Motion And Meaning

And I’m Cennydd Bowles. In our last episode, if you were listening, you’ll have heard us talk a lot about prototyping tools and boy, there are quite a lot of them, so that was a fairly long and deep episode…

Yeah! We’re going to continue that theme of being quite full-on and deep here, or relatively so; we’re going to cover physics today. We’re going to cover some simple physics of motion which is perhaps not a subject that a lot of people know a huge amount about or even may have learned about it at school or something like that and maybe it’s just slipped from your memory. I have one advantage here, in that I have a Physics degree, which has been utterly useless for my career, to be honest. Until now. Until this episode of this very podcast, I think. So I’m going to try and talk through some of the things that I remember about basic mechanics and how those translate to motion in digital systems. A couple of caveats here: the first is, essentially, that this was fifteen years ago that I studied this stuff!

It was a while ago, so you never know. I don’t think anything has fundamentally changed, but my brain may not have quite the capacity for this stuff that it used to! And clearly, we’ll be talking about simple stuff here; we’re not going to be going into detailed stuff around quantum mechanics and non-euclidean spaces and inertial frames of reference and so on.

Frankly, nor do I these days! Simple pushes and pulls and some simple mechanics. But hopefully there’ll be some things in here that will help you, the listener, make the motion in your products feel that bit more natural, feel that bit more appropriate for what you’re trying to do.

That sounds good. I’ll try and provide relatively smart answers if I can. I think you’re right, a lot of people will have played around with friction and things like that in systems; we talked about some of the physics modelling in Hype, for instance, in the last episode. Having some knowledge of what those actually correspond to is useful but I wouldn’t want anyone to come out of this episode thinking, I’ve got to calculate everything! It’s always going to be, do it by eye, see what works best, but understanding some of those principles will hopefully get you just that little bit closer.

Yeah, let’s hope so. OK, so I’m going to kick off, we’ll start talking about Newton’s Laws which hopefully ring a bell for a lot of folks in the back of their heads there. We’ll start with the idea of force, which is really what sits behind all motion. All motion comes from some kind of force and a force is just a push or a pull and that can be physical: it can be a donkey pulling a cart; it can be me kicking a football or it can be essentially invisible; it can be something like gravity or it can be friction and so on. But all of these are forces acting on the bodies, the elements within an environment. Without force, there is no motion.

Uh-huh, yep. Absolutely. And even when you’re moving yourself, you’re propelling, you’re swinging your leg forward through contraction of muscle and so on; that exerts force on your leg or whatever it is. There are complex forces that come into play as well when you step on the ground, you exert a force on the ground, the ground exerts a force on you; there’ll be resistance, there’ll be friction, which are all forces as well. So you can draw these kind of force diagrams and you’re trained to do this in the UK, GCSE Science level which is sixteen year old exams for us and you get quite familiar with drawing these force diagrams and what you quickly learn is, there are forces everywhere!

Something that looks simple…yeah, there are just forces that act in all sorts of different ways. What we’re not suggesting here is that you model all of them, but just you’re aware of some of them. So, Newton’s First Law essentially flips what I said about force causes motion. It says that without forces, motion continues uniformly. And it’s stated thusly which is: ”Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.”

Great! This is sometimes known as the Law of Inertia and it’s most simple application is actually if you think about something in space and we’ve seen the films where an astronaut’s cut adrift and they’re just flying through space and you know they’ll be doing that forever because that’s the way it works; there’s actually no force in space that’s going to slow them down, so that motion will continue unimpeded. Over time, they may collide with asteroids or who knows, which in turn there’ll finally be some forces there, but that’s an example of Newton’s First Law; this motion continues in the absence of a force. Equally, motion with a velocity of zero, i.e. no motion at all, stays as no motion until there’s a force applied. In the environment of Earth, there are always forces acting in the other direction, however, so if you fire a cannon ball, you know gravity is going to pull it down; you know air resistance is going to get in the way. When it hits the ground it’ll make a little divot; it will bounce back at a much lower rate than it impacted and so on. Earth has all these complex resistive forces as well, so that’s why when you have an astronaut floating through space, the same would not happen in Earth because you have these forces that will essentially drag that motion backward or slow it down and so on.

