out of the
way fast enough, right, like you want the water to
(20:58):
push back, Yes, exactly, you need the water to push
back and so like when you jump off a diving
board and you land totally flat on the water, right,
If you've done that, you know that water can feel
like concrete, right, like belly flop, Like a belly flop, yeah,
a belly flop exactly. If you fall off a bridge,
for example, from high enough, you can die landing in water, right,
because water just can't get out of the way fast enough,
(21:20):
and so it pushes back on you just sort of
the way the ground does right when you land on it.
It doesn't get out of the way fast enough. Yeah,
Like if you're falling through air, Air is much much
less viscous than water, and it gets out of the way, right,
It just moves out of the way. I mean, there's
still some friction there, which is why you have a
terminal velocity, but it gets out of the way fast
enough for you to pass through it, right. But water
is much much more viscous, so it provides much much
(21:43):
more drag. And if you hit the water flat, then
you're gonna get a big force the other direction, um,
which is gonna make your belly smart or make your
rock skip. So that's the key, right, is that the
water pushes back up on the rock. You can't skip
rocks on clouds or air. You can skip rocks on air, actually,
I mean you can skip spaceships on air, but it
(22:03):
takes much much higher speed. Yeah, well this is a
perfect point to take a break. So it's about the
viscosity of the liquid. So the more viscous it is,
(22:25):
the easier it is to bounce it. Yes, you need
a flat object and a viscous liquid. And that's a
flat object, because if it's not flat, it's not flat.
Like I mean, the most extreme thing is you drop,
like you know, drop something really thin and sharp into
the water, it's just going to separate the water and
sink down to the bottom. Right, So you need to
drop something flat so that it into the water can't
(22:46):
move out of the way. You need to belly flop, Yes, exactly.
Skipping stones should be called belly flopping stones, don't. They
have funny names for this in other countries, like what
they call in England. Um, in England they call it
ducks and drakes, which makes absolutely no sense to me, Like,
are you shooting ducks? Like what's going on? Well, like,
if you're you're throwing stones at a leg to skip
(23:07):
to create those ripples and skips. That that's called ducks
and drakes. Ducks and drake's. Yeah, somebody out there in
England explain that to us. In France they have a
clever name for it. They call it rick o'che, which
you know makes some sense, right, your rock is ricocheting
off the surface of the water. What are some other
funny words? Well, these are words that don't even make
sense to me. Like in Ireland they call it stone scuffing,
the way you would scuff your shoes and get marks
(23:28):
on them, or something you got to say with an
Irish saxon. I can't do an Irish accent, not at all.
But every language has a word for it. And the
funniest one is that according to the Oxford English Dictionary, Right,
so according to the Brits, the way Americans say it
is they say it's called dapping, and that's not a
word I've ever heard. So, like, what are the Brits
(23:48):
doing over there telling us what we call skipping stones
and they're getting totally wrong. Well, it's all just ducks
and drakes to them, So they got no credibility over there?
Is that where the dap comes from? Where am I
thinking of the dead. I am not culturally relevant enough
to answer that question. But it's more than just the viscosity, right,
and more than just having silly name. Okay, so it's
(24:09):
more than just having um substance to the thing you're
trying to skip on, which which water does it has
enough fixed causity to skip stone? That's right? Because you
know when you jump off a diving board and belly flop,
you don't skip right, So there's a difference between belly
flopping and skipping. And then what else is going on here?
So another key is the angle. Right when you throw
(24:29):
the stone, you want it the front edge of the
stone to be higher than the back edge of the stone.
So you want the stone when it hits the water
for the trailing edge to hit first, all right, And
the reason is that you're pushing down on the water
and if you have that like a like a wheelie,
like you're you're tilted back. Yes, exactly, exactly, Though I
(24:51):
do not recommend trying to do this with your motorcycle
or your mountain biketycle, or do it and send us
a video. That would be awesome. I think that's called
geezs and gooses in England. I think you're probably right.
And um, what happens when you do that is then
you push down the water at an angle, and the
water creates kind of a ramp, right because you've you've
(25:14):
pushed it down to the back end is further down
than the front end, and so you have a ramp
that slopes up. Okay, and here's where the speed comes.
You've got to be moving fast. So you push down
on the water to create a ramp, and then that
ramp launches you back up into the air at the
same angle. You're not really bouncing then, it's more like
you're going into the water and then creating a ramp
(25:34):
for you to take off of. Yeah, well you're doing
both at the same time. One of my favorite things
about sort of projectile motion in physics is how you
can break things up into two dimensions that are totally independent. Right,
motion in one dimension is independent from motion in the
other dimension. Somebody in the question intersection said that the
reason that you can skip a rocket is because it's
moving really fast, um, in one direction, and so it
(25:56):
doesn't fall down, which is totally not true. Right, Like
you shoot the bullet, you know, it is moving really
really fast parallel to the surface of the earth. But
it's also at the same time falling and falling towards
the surface of the earth is not affected by your
motion parallel to the surface. And so you've got two
things going on here. One is you're bouncing perpendicular to
the surface of the water, right, but you're also moving parallel,
(26:19):
So you've got the belly flop in the perpendicular direction,
and you've got this little ramp action in the parallel direction. Well,
what happens if if you don't hit the water at
a good angle, like let's say your head, you can
hit the water kind of leaning forward instead of leaning back,
then you end up the front edge of your rock
will dig into the water and it'll slow it down
(26:39):
and it'll flip, and then you don't don't then you
don't get any more skips. You don't create this little ramp. Yeah,
because remember the key to getting a lot of skips
is doing the same thing over and over again. So
you want that when you leave the water, you're at
the same angle as when you hit the water, right,
so that you can hit the water again at that
same angle. So anything that flips you over start to spinning,
(27:00):
you're losing your energy and you're not gonna hit the
water at the right angle the next skip. Well, I
would have thought the spin was really important because without
the spin, wouldn't you lose that angle really quickly with
all the water slushing around. Yeah, spinning is important, and
nobody wins the stone Skipping World Championship without a really
good spin. But it's not technically necessary. But yeah, it's
really important to maintaining the angle and to maintaining your speed.
