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What is the physics behind skipping stones? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, Daniel, what did it like to look at the
0:10
Speaker 1: world through the eyes of a physicist? Oh, it's amazing.
0:14
Speaker 1: It's red, there's green, and then there's blue. It's really
0:17
Speaker 1: quite beautiful. I mean, like, are you always trying to
0:20
Speaker 1: analyze and understand everything? Like it's the world just a
0:24
Speaker 1: giant mess of puzzles and interesting phenomenon pretty much. It's
0:29
Speaker 1: hard to turn that part of your brain off once
0:31
Speaker 1: you've turned it on. I mean, don't you ever want
0:32
Speaker 1: to just sit back, relax and just live in the
0:35
Speaker 1: moment and experience things? Yeah? Every time I do that,
0:39
Speaker 1: I think, how does that work? Living in a moment?
0:41
Speaker 1: What do you relaxation? Anyway? I am handmade cartoonists and
1:01
Speaker 1: the creator of PhD comics. Hi, I'm Daniel Whitson. I'm
1:04
Speaker 1: a particle physicist and I'm incapable of fully relaxing. You
1:08
Speaker 1: think that's a personal trade or is that just comes
1:10
Speaker 1: with the job of being a physicist? Yeah? I can't
1:12
Speaker 1: tell about the cause and effect there? Am I that
1:14
Speaker 1: way because I became a physicist? Or did I become
1:17
Speaker 1: a physicist because I am that way? Causation versus correlation?
1:21
Speaker 1: That's right? But you know, sometimes you can do both.
1:24
Speaker 1: Sometimes you can enjoy looking at the world and unraveling
1:27
Speaker 1: its mysteries. You know. Uh, some people do puzzles for fun,
1:30
Speaker 1: and for me, the universe is my puzzle. Well, hopefully
1:33
Speaker 1: that's why people are listening in to this Welcome to
1:35
Speaker 1: our podcast Daniel and Jorge Explain the Universe, a production
1:39
Speaker 1: of My Heart Radio, in which we look at the
1:42
Speaker 1: universe like a big puzzle and we try to take
1:43
Speaker 1: it apart for you and hopefully entertain you along the
1:46
Speaker 1: way with our terrible dad jokes and attain you and
1:48
Speaker 1: also maybe ruin your vacations or your relaxation times, or
1:53
Speaker 1: enhance your relaxation by showing you a whole other dimension
1:56
Speaker 1: behind the simple plane of reality that everybody else experiences.
2:00
Speaker 1: Do you feel like sometimes you are like a neo
2:03
Speaker 1: in the matrix where you can see the code behind
2:07
Speaker 1: everything around you. No, but I can totally do that
2:09
Speaker 1: kick move. You can dodge bullets and look like Keanu Reeves. Hey,
2:13
Speaker 1: I've never been shot. Okay, I can say that you
2:16
Speaker 1: successfully dodge every bullet that's been fired at you, every
2:21
Speaker 1: single bullet fired at anybody I successfully dodged. Oh wow,
2:25
Speaker 1: that that's a lot of bullets of bullets. Yeah. So
2:28
Speaker 1: today on the podcast will be tackling a phenomenon that
2:31
Speaker 1: maybe hopefully a lot of people have done or tried
2:34
Speaker 1: to do, or look at and think, hey, that's pretty cool. Yeah.
2:38
Speaker 1: I think a lot more people have tried to do
2:39
Speaker 1: it than successfully done it. Something you might do when
2:42
Speaker 1: you're out for a relaxing stroll or maybe out on
2:45
Speaker 1: vacation or just killing time or having some thoughts to yourself.
2:49
Speaker 1: So to be on the program, we'll be talking about
2:56
Speaker 1: physics of skipping stones. That's right. Why is that even possible?
3:02
Speaker 1: Why is it that you can bounce a rock on water?
3:05
Speaker 1: I mean it seems sort of crazy. If you had
3:06
Speaker 1: never seen it, but somebody described it to you, you
3:09
Speaker 1: would be pretty skeptical. Yeah, it's pretty cool. Um. And
3:12
Speaker 1: this is you know, when you're walking by a lake
3:14
Speaker 1: or a pond, or maybe just even like a fountain. Um,
3:18
Speaker 1: you might have seen people or tried to do it yourself.
3:20
Speaker 1: As you pick up a little stone, you throw it
3:22
Speaker 1: and it skips along the surface of the water, sometimes
3:25
Speaker 1: a lot. Yeah, what's your personal record? Jorge probably made
3:28
Speaker 1: like ten skips and a stone. Maybe Ten's kind of
3:32
Speaker 1: hard to count, right, isn't it hard to count? Especially
3:35
Speaker 1: at the end there they get faster and faster. My
3:37
Speaker 1: son likes to skip stones. I think his record is eight,
3:40
Speaker 1: and mind is something more like five or six. Is
3:43
Speaker 1: this a profession some people have, Like are their professional
3:45
Speaker 1: stone skippers? There certainly are competitions where people fight voraciously
3:49
Speaker 1: for the championship. I don't know if they get paid
3:51
Speaker 1: for it, but they do pretty well. The current world
3:54
Speaker 1: record for most number of skips is eight eight eight skips.
3:59
Speaker 1: So you throw a rock at some water, a body
4:02
Speaker 1: of water, and it skips eighty eight times. It's unbelievable, Like,
4:06
Speaker 1: how do they even? They must have like a camera,
4:08
Speaker 1: like the kind of used that really fancy tennis matches.
4:11
Speaker 1: How far is that like when you throw it? Yeah,
4:14
Speaker 1: it's actually interesting. In the US they have stone skipping
4:16
Speaker 1: competitions and there they count the number of skips, whereas
4:20
Speaker 1: if you go to sort of international world championships stone skipping,
4:24
Speaker 1: they don't care about the number of skips. They care
4:26
Speaker 1: about the distance and the furthest stone anybody's ever thrown
4:30
Speaker 1: via skipping is a hundred and twenty one. It's longer
4:35
Speaker 1: than a soccer field. It's like a seventh of a kilometer. Yeah,
4:39
Speaker 1: it's huge, It's unbelievable. Um. So somebody has really figured
4:43
Speaker 1: out how to do this thing. And you know, when
4:45
Speaker 1: you're out there and you're skipping stones, you sort of
4:47
Speaker 1: get a feeling for You're like, Okay, I need this
4:49
Speaker 1: kind of stone. I gotta swing it this kind of way.