I think what I take from this, from Newton’s First Law is when I’m thinking about a design, thinking about an animation I’m going to do, I’m asking myself, what are the forces involved here? What’s actually happening? What’s giving something a push or what’s giving something a pull? If it’s a push, how can I signify that? Could I even do it by deforming on one side, like kicking a football, that kind of thing, does it compress before it goes out, or is it a pull, in which case maybe it elongates, maybe like a tug-of-war, like a rope or the guy with a hook on the side of a comedy stage…

Getting rid of a bad performer, is it that squash and stretch that we talked about way back in Episode 2 there. So I just started to think about, on a very basic level, where are the pushes and pulls coming from? What’s causing this motion in the first place because that might change the way that that motion happens, I suppose.

Yeah, maybe you’re right there I think. You can also then consider, is it a constant force or a one-off force? If you think of coming back to the football example, a kick would be more or less a one-off force; the ball would compress slightly and then it’s left the foot and it’s gone and it will follow its own trajectory, versus me lifting the football and running with it, which is then a constant force and the ball just behaves in different ways there; the acceleration will be different: we’ll come onto that in a second. So you can think about how you model those types of forces as well and the impact they’re going to have and the trajectory that’s going to come from those two.

Sure, OK. That kind of sets up Newton’s Second Law, quite well, actually. The short answer is, it is not true that things fall at different rates if they’re different mass. All other things being equal, so assuming that they’re made of the same material and they have the same shape, if you drop a bowling ball, it will fall at the same rate as the empty shell of a bowling ball. Actually it wouldn’t because of air resistance, but a bowling ball made of a lighter material; a sixteen pound versus an eight pound: they will hit the floor at the same time and actually Galileo did an experiment to this on the Leaning Tower of Pisa, where he dropped two similar masses; of different mass but similar shape and property and demonstrated, lo and behold, they do actually hit the ground at the same time. I’ll talk a bit about Newton’s Second Law before I come to explaining why that happens because you kind of need to know.

So, Newton’s Second Law is essentially just an equation and it’s a really simple one: f=ma, so it says that the relationship between an object’s mass, its acceleration and the force that’s applied to it are related in those ways. So, if you have a constant force and you have a large mass, something that’s heavy, it’s going to accelerate slowly; you have something that’s very light, it’s going to accelerate very rapidly, very easily, which makes obvious intuitive sense there. Where it gets a bit complex is, there is a bit of a pitfall here when people think about gravity because you think, well, gravity’s constant, right? So, if the mass is different, then the acceleration is going to be different as well. What actually is happening is the force exerted by gravity is not constant on these objects; the force is different. Basically, it exerts more force on the heavy object than it does on the lighter object. We don’t exactly know how it does this! One of the things we’re still, I think Physics is still trying to figure out is, what exactly at the sub-atomic level causes gravitational pull? And there’s some theory of exchange of gravitons and all these sort of theoretical sub-atomic particles. I think they’re called gravitons: I may have just embarrassed myself there!

Well, who knows? And it’s kind of counter-intuitive. You think: how does the world know to exert more pull on this one because it’s heavier? And those are the sort of questions that I would dread receiving were I a physics teacher because I do not know the answer, but actually what happens is, the force is larger, so that actually essentially it equalises out so if the mass is higher, the force is equally high, so the acceleration is actually the constant thing. So those would hit the ground at the same time, in that scenario.

It gets more complex when you have things like feathers or hollow bodies where air resistance particularly becomes a big factor.

The shape and the material as well. I actually did my Masters…not my Masters dissertation; my Bachelors dissertation was about flutter; so you drop a feather and it kind of goes side to side in the air and so analysing why that happens and that is fascinating and definitely beyond the scope of this podcast, but suffice to say it’s the shape, it’s the length of the object that actually causes that kind of side to side motion. And if it’s particularly small, it will actually tumble side over side rather than swing side to side. It’s very much like pendular motion; it’s very much like walking, there’s a lot of physical comparisons there. So, one of the nice things about f=ma is you can use that to calculate how velocities are going to change under force. A little bit of technical stuff here, which is just a bit of pedantry, which is to say that velocity is actually a different thing than speed.