(27:24):
So there's a lot of different things going on when
just one simple rock hits the water, all right, and
then um so viscosity angle, and then what's the last
critical ingredient? Last critical ingredient is speed? Right? If you
are not moving fast enough, then you sink when you
hit the water because the ramps are dissolves, right, that
ramps is not doesn't stick around for very long. You
hit the water right, and you bounce back because the
(27:46):
water doesn't have a lot of time to move out
of the way. But um, you want to get off
of that ramp as soon as you can, because just
pass the ramp as a little divid right, and so
you want to get off of that ramp, and so
you need enough speed. You need to me moving faster
than the water is, of course moving faster horizontally or
does it help to go fast a little bit, you know,
(28:08):
into the water or do you get that anyways, you
need both. You need if you if you're not moving
fast enough perpendicular to the water, then you won't get
the bounce right. You need to be hitting the water
with enough speed that the water can't get out of
the way and pushes back at you. You also need
horizontal velocity so you can get off that ramp before
it dissolves. But if you're going to slow then you know,
(28:29):
you push the water into a ramp, but then you
you don't take off, You just kind of move exactly.
That's exactly the sound it makes. Why you must have
done these experiments. That's what it sounds like in every
slow motion video clip I've seen on YouTube. But every
time you bounce, you also do lose energy. It's impossible
not to because this friction and so you hit the
(28:52):
water and it slows you down a little bit. So
every bounce is in the best case scenario, like of
the neck of the previous ounce, And so that's why
you know, they get faster and faster. The time between
bounces it gets smaller and smaller because you're not going
as fast, you're not going as far up, so it
doesn't take as much time to come back down. So
that's pretty good. Like you only lose in each skip
(29:15):
or is that like a you know, is that is
that what the pros do? Or is that what happens
when anyone skip stones? Is that it's always now that's
the like top level performance, like eighty yeah, And that's
the key to getting a lot of skips is losing
the smallest amount of energy every skip, which means hitting
the water at just the right angle. And it's one
(29:37):
of these things where it's very sensitive. You're slightly off
of the optimal angle, you're gonna lose a lot more
energy in the first skip, and then you're gonna be
off on the next angle and the next angle, and
pretty soon you know you're sinking to the bottom of
the lake. That's pretty impressive. So that's how they get
to like a hundred yeah, exactly eight skips, which still
blows my mind. It feels like that would take like
(29:57):
an hour, you know, Like you throw the rock and
then you're just like and minutes later, still scount counting
skips or something. Well, towards the end, it's it's it's
not like skips skips skip. It's like, yeah, exactly, it's
like stuttering skips at the end, which is what tells
me what these really like massively competitive events. They must
have some sort of camera that's taking pictures to count
these skips. They can't have like a super spot or
(30:19):
something just using their eyeballs. You think it's at that
high tech. You think they have like corporate sponsors and
pro athletes, probably some Swiss watch companies you know, sponsoring it, etcetera. Yeah,
but the top guys have groupies and travel around and
fancy busses and all sorts of stuff. All right, So
then have you tried this? Since you're learning the secrets here,
have you tried and does it help? Did it help
(30:40):
you skip stones better? No? And that's the thing about physics.
It tells you how the universe works, It gives you inside,
but it doesn't help you actually navigate the situation. You know.
For example, we did that whole podcast episode about how
to ride a bike, right that just made me ride
bikes any better, especially because we learned that physics doesn't
know why why bicycles. Allan's right, but you can still
(31:01):
ride a bike. So there's this disconnect between understanding something
and being able to do it. Well, you know, it
did help me. I mean after learning all of this,
I did try it, and it does help. Like if
you know that you have to throw it at the
water kind of at a little bit of an angle
leaning bag that's a huge hill, like because you know
before you I would try to like throw it at
(31:23):
the water so it's perfectly parallel to the water. I
thought that was the secret, you know, but like once
you know you have to kind of lean it back
a little bit. It does help you skip better. All right,
well you go, I skips, but you know, physics has
improved your life for her for the first time, it
ruined my vacation, but it increase my standing in the
world rankings of It's amazing to me that the universe
(31:44):
is mathematical, that we can understand it at all, that
it seemed to follow these rules and we can write
down on paper manipulate with our minds um. It's amazing
that that works, and it's a joy to see what
it happens. You know that we can actually isolate the
critical elements and understand something. So the next time you
see something weird, the next time you see something you
don't understand, you know, think about the physics of it,
what's making that happen? Um, what's going on inside is
(32:07):
there's some microscopic explanation that can help you understand why
those ducks are chasing that funny bird or you know
why those clouds form in that weird way. And there's
physics all around us. And if you don't understand, just
send the question to Daniel and we'll answer it on
the podcast that's right before after I do a bunch
of experiments at Jorge's house skipping gold coins on my Hey,
(32:29):
can I borrow a few of those gold coins please?
I gotta do some experiments. Whoops, they sank to the
bottom of the lake. Sorry about that? All right? Well,
we hope you enjoyed Dad, And as Neo says, whoa,
thanks for tuning in. See you next time. Before you
(32:52):
still have a question after listening to all these explanations,
please drop us a line. We'd love to hear from you.
You can find us on Facebook, Twitter, and Instagram at
Daniel and Jorge that's one word, or email us at
feedback at Daniel and Jorge dot com. Thanks for listening
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio. For more podcast from
(33:14):
my heart Radio, visit the i heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. Yeah.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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