4:51
Speaker 1: It's sort of like throwing a frisbee a little bit.
4:53
Speaker 1: You sort of feel it out with your mind. This
4:55
Speaker 1: is when it gets it in your wrist? Is it
4:57
Speaker 1: in your you know, our movement? Yeah? About the water, yeah,
5:02
Speaker 1: and about the perfect stone. And most people just sort
5:05
Speaker 1: of figured out intuitively, right, they try this, they try that,
5:09
Speaker 1: and eventually sort of get the hang of it and
5:10
Speaker 1: they perfect it right, And that that's how most people
5:14
Speaker 1: approach the world. Right. You learn how to walk, not
5:17
Speaker 1: by like thinking about the physics of walking. You learn
5:19
Speaker 1: how to ride a bike just by sort of trying
5:21
Speaker 1: it out. Your rain gets trained to sort of learn
5:24
Speaker 1: the physics intuitively. You don't like write down equations. Um,
5:28
Speaker 1: but it is possible also to take this really fun
5:30
Speaker 1: vacation activity and sort of ruin it by you know,
5:33
Speaker 1: turning into an equation. No, it doesn't make you want
5:36
Speaker 1: to appreciate it more, you know, wants you to understand
5:40
Speaker 1: the physics behind it. Depends on what the answer is. Right.
5:43
Speaker 1: If the answer is something oh hum, then you know,
5:45
Speaker 1: maybe you've ruined the magic. But if the answer is
5:47
Speaker 1: there's some crazy surprising physics in there, then yeah, then
5:51
Speaker 1: you know, doing the physics has revealed something fascinating about
5:54
Speaker 1: the universe. So we're in for some crazy surprising physics.
5:57
Speaker 1: Turns out that the way most people think skipping stones
6:00
Speaker 1: works is not the way it actually works. Well, I
6:03
Speaker 1: think I've got the trick of it, but maybe we'll see.
6:06
Speaker 1: We'll see what I'm doing corresponds to the physics of it.
6:09
Speaker 1: All right, I'll give you a physics grade at the
6:11
Speaker 1: end of it. All right, Well, it is kind of complicated,
6:13
Speaker 1: and we were wondering how many people out there knew
6:16
Speaker 1: what it takes, or how it works, or what's actually
6:19
Speaker 1: happening when you skip a stone. So I walked around
6:22
Speaker 1: the streets of Aspen, Colorado, a beautiful town in the
6:25
Speaker 1: mountains with lots of stones and lots of little lakes
6:28
Speaker 1: and ponds, so people could have plenty of chance to
6:31
Speaker 1: practice their skipping and I asked, folks, do you know
6:34
Speaker 1: why it's possible to skip a stone? Then most people respond, oh,
6:37
Speaker 1: I don't skip stones. I just they're a gold bulls
6:43
Speaker 1: skip stones, I get stoned. That's a different physics episode
6:50
Speaker 1: probably somewhere and asking you can buy like a nine
6:53
Speaker 1: dollar gold plated perfect skipping stone. So you went out
6:56
Speaker 1: there and you ask people in the street if they
6:58
Speaker 1: knew why is it possible to skip stones on a lake?
7:02
Speaker 1: To think about it for a second and then listen
7:04
Speaker 1: to what people had to say. Curface resistance tension should
7:07
Speaker 1: be more friction gravity because it's hydroplinion, it's like tight tension.
7:13
Speaker 1: Or if you' but if you put a rock on
7:15
Speaker 1: the water, the supertension velocity as a flat surface, so
7:20
Speaker 1: it's going to spin across the thought if it had
7:22
Speaker 1: jagged edges, it was catching it pulled into the water,
7:26
Speaker 1: gravity gravity would pull it down. Yeah, okay, I don't know.
7:31
Speaker 1: All right, A lot of great answers here, a lot
7:33
Speaker 1: of your favorite answers, right, gravity gravity, Yes, always works.
7:38
Speaker 1: You can also say the Big Bang or physics, physics,
7:45
Speaker 1: and that's usually how things work. Usually physics is the
7:47
Speaker 1: way everything works. So yeah, that's a solid answer. I know,
7:50
Speaker 1: we've got lots of people saying things like surface tension, right,
7:54
Speaker 1: or hydroplaning or just jeez, I have no idea. Some
7:58
Speaker 1: people had really never thought about it before. Or That's
8:00
Speaker 1: one of my favorite moments is when I spark in
8:02
Speaker 1: their minds this moment of curiosity, because then they always
8:05
Speaker 1: turn around they ask me. They're like, well, what tell
8:07
Speaker 1: me the answer? And now now you asked me, I
8:08
Speaker 1: have to know. Yeah, a lot of answers like surface tensions,
8:12
Speaker 1: the speed, the shape of the rock, what's what's going on? Yeah,
8:15
Speaker 1: so based just on those answers, you might guess that
8:17
Speaker 1: the answer is pretty complicated. There might be a lot
8:20
Speaker 1: of things going on, right, some physics magic, some physics
8:24
Speaker 1: not magic. And it turns out it is pretty complicated.
8:27
Speaker 1: We should do magical physics. That can I say magical
8:30
Speaker 1: physics physics ruining magic for hundreds of years, magic adjacent physics.
8:35
Speaker 1: That's right. Well, the most popular answer that you heard
8:37
Speaker 1: there is surface tension. And that makes some sense, right,
8:41
Speaker 1: is it the surface tension? Turns out it's not surface tension. Right.