You often think of these, we use them pretty much interchangeably; ten metres a second velocity or speed. Velocity actually has a direction associated to it; it’s a vector. So, to be technically direct, velocity is ten metres per second due west on a bearing of 190 or directly up in the air or whatever it is. So, when we’re saying that a force causes a change in acceleration, that’s actually…sorry, it just causes an acceleration, that actually is just any change of velocity. That doesn’t mean it has to slow down or speed up; it can just change direction. So, if I’ve got, I don’t know, let’s say a ping-pong ball travelling in a straight line and I blow on it from the side, it’s going to deflect in the air. It’s probably not going to slow down; it might do just a tiny bit but my breath is not going to be enough but it will deflect it; that’s a change in velocity, that is an acceleration there, even though it’s not travelling any faster; it’s the direction that’s changed and that counts as an acceleration, so I’m actually putting that force in from the side. That’s a little bit overly pedantic, I think, but it means you can bring that into some design thinking when you’re thinking about the properties you want to convey and this is where it comes down to what you were saying before is, we can use this principle to explain the mass something has; is it a heavy object? What’s the force being applied to it? And then, where’s that force coming from because that may push it; it may speed it up, it may slow it down if there’s something like friction, or it may just deflect it, it may just move it left or right or up and down even. So, all of those things can help to suggest what’s happening in the interface. Are the forces coming in this direction because look, that thing’s moved and it’s moved a long way, so it’s either a very light object or a very strong force.

You’ll always find in a deflection, I think you probably always lose just a tiny bit of speed; I don’t think you could ever do it without losing any because of just the friction but it could be negligible.

So, moving on, I said was going to cover Newton’s Laws and there are three of them, so we’re pretty much done with Newtonian stuff here. Newton’s Third Law is probably the most famous one, which is for every action there’s an equal and opposite reaction. That’s the one a lot of people have heard…

It’s kind of the punchiest one; Newton Two is just an equation really, so that’s not very memorable, equal and opposite reaction applies to a ton of things but it’s really about what we call momentum which is represented by the letter P in physics and it’s a combination, the factor basically, of mass and velocity. So we think about momentum and we know that, say, a bulldozer has a lot of momentum. If you’re hit by a bulldozer, you’ll know about it, even if it’s going ten miles an hour, you’ll know about it a lot more probably than if you’re hit by a child’s trike going fifteen miles an hour because the momentum is so much more. The mass combined with the velocity is essentially the thing that does the damage there.

The idea of this equal and opposite reaction basically says that momentum is conserved. This idea of conservation of momentum basically means, if you’ve got something going one way, generally, something will go the other way to balance it out. So, rockets use this. In space films you see, again, astronauts who are cut loose and they’ve got their little jet-packs on and they can use their thrust nozzles or whatever to right themselves; or Wall-E with his fire extinguisher in Wall-E. That’s the conservation of momentum right there; essentially, particles go that way, I go the other way. And you can use the mathematics; you can say mass x volume on one side, mass x volume on the other and they should balance out to be zero, basically. Another example would be a bullet from gun. You have a lot of momentum going in one direction and you have to have the same amount going the other direction backwards; that’s why you have recoil. So, a bullet has very low mass, very high velocity; the gun has very high mass and very low velocity so therefore the recoil is only an inch or so…

Right, exactly. Which is why actually guns need to be a little bit heavy; if you made a lighter gun it would actually recoil more dangerously; it would recoil with higher velocity because of the conservation of momentum, so it’s actually beneficial to have a bit of bulk; another reason why you put your shoulder behind, I believe: I’m not a gun enthusiast but I believe you put your shoulder behind it because then it essentially adds all of your mass to that equation.

Very likely! Exactly. So that’s all coming down to the conservation of momentum. The reason I think this is an interesting one for digital design is, it kind of comes back to the choreography stuff that we talked about in Episode 4, which is essentially if this thing is moving, does something else need to move too?

Yeah.

Yeah, that’s it. If you’ve got a ton of stuff moving all the way to the right, is there something visible that should move left to balance the physics? The answer may be no, because the things that can move, that could just be air, it could be dust; we’re talking about physical environment here.

No, exactly. You can still throw a ball and you don’t feel the recoil, but there technically is one there. So, it’s not something you need to go, oh well, this is moving, therefore I need something going the other way. But it’s just an option. If you’re going for more of a kind of propulsion model: I’ve given this a kick or this moves this out the way, at that collision, at that point of impact where the force is applied, should there be a recoil? Anyway, it’s something that you can think about. There are some tools that will do that for you when you’re modelling collisions and so on: again, something like Hype has some of that collision physics built in as well, so those are your three mail laws of Newtonian Mechanics, as it’s known.

I just want to talk about one or two other things. I want to talk about friction, if that’s OK?

I think again, there’s no definitive answer. Pretty much most of our interfaces are essentially ice; there’s almost zero friction. Something starts sliding off screen and it’ll keep going; or sometimes it’ll even speed up, which of course suggests it’s being pushed because there has to be a force being applied there. But maybe there’s something interesting we could do about saying, well what is the friction of our screen? What material are we trying to model here? It may be that we say, it doesn’t matter, it can be ice, it can essentially have what’s called a co-efficient of friction of zero and that’s just basically a multiplication factor, like anything around one, two, three is very friction-y: a rug or something like that, or sandpaper or something.

Yeah. And then the other end would be something like ice or glass that has very low friction that would have a co-efficient of friction nearer zero. There’s a case for saying perhaps you could even have different surfaces within your app so if we’ve got a button or an avatar that’s moving between boundaries, a white bit and a green bit, it depends how far you’re taking it; you might be in this realm where you want to actually model physical properties. Here’s an example: dealing cards onto a table. Let’s say you have a white and a green bit and cards are sliding across the screen. The white bit is probably air, that’s what you’re modelling. Low friction, it’ll move fairly unimpeded. When it hits the green stuff, it’ll slow down, it’ll stick a lot more quickly, so maybe you model it and maybe that transition helps to communicate the properties of those surfaces. Oh, that’s the table; oh, that’s the air and the dealer’s sort of flinging them onto the baize there.

Yeah, exactly. All motion happens through a medium essentially, and the medium has an impact. In space, the medium is essentially zero, the odd cosmic ray or the odd asteroid aside. But certainly on Earth, you’re travelling through air, which is surprisingly sticky actually. Or you’re travelling through water or you’re travelling on a surface or whatever, and those media have an effect on motion, so again, it’s optional, but you can choose to model that stuff if you want that to represent something to bring more of that kind of physical feel to the work you’re doing. Most people, frankly, don’t bother with digital stuff and I can see why; it’s extra work. But there may be…

Yeah. Coming back to the card dealing example: if it’s important that your user understands the green bit is the baize because that’s where you’re looking at, because that’s where your hand is dealt and so on and that’s where you should be paying attention, then that might be a good thing to do. You’ll have visual cues as well: you might have a little cutesy poker table with dogs sitting round it or whatever, but give it a bit of friction as well and that’ll help communicate what’s happening there.

Motion through air, I’ll just talk about very quickly, just maybe as a last sort of thing. Air resistance is surprisingly sticky, as I say, and that actually varies with the square of the velocity, so what that means is, at high speed, air resistance gets a lot more resistant the faster you go, which is why racing teams spend so much money on the aerodynamics of their vehicles to reduce the air resistance and so on. Air is a fluid, technically, although it’s a gas, gas is actually a fluid and fluid has viscosity, essentially how much does it resist flowing? How much does it stick. Consider water pouring…a glass of water versus a glass of honey. Essentially that’s viscosity: that’s essentially resistance and friction happening right there. So there may be benefit, if you’re trying to convey that something’s moving really quickly, it’s actually likely, almost counter-intuitively, it’s actually likely to have more air resistance countering it at higher speeds.

Yep.

Yes, it would. But only at high velocities. Fluid dynamics is really complex and way beyond the graduate level that I studied.

And I don’t even pretend to understand it but again, if you’re trying to model something going really fast, kind of almost counter-intuitively, you may want to model some resistance, some air resistance there, because actually we’re expecting it because that’s how the real world behaves. And there are different ways you could do that: you’d either just slow it down or even you could do, when things re-enter the atmosphere, satellites and all this sort of stuff, they start to glow around the edges, these other signs of friction.

Yeah, that might be a bit tacky to do it on a button as it flies off-screen, but if you’re modelling something else, you never know: maybe that friction is seen in a different way. Or maybe it’s sound or light and heat and so on, different ways that can manifest.