8:44
Speaker 1: That's a tempting answer because you think, oh, the water
8:47
Speaker 1: is sort of holding the surface together. There is some
8:50
Speaker 1: resistance there and you may have even seen like bugs
8:53
Speaker 1: skimming on the surface of water, or you can put
8:55
Speaker 1: a paper clip down on water and it can sit
8:57
Speaker 1: on the surface tension, right, So the a sense that
9:00
Speaker 1: the water is doing something there to hold something up, right,
9:04
Speaker 1: But it turns out service tension has nothing to do
9:06
Speaker 1: with skipping stones. Maybe step us through a little bit.
9:08
Speaker 1: What is surface tension? Yeah, service tension. It happens when
9:11
Speaker 1: you have a liquid and that liquid is attracted to itself, right,
9:15
Speaker 1: and so it wants to clump together. So for example,
9:17
Speaker 1: if you put a drop of water on a table,
9:20
Speaker 1: it doesn't just run totally flat, which is what you
9:23
Speaker 1: would expect from a smoothly flowing liquid under gravity, right
9:27
Speaker 1: gravity anything. It doesn't spread out, you know, out to
9:30
Speaker 1: cover the whole table exactly. It stays together one one
9:34
Speaker 1: molecule thick covering coding. But it stays as a droplet, right,
9:39
Speaker 1: that's right. It stays as a droplet. And the reason
9:42
Speaker 1: is that the molecules are attracted to each other. There's
9:44
Speaker 1: some small force pulling the molecules to each other. And
9:47
Speaker 1: we've talked on another episode of about how water is
9:49
Speaker 1: a dielectric, and so it's neutral, right, the positive negative
9:53
Speaker 1: charges balanced overall, but the positive negative charges aren't on
9:57
Speaker 1: the same side of the water molecule, and so they
9:59
Speaker 1: can line a in this way where the positive end
10:01
Speaker 1: of one water molecule attracts the minus end of another one.
10:04
Speaker 1: They can tug on each other a little bit. So
10:06
Speaker 1: they like to stay together. They're like a little scared
10:09
Speaker 1: crowd of children or something. They clump together in fear.
10:13
Speaker 1: And it's like having a handful of magnets at a distance.
10:16
Speaker 1: Maybe they are neutral to each other, but if you
10:20
Speaker 1: get them closed, they'll they'll start to clump together. Yeah,
10:22
Speaker 1: maybe a handful of magnets was a better analogy than
10:24
Speaker 1: a clump of scared children. Probably where are they scared
10:30
Speaker 1: because you're coming out them with a clump of of
10:32
Speaker 1: of magnets Because I'm holding a rock right now. M Yeah,
10:36
Speaker 1: so it clumps together. That's what surface tension is, is
10:38
Speaker 1: the water clumping together um and so it um And
10:41
Speaker 1: that happens also on the surface, right, not just inside,
10:44
Speaker 1: but also on the surface. It's kind of like why
10:46
Speaker 1: in space, if you see a blob of water, it
10:49
Speaker 1: doesn't just spread out like a gas, it stays as
10:51
Speaker 1: a blob of water. Yeah, exactly, because it's attracted to itself.
10:55
Speaker 1: So that causes surface tension. Right, that's what surface tension.
10:57
Speaker 1: And wait, what is it? What is it called surface tension?
11:00
Speaker 1: Like what's happening at the surface. It's called surface tension
11:02
Speaker 1: because at the surface, if you poke it, it's it
11:05
Speaker 1: sort of holds together, like it creates this sort of
11:07
Speaker 1: sheet at the surface that resists being torn apart or penetrated.
11:11
Speaker 1: It's the same way that like the wall holds itself together. Right,
11:14
Speaker 1: you have bonds between the molecules that are holding the
11:16
Speaker 1: wall together. These bonds are much much weaker, which is
11:19
Speaker 1: why water in this case is a fluid. Right, it's
11:22
Speaker 1: just a It's just a weaker version of those same
11:24
Speaker 1: bonds that hold all matter together, right, right, But there's
11:28
Speaker 1: something special about the surface, isn't it? Like the isn't
11:31
Speaker 1: it or maybe I'm wrong, is that the molecules on
11:34
Speaker 1: the surface are sort of holding on a little bit
11:36
Speaker 1: tighter to each other than the ones like inside of
11:39
Speaker 1: the water could be you tell me you're the engineer.
11:42
Speaker 1: They were completely lost. Now. But the key thing when
11:46
Speaker 1: it comes to understanding skipping stones. Is you know, could
11:49
Speaker 1: you use surface tension to skip a stone, because you can.
11:52
Speaker 1: You can use it to float a bug, right, Like
11:55
Speaker 1: some bugs stand on water because the water refus uses
12:00
Speaker 1: to sort of be broken apart. That's right. Yeah, the
12:02
Speaker 1: water holds itself together and creates a surface, right. Um,
12:06
Speaker 1: But that only really works for very, very low weight
12:09
Speaker 1: objects because the surface tension is not very strong. I mean,
12:12
Speaker 1: you touch a drop of water and it clings to
12:14
Speaker 1: you instead, right, It's very easy to break surface tension.
12:17
Speaker 1: And if you just if it was just surface tension,
12:19
Speaker 1: then you could be able to take a rock and
12:20
Speaker 1: put it on water and it would float there, right.
12:23
Speaker 1: But a rock that you could easily skip, if you
12:25
Speaker 1: put it on the surface of the water, it would
12:27
Speaker 1: just sink through. Some people might be wondering, is it
12:29
Speaker 1: the surface tension that's creating some kind of trampoline on
12:33
Speaker 1: the water that's maybe causing the stone to skip exactly.
12:37
Speaker 1: It's kind of that was kind of the scenario we're
12:39
Speaker 1: trying to debunk. Yeah, that's definitely not happening, because if
12:42
Speaker 1: you put a stone on a trampoline, right, it doesn't
12:44
Speaker 1: fall through the trampoline, right, So you know, the trampoline
12:47
Speaker 1: is strong enough to bounce the stone back up. But
12:50
Speaker 1: if you put a stone just lay it sort of
12:52
Speaker 1: gently on the surface of the water, you know what's
12:54
Speaker 1: gonna happen. It's not gonna sit there. It's gonna sink
12:56
Speaker 1: unless your stone weighs the same as a paper clip, right,
12:58
Speaker 1: which case you probably can't it. But anyway, but a
13:01
Speaker 1: stone that you could skip won't sit gently on the water.
13:04
Speaker 1: Um So service tension is not enough to provide sort
13:07
Speaker 1: of trampoline effect there, even if it's going fast. I
13:10
Speaker 1: could maybe imagine somebody thinking you throw a stone so
13:13
Speaker 1: fast at a body of water that it actually kind
13:16
Speaker 1: of bounces back from the hitting that service tension. It
13:19
Speaker 1: does bounce back, but not because of the service tension, right.
13:22
Speaker 1: And you can see this because you can skip stones
13:25
Speaker 1: or skip objects on things without as much surface tension.
13:29
Speaker 1: For example, you can even skip things off like the atmosphere.
13:32
Speaker 1: You know, sometimes when spacecraft are trying to re enter
13:35
Speaker 1: the atmosphere, there's an angle they have to enter it
13:38
Speaker 1: where if they go too steep then they heat up
13:40
Speaker 1: too much, but if they go too shallow, then they
13:42
Speaker 1: skip off the atmosphere back into space. Oh you can
13:46
Speaker 1: skip spaceships. Yeah, you can skip spaceships. I don't think
13:49
Speaker 1: it's a good idea. I don't think anybody's ever done
13:51
Speaker 1: it on purpose. I think it's a is there a
13:53
Speaker 1: world record for that one or a competition? Yeah one,
13:56
Speaker 1: And that guy bounced out into space and nobody's ever
13:58
Speaker 1: seen him before. They said, they said the diploma, but
14:04
Speaker 1: they didn't get it. You can also skip on on
14:06
Speaker 1: surfaces that have no tension at all, Like you can
14:08
Speaker 1: skip rocks on sand dunes, right, and sand doesn't have
14:11
Speaker 1: any attraction to itself. There's no surface tension on sand. Dudes,
14:14
Speaker 1: you don't get drops of sand, right, So you see
14:18
Speaker 1: the skipping affected area in places with more or less
14:20
Speaker 1: surface tension. And the bottom line, though, is the service
14:23
Speaker 1: tension is just super weak and it can't contribute at all.
14:26
Speaker 1: All right, So it's not surface tension that's helping you
14:29
Speaker 1: or helping us skip stone. So there must be other things.
14:32
Speaker 1: So let's get into it. But first let's take a
14:35
Speaker 1: quick break. All right. We're talking about why is it
14:51
Speaker 1: possible to skip stones on a lake, Like what's the
14:54
Speaker 1: physics of it? And so we talked about how it's
14:57
Speaker 1: not surface tension which is what a lot of people gets. Um,
15:00
Speaker 1: And so maybe, uh, it might be something else. So
15:03
Speaker 1: I would guess it maybe has something to do with
15:05
Speaker 1: the spin that you put into the stone. Is that
15:08
Speaker 1: kind of what's happening. It feels like a good answer, right,
15:11
Speaker 1: because when you're skipping stones, you notice that if you
15:14
Speaker 1: spin them really fast, they skip better. Right. We will
15:17
Speaker 1: talk about it in a moment how the stone spinning
15:20
Speaker 1: actually does help it skip, but it's not actually necessary.
15:23
Speaker 1: I mean the spin what it does is it helps
15:25
Speaker 1: make it fly straight, sort of like a frisbee, so
15:28
Speaker 1: that it like it hits the water at a good
15:29
Speaker 1: angle and in a good direction and all that stuff.
15:33
Speaker 1: But it's not actually necessary. You can skip a stone
15:35
Speaker 1: without spinning. It's possible. I always thought that it was
15:38
Speaker 1: something to do with like conservation of angular momentum. You know,
15:42
Speaker 1: like if you spin it, it wants to keep spinning
15:44
Speaker 1: and that keeps it kind of level, which then kind
15:47
Speaker 1: of makes it graze the surface of the water. Yeah,
15:50
Speaker 1: I mean you're getting there. It's not actually necessary for spinning,
15:53
Speaker 1: but you're right, it keeps it sort of flat, right, Um,
15:56
Speaker 1: just the way a frisbee flies better when it's spinning
15:58
Speaker 1: for the same reason as you say, angular momentum. It
16:01
Speaker 1: doesn't like to tip over or it takes more of
16:03
Speaker 1: a force to tip it over. Um. And it turns
16:05
Speaker 1: out that you skip best when you hit the water
16:07
Speaker 1: at just the right angle, and so spinning the stone
16:11
Speaker 1: does help it hit at that right at that angle
16:13
Speaker 1: that the good angle for for skipping. But it's not
16:16
Speaker 1: actually necessary. You can skip a stone without spinning it.
16:19
Speaker 1: It is possible. While you're blowing my mind here, you
16:22
Speaker 1: can skip stones on sand and you can do it
16:25
Speaker 1: without spinning. Yeah, yeah, exactly. Skipping turns out to be
16:29
Speaker 1: quite the fascinating physics topic. Um. Alright, so then if
16:32
Speaker 1: it's not surface tension or spinning, what's, um, what's happening
16:37
Speaker 1: when we skip stones? Well, this is actually really interesting.
16:40
Speaker 1: And it turns out there's a physicist about twenty years
16:42
Speaker 1: ago whose son asked him, Hey, Dad, how is it
16:46
Speaker 1: that you can skip stones? What's the physics of it?