Yeah, absolutely. And I think we talked about the idea of magical realism earlier, didn’t we?

Yeah, and it’s kind of similar to the cartoonists of old found that you don’t have to obey physics: you can stretch it, you can tweak it and there’s a lot of fun to be had in doing that, but probably for 5% of your interactions. If you just ignore gravity for your cartoon, unless it’s set in space, it’s going to look really stupid, so you need …it’s one of those you need to understand the rules to understand when they’re best broken, I suppose.

Exactly. The last thing I want to cover really quickly is gravity. And actually, one thing people tend not to realise about gravity is it actually happens between any object. Any two objects will have a gravitational pull.

We kind of think it’s the Earth pulls the Moon to it or you drop an apple, that sort of stuff. We know planets have gravity. Everything has gravity so the elephants in London Zoo right now are attracting me towards them through gravitational forces.

Likewise, I’m attracting them through gravity. But at a very, very, very weak level. Physically the atoms in my body probably don’t really move significantly…

But theoretically, if there were no friction, if we were spherical lions in a vacuum, all this sort of stuff, then eventually in millions and millions of years, we would end up next to each other! But the attraction between those bodies, you mostly see it around planets and stuff because they’re really, really big. It’s related to the mass, so, heavy stuff attracts more and then it’s related to the distance and particularly it’s very, very weak at long range and very strong at short range, so that’s what causes the acceleration that you see when you drop something: it starts slowly, the acceleration brings it in and it’s actually fastest right at the point of impact.

But the Earth’s gravitational pull is different at different radii from the centre of the Earth as well so it’s strongest on sea level or below it and it’s much weaker in space, which again is why you can have things in orbit at however many tens of thousands of kilometres or whatever they are. But essentially, that’s the thing to be aware of, that it’s a function of mass so if you’ve got two things that you need to represent as heavy objects, they’ll actually be slightly drawn toward each other and maybe even just as they move, they’ll attract ever so slightly.

There’s a terrific game which I’d recommend anyone who wants to look at this kind of stuff, it’s a game for iOS called Eliss; E-L-I-S-S and essentially it’s around planetary motion and it has this 8-bit retro aesthetic and planets are drawn towards each other and you can split them up and you have to move them into certain spots with multi-touch. And it’s a great game but it has light gravitational pull; planets start to come together or there’s a black hole and it will draw all these masses together and then you’re trying to put all the fingers to hold them apart and so on.

More about gravity at a cosmic level; I really recommend Eliss; it’s a terrific game. You may not have to model quite that much in your ecommerce application.

Yeah, I think it’s really interesting stuff anyway and you never know, there may be some interesting and unusual explorations that you can come to as a result of knowing how that works.

OK, goodness me, that’s a kind of whistle-stop tour of some physics there! I have long term plans to write some of this stuff up, so have a look out. If I can use the benefits of my expensive education for other people’s gain at some point, then that’s hopefully a good thing. Anyway, it’s definitely worth poking around if it’s interesting. Wikipedia’s got some decent articles on this and we’ll try and dig out some other resources.

Yeah, diagrams and things like that. Like I say, the day to day different it makes may be very small but hopefully things like some air resistance, some friction, things falling under gravity and how you depict force and mass; hopefully we’ve covered some stuff there that’s going to be useful for you when you’re doing that kind of work anyway.

Yeah, do you want to go first?

We’ll see!

I do have a Medium article, I’m afraid:

It’s on spatial interfaces; it’s by Pasquale D’Silva who we’ve mentioned I think a couple of times in this series. Pasquale has done a really good job of talking essentially about how you use spatial metaphor and it touches along some of the similar stuff we were talking around choreography and saying, if things exit stage left, they should enter stage left and so on. But it’s a nice post with some good examples as well; I think he’s chosen some nice illustrations, some nice graphics.

So I recommend that one. As always, we’ll put the links to those articles in the show notes and put them on the website so you can have a chance to check them out for yourself.

You’ve been listening to Episode 7 of Motion & Meaning with Val Head and Cennydd Bowles. You can find out more about this show at motionandmeaning.io and we’d love to hear your feedback on Twitter and we’re @MotionMeaning there. See you again soon.