16:48
Speaker 1: And he was like, oh, I'm sure it's pretty simple,
16:50
Speaker 1: And he started digging into it, and turns out it's
16:52
Speaker 1: pretty complicated, and he spent months studying this and doing experiments,
16:57
Speaker 1: and he finally came up with an equation, an equation
16:59
Speaker 1: that predict how many skips you'll get from a stone
17:03
Speaker 1: as a function of the angle of the stone, the
17:06
Speaker 1: velocity of the stone, and the speed of skipping, the
17:09
Speaker 1: weight of the stone, all this stuff, And so he
17:12
Speaker 1: was able to isolate sort of what are the important
17:15
Speaker 1: factors in getting a stone to skip? So wait, his
17:17
Speaker 1: son asked him a question, like hey, Dad, and then
17:21
Speaker 1: three years later he's like, here it is. Here's a
17:24
Speaker 1: paper full of equations. That's what you wanted, right, I
17:29
Speaker 1: just wanted yes or no. After that, the kid probably
17:32
Speaker 1: totally forgot about it. He's like, oh, I don't care whatever, Um,
17:35
Speaker 1: that's what it's like to have businesses for a parent.
17:37
Speaker 1: The kid is still waiting by the lake with the stone,
17:39
Speaker 1: and Dad, show me how you just gotta finish this equation.
17:44
Speaker 1: Did he figure this out from like equations or was
17:48
Speaker 1: this all experimental like I'm gonna change try different stone
17:51
Speaker 1: sizes and different angles and different velocities, or a little
17:55
Speaker 1: bit of book. It was a little bit of both.
17:56
Speaker 1: He started out theoretically. He was like, can I understand
17:59
Speaker 1: the forces involved and what we do all the time
18:01
Speaker 1: in physics, which is critical? Is he simplified the problem.
18:04
Speaker 1: He's like, Okay, let me assume the stone is perfectly flat.
18:07
Speaker 1: Let me assume assume it's a perfect circle. Let me
18:10
Speaker 1: assume it hits the water at this angle or whatever.
18:13
Speaker 1: He made some assumptions to simplify the problem, and then
18:15
Speaker 1: he was able to write down equations that he thought
18:18
Speaker 1: would describe the forces involved and that let him make predictions.
18:22
Speaker 1: He's like, okay, if these are the forces involved, right,
18:24
Speaker 1: and he knows how the forces change is a function
18:27
Speaker 1: of the angle you throw it in the speed, then
18:29
Speaker 1: I can make predictions for how many times the stone
18:32
Speaker 1: will skip. Then he went out and checked it and
18:34
Speaker 1: he actually did some experiments to see if his equations
18:37
Speaker 1: were valid. Yeah, exactly. I want to see what that
18:41
Speaker 1: peer review process was like. You know, did somebody go
18:44
Speaker 1: out there with a bunch of stones and his paper
18:46
Speaker 1: and try this thing out? Um, I'm not quite sure,
18:49
Speaker 1: but he published the paper. It's a guy from the
18:50
Speaker 1: University of Lyon in France. And in his paper he
18:54
Speaker 1: suggests that the critical variables are, of course, the flatness
18:57
Speaker 1: of the stone, the viscas city of the fluid, right,
19:01
Speaker 1: the viscosity of the liquid, which is not the same
19:03
Speaker 1: thing as surface tension, the angle of the stone, and
19:07
Speaker 1: its speed, So neither none of those things involved surface
19:11
Speaker 1: tension or spinning. Yeah, and spinning again helps because it
19:14
Speaker 1: helps you get the right angle right, and it's also
19:16
Speaker 1: easier to throw throw a rock really fast if you're
19:19
Speaker 1: spinning it sort of out the tip of your finger
19:21
Speaker 1: as you throw it. So spinning is sort of like
19:23
Speaker 1: a supplemental thing. It's not necessary, but it's helpful. I'm
19:26
Speaker 1: waiting for the twist in the story where you say,
19:29
Speaker 1: and then he became the world record horbholder. So in
19:32
Speaker 1: the end, it's not really surface tension. It's much more
19:35
Speaker 1: about like the viscosity of the water, right, because you know,
19:39
Speaker 1: think about like meaning like the thickness of it, or
19:41
Speaker 1: how goopy it is. Yeah, how goopy it is. Right,
19:44
Speaker 1: water not only is it attracted to itself, which caused
19:47
Speaker 1: the surface tension. It's a fairly fairly small force, but
19:50
Speaker 1: it's also kind of viscous, right, It's like, you know,
19:52
Speaker 1: there's a bit of goopiness to it, like honey is
19:55
Speaker 1: goopier of course in water, right, And this comes from
19:58
Speaker 1: how the molecules would rub each other, like the friction
20:01
Speaker 1: the molecule to molecule friction, So it's a different physics, right.
20:05
Speaker 1: Surface tension is about attraction of the molecules. This is
20:08
Speaker 1: about how the molecules flow past each other. If something
20:10
Speaker 1: is really viscous, like tar, or like a giant vat
20:14
Speaker 1: of honey, like I would imagine if I throw us
20:16
Speaker 1: kipping stone on top, which is kind of blurb kind
20:19
Speaker 1: of you know, when it's skip or would it. I
20:21
Speaker 1: don't know, that's a good question, I think. Um. I mean, obviously,
20:25
Speaker 1: if it's really viscous, then it's a flat surface, then
20:27
Speaker 1: it will skip, right, So there's probably some point duper
20:32
Speaker 1: super duper but not sticky, yeah, exactly. And you know,
20:37
Speaker 1: you can come up with examples of liquids that are
20:39
Speaker 1: really viscous but don't have a lot of surface tension,
20:42
Speaker 1: or the other way around. So there's a complicated relationship
20:45
Speaker 1: between surface tension and viscosity. But in the end, what
20:48
Speaker 1: you need is viscosity. And the reason is that when
20:51
Speaker 1: the stone hits the water, you want the water to
20:53
Speaker 1: not be able to sort of flow out of the
20:55
Speaker 1: way fast enough, right, like you want the water to
20:58
Speaker 1: push back, Yes, exactly, you need the water to push
21:01
Speaker 1: back and so like when you jump off a diving
21:03
Speaker 1: board and you land totally flat on the water, right,
21:07
Speaker 1: If you've done that, you know that water can feel
21:09
Speaker 1: like concrete, right, like belly flop, Like a belly flop, yeah,
21:12
Speaker 1: a belly flop exactly. If you fall off a bridge,
21:14
Speaker 1: for example, from high enough, you can die landing in water, right,
21:18
Speaker 1: because water just can't get out of the way fast enough,
21:20
Speaker 1: and so it pushes back on you just sort of
21:22
Speaker 1: the way the ground does right when you land on it.
21:25
Speaker 1: It doesn't get out of the way fast enough. Yeah,
21:27
Speaker 1: Like if you're falling through air, Air is much much
21:30
Speaker 1: less viscous than water, and it gets out of the way, right,
21:32
Speaker 1: It just moves out of the way. I mean, there's
21:34
Speaker 1: still some friction there, which is why you have a
21:36
Speaker 1: terminal velocity, but it gets out of the way fast
21:38
Speaker 1: enough for you to pass through it, right. But water
21:40
Speaker 1: is much much more viscous, so it provides much much
21:43
Speaker 1: more drag. And if you hit the water flat, then
21:45
Speaker 1: you're gonna get a big force the other direction, um,
21:48
Speaker 1: which is gonna make your belly smart or make your
21:51
Speaker 1: rock skip. So that's the key, right, is that the
21:54
Speaker 1: water pushes back up on the rock. You can't skip
21:57
Speaker 1: rocks on clouds or air. You can skip rocks on air, actually,
22:01
Speaker 1: I mean you can skip spaceships on air, but it
22:03
Speaker 1: takes much much higher speed. Yeah, well this is a
22:06
Speaker 1: perfect point to take a break. So it's about the
22:21
Speaker 1: viscosity of the liquid. So the more viscous it is,
22:25
Speaker 1: the easier it is to bounce it. Yes, you need
22:27
Speaker 1: a flat object and a viscous liquid. And that's a
22:31
Speaker 1: flat object, because if it's not flat, it's not flat.
22:34
Speaker 1: Like I mean, the most extreme thing is you drop,
22:36
Speaker 1: like you know, drop something really thin and sharp into
22:39
Speaker 1: the water, it's just going to separate the water and
22:41
Speaker 1: sink down to the bottom. Right, So you need to
22:43
Speaker 1: drop something flat so that it into the water can't
22:46
Speaker 1: move out of the way. You need to belly flop, Yes, exactly.
22:49
Speaker 1: Skipping stones should be called belly flopping stones, don't. They
22:52
Speaker 1: have funny names for this in other countries, like what
22:55
Speaker 1: they call in England. Um, in England they call it
22:58
Speaker 1: ducks and drakes, which makes absolutely no sense to me, Like,
23:01
Speaker 1: are you shooting ducks? Like what's going on? Well, like,
23:04
Speaker 1: if you're you're throwing stones at a leg to skip
23:07
Speaker 1: to create those ripples and skips. That that's called ducks
23:09
Speaker 1: and drakes. Ducks and drake's. Yeah, somebody out there in
23:12
Speaker 1: England explain that to us. In France they have a
23:14
Speaker 1: clever name for it. They call it rick o'che, which
23:16
Speaker 1: you know makes some sense, right, your rock is ricocheting
23:19
Speaker 1: off the surface of the water. What are some other
23:21
Speaker 1: funny words? Well, these are words that don't even make
23:22
Speaker 1: sense to me. Like in Ireland they call it stone scuffing,
23:25
Speaker 1: the way you would scuff your shoes and get marks
23:28
Speaker 1: on them, or something you got to say with an
23:29
Speaker 1: Irish saxon. I can't do an Irish accent, not at all.
23:34
Speaker 1: But every language has a word for it. And the
23:36
Speaker 1: funniest one is that according to the Oxford English Dictionary, Right,
23:40
Speaker 1: so according to the Brits, the way Americans say it
23:43
Speaker 1: is they say it's called dapping, and that's not a
23:45
Speaker 1: word I've ever heard. So, like, what are the Brits
23:48
Speaker 1: doing over there telling us what we call skipping stones
23:51
Speaker 1: and they're getting totally wrong. Well, it's all just ducks
23:54
Speaker 1: and drakes to them, So they got no credibility over there?
23:58
Speaker 1: Is that where the dap comes from? Where am I
24:00
Speaker 1: thinking of the dead. I am not culturally relevant enough
24:03
Speaker 1: to answer that question. But it's more than just the viscosity, right,
24:07
Speaker 1: and more than just having silly name. Okay, so it's
24:09
Speaker 1: more than just having um substance to the thing you're
24:12
Speaker 1: trying to skip on, which which water does it has
24:15
Speaker 1: enough fixed causity to skip stone? That's right? Because you
24:18
Speaker 1: know when you jump off a diving board and belly flop,
24:21
Speaker 1: you don't skip right, So there's a difference between belly
24:24
Speaker 1: flopping and skipping. And then what else is going on here?
24:26
Speaker 1: So another key is the angle. Right when you throw
24:29
Speaker 1: the stone, you want it the front edge of the
24:32
Speaker 1: stone to be higher than the back edge of the stone.
24:35
Speaker 1: So you want the stone when it hits the water
24:37
Speaker 1: for the trailing edge to hit first, all right, And
24:41
Speaker 1: the reason is that you're pushing down on the water
24:44
Speaker 1: and if you have that like a like a wheelie,
24:47
Speaker 1: like you're you're tilted back. Yes, exactly, exactly, Though I
24:51
Speaker 1: do not recommend trying to do this with your motorcycle
24:53
Speaker 1: or your mountain biketycle, or do it and send us
24:57
Speaker 1: a video. That would be awesome. I think that's called
25:00
Speaker 1: geezs and gooses in England. I think you're probably right.
25:05
Speaker 1: And um, what happens when you do that is then
25:08
Speaker 1: you push down the water at an angle, and the
25:10
Speaker 1: water creates kind of a ramp, right because you've you've
25:14
Speaker 1: pushed it down to the back end is further down
25:16
Speaker 1: than the front end, and so you have a ramp
25:18
Speaker 1: that slopes up. Okay, and here's where the speed comes.
25:22
Speaker 1: You've got to be moving fast. So you push down
25:24
Speaker 1: on the water to create a ramp, and then that
25:26
Speaker 1: ramp launches you back up into the air at the
25:29
Speaker 1: same angle. You're not really bouncing then, it's more like
25:32
Speaker 1: you're going into the water and then creating a ramp
25:34
Speaker 1: for you to take off of. Yeah, well you're doing
25:37
Speaker 1: both at the same time. One of my favorite things
25:39
Speaker 1: about sort of projectile motion in physics is how you
25:42
Speaker 1: can break things up into two dimensions that are totally independent. Right,
25:45
Speaker 1: motion in one dimension is independent from motion in the
25:48
Speaker 1: other dimension. Somebody in the question intersection said that the
25:51
Speaker 1: reason that you can skip a rocket is because it's
25:53
Speaker 1: moving really fast, um, in one direction, and so it
25:56
Speaker 1: doesn't fall down, which is totally not true. Right, Like
25:59
Speaker 1: you shoot the bullet, you know, it is moving really
26:01
Speaker 1: really fast parallel to the surface of the earth. But
26:04
Speaker 1: it's also at the same time falling and falling towards
26:07
Speaker 1: the surface of the earth is not affected by your
26:09
Speaker 1: motion parallel to the surface. And so you've got two
26:12
Speaker 1: things going on here. One is you're bouncing perpendicular to
26:16
Speaker 1: the surface of the water, right, but you're also moving parallel,
26:19
Speaker 1: So you've got the belly flop in the perpendicular direction,
26:22
Speaker 1: and you've got this little ramp action in the parallel direction. Well,
26:25
Speaker 1: what happens if if you don't hit the water at
26:28
Speaker 1: a good angle, like let's say your head, you can
26:30
Speaker 1: hit the water kind of leaning forward instead of leaning back,
26:35
Speaker 1: then you end up the front edge of your rock
26:37
Speaker 1: will dig into the water and it'll slow it down
26:39
Speaker 1: and it'll flip, and then you don't don't then you
26:41
Speaker 1: don't get any more skips. You don't create this little ramp. Yeah,
26:45
Speaker 1: because remember the key to getting a lot of skips
26:47
Speaker 1: is doing the same thing over and over again. So
26:50
Speaker 1: you want that when you leave the water, you're at
26:52
Speaker 1: the same angle as when you hit the water, right,
26:55
Speaker 1: so that you can hit the water again at that
26:57
Speaker 1: same angle. So anything that flips you over start to spinning,
27:00
Speaker 1: you're losing your energy and you're not gonna hit the
27:02
Speaker 1: water at the right angle the next skip. Well, I
27:04
Speaker 1: would have thought the spin was really important because without
27:06
Speaker 1: the spin, wouldn't you lose that angle really quickly with
27:09
Speaker 1: all the water slushing around. Yeah, spinning is important, and
27:13
Speaker 1: nobody wins the stone Skipping World Championship without a really
27:17
Speaker 1: good spin. But it's not technically necessary. But yeah, it's
27:20
Speaker 1: really important to maintaining the angle and to maintaining your speed.
27:24
Speaker 1: So there's a lot of different things going on when
27:26
Speaker 1: just one simple rock hits the water, all right, and
27:28
Speaker 1: then um so viscosity angle, and then what's the last
27:31
Speaker 1: critical ingredient? Last critical ingredient is speed? Right? If you
27:36
Speaker 1: are not moving fast enough, then you sink when you
27:38
Speaker 1: hit the water because the ramps are dissolves, right, that
27:40
Speaker 1: ramps is not doesn't stick around for very long. You
27:43
Speaker 1: hit the water right, and you bounce back because the
27:46
Speaker 1: water doesn't have a lot of time to move out
27:48
Speaker 1: of the way. But um, you want to get off
27:51
Speaker 1: of that ramp as soon as you can, because just
27:54
Speaker 1: pass the ramp as a little divid right, and so
27:56
Speaker 1: you want to get off of that ramp, and so
27:58
Speaker 1: you need enough speed. You need to me moving faster
28:01
Speaker 1: than the water is, of course moving faster horizontally or
28:05
Speaker 1: does it help to go fast a little bit, you know,
28:08
Speaker 1: into the water or do you get that anyways, you
28:11
Speaker 1: need both. You need if you if you're not moving
28:14
Speaker 1: fast enough perpendicular to the water, then you won't get
28:17
Speaker 1: the bounce right. You need to be hitting the water
28:19
Speaker 1: with enough speed that the water can't get out of
28:21
Speaker 1: the way and pushes back at you. You also need
28:23
Speaker 1: horizontal velocity so you can get off that ramp before
28:26
Speaker 1: it dissolves. But if you're going to slow then you know,
28:29
Speaker 1: you push the water into a ramp, but then you
28:32
Speaker 1: you don't take off, You just kind of move exactly.
28:36
Speaker 1: That's exactly the sound it makes. Why you must have
28:38
Speaker 1: done these experiments. That's what it sounds like in every
28:42
Speaker 1: slow motion video clip I've seen on YouTube. But every
28:47
Speaker 1: time you bounce, you also do lose energy. It's impossible
28:49
Speaker 1: not to because this friction and so you hit the
28:52
Speaker 1: water and it slows you down a little bit. So
28:54
Speaker 1: every bounce is in the best case scenario, like of
28:58
Speaker 1: the neck of the previous ounce, And so that's why
29:01
Speaker 1: you know, they get faster and faster. The time between
29:04
Speaker 1: bounces it gets smaller and smaller because you're not going
29:06
Speaker 1: as fast, you're not going as far up, so it
29:09
Speaker 1: doesn't take as much time to come back down. So
29:11
Speaker 1: that's pretty good. Like you only lose in each skip
29:15
Speaker 1: or is that like a you know, is that is
29:18
Speaker 1: that what the pros do? Or is that what happens
29:20
Speaker 1: when anyone skip stones? Is that it's always now that's
29:23
Speaker 1: the like top level performance, like eighty yeah, And that's
29:27
Speaker 1: the key to getting a lot of skips is losing
29:30
Speaker 1: the smallest amount of energy every skip, which means hitting
29:34
Speaker 1: the water at just the right angle. And it's one
29:37
Speaker 1: of these things where it's very sensitive. You're slightly off
29:40
Speaker 1: of the optimal angle, you're gonna lose a lot more
29:42
Speaker 1: energy in the first skip, and then you're gonna be
29:44
Speaker 1: off on the next angle and the next angle, and
29:46
Speaker 1: pretty soon you know you're sinking to the bottom of
29:48
Speaker 1: the lake. That's pretty impressive. So that's how they get
29:51
Speaker 1: to like a hundred yeah, exactly eight skips, which still
29:55
Speaker 1: blows my mind. It feels like that would take like
29:57
Speaker 1: an hour, you know, Like you throw the rock and
29:59
Speaker 1: then you're just like and minutes later, still scount counting
30:02
Speaker 1: skips or something. Well, towards the end, it's it's it's
30:04
Speaker 1: not like skips skips skip. It's like, yeah, exactly, it's
30:08
Speaker 1: like stuttering skips at the end, which is what tells
30:10
Speaker 1: me what these really like massively competitive events. They must
30:14
Speaker 1: have some sort of camera that's taking pictures to count
30:16
Speaker 1: these skips. They can't have like a super spot or
30:19
Speaker 1: something just using their eyeballs. You think it's at that
30:21
Speaker 1: high tech. You think they have like corporate sponsors and
30:25
Speaker 1: pro athletes, probably some Swiss watch companies you know, sponsoring it, etcetera. Yeah,
30:30
Speaker 1: but the top guys have groupies and travel around and
30:33
Speaker 1: fancy busses and all sorts of stuff. All right, So
30:35
Speaker 1: then have you tried this? Since you're learning the secrets here,
30:38
Speaker 1: have you tried and does it help? Did it help
30:40
Speaker 1: you skip stones better? No? And that's the thing about physics.
30:44
Speaker 1: It tells you how the universe works, It gives you inside,
30:47
Speaker 1: but it doesn't help you actually navigate the situation. You know.
30:50
Speaker 1: For example, we did that whole podcast episode about how
30:52
Speaker 1: to ride a bike, right that just made me ride
30:54
Speaker 1: bikes any better, especially because we learned that physics doesn't
30:57
Speaker 1: know why why bicycles. Allan's right, but you can still
31:01
Speaker 1: ride a bike. So there's this disconnect between understanding something
31:05
Speaker 1: and being able to do it. Well, you know, it
31:07
Speaker 1: did help me. I mean after learning all of this,
31:09
Speaker 1: I did try it, and it does help. Like if
31:12
Speaker 1: you know that you have to throw it at the
31:14
Speaker 1: water kind of at a little bit of an angle
31:17
Speaker 1: leaning bag that's a huge hill, like because you know
31:20
Speaker 1: before you I would try to like throw it at
31:23
Speaker 1: the water so it's perfectly parallel to the water. I
31:26
Speaker 1: thought that was the secret, you know, but like once
31:28
Speaker 1: you know you have to kind of lean it back
31:30
Speaker 1: a little bit. It does help you skip better. All right,
31:32
Speaker 1: well you go, I skips, but you know, physics has
31:36
Speaker 1: improved your life for her for the first time, it
31:38
Speaker 1: ruined my vacation, but it increase my standing in the
31:41
Speaker 1: world rankings of It's amazing to me that the universe
31:44
Speaker 1: is mathematical, that we can understand it at all, that
31:47
Speaker 1: it seemed to follow these rules and we can write
31:49
Speaker 1: down on paper manipulate with our minds um. It's amazing
31:52
Speaker 1: that that works, and it's a joy to see what
31:54
Speaker 1: it happens. You know that we can actually isolate the
31:56
Speaker 1: critical elements and understand something. So the next time you
31:59
Speaker 1: see something weird, the next time you see something you
32:01
Speaker 1: don't understand, you know, think about the physics of it,
32:03
Speaker 1: what's making that happen? Um, what's going on inside is
32:07
Speaker 1: there's some microscopic explanation that can help you understand why
32:10
Speaker 1: those ducks are chasing that funny bird or you know
32:13
Speaker 1: why those clouds form in that weird way. And there's
32:16
Speaker 1: physics all around us. And if you don't understand, just
32:18
Speaker 1: send the question to Daniel and we'll answer it on
32:21
Speaker 1: the podcast that's right before after I do a bunch
32:24
Speaker 1: of experiments at Jorge's house skipping gold coins on my Hey,
32:29
Speaker 1: can I borrow a few of those gold coins please?
32:31
Speaker 1: I gotta do some experiments. Whoops, they sank to the
32:33
Speaker 1: bottom of the lake. Sorry about that? All right? Well,
32:38
Speaker 1: we hope you enjoyed Dad, And as Neo says, whoa,
32:41
Speaker 1: thanks for tuning in. See you next time. Before you
32:52
Speaker 1: still have a question after listening to all these explanations,
32:54
Speaker 1: please drop us a line. We'd love to hear from you.
32:57
Speaker 1: You can find us on Facebook, Twitter, and Instagram at
33:01
Speaker 1: Daniel and Jorge that's one word, or email us at
33:04
Speaker 1: feedback at Daniel and Jorge dot com. Thanks for listening
33:08
Speaker 1: and remember that Daniel and Jorge Explain the Universe is
33:10
Speaker 1: a production of I Heart Radio. For more podcast from
33:14
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33:18
Speaker 1: or wherever you listen to your favorite shows. Yeah.