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- This is actually one of those videos
where I explain a thing by making a 2D version of that thing
because the 2D version is easier to understand.
Like these two videos,
link in the description and in the card.
The difference with this video is that
it's not about a hydrodynamic mechanism,
it's about a structure.
A type of structure called a tensegrity structure,
like this beautiful tensegrity table here.
I got this from a company called Stirling Engines .co.uk.
It's also where I got my Stirling Engine,
which you'll recognize from my video explaining entropy,
link in the description for that as well.
Stirlingengine.co.uk aren't sponsoring this video,
I just really like the things that they make.
But anyway, in this video
I'm going to explain how a tensegrity table
like this works by making a 2D version of it.
But first let's clarify,
what's the criteria for a tensegrity structure?
Well, the first thing is it needs to have components
that are in compression and components that are in tension.
So these rods here are in compression.
They're being compressed.
They're made of aluminum.
They're really good at that.
And they've got these wires here that are in tension.
Wires are really good in tension.
The other criteria for a tensegrity structure
is there needs to be at least two separate parts
that are in compression,
that are entirely separate from each other,
and only joined by parts that are in tension.
So here, for example, the bottom part
and the top part are separate from each other.
They're not touching except for
these wires that are in tension.
So this is an example of a tensegrity structure as well.
You've got these six wooden rods that are in compression.
They're only joined together
by these elastic parts in tension.
So the wooden parts aren't touching each other at all.
And I'll tell you a bit more about this later on
'cause there's some really interesting stuff
about this particular structure.
But first, just to go back to the table,
it's really counterintuitive, isn't it?
It's all being held together by wires.
Like, how is that possible?
And that's what I want to explain in this video.
So here's the 2D version.
And I've built it with elastic bands instead of wires
because I want you to see how they bend
and stretch when you try to collapse the structure.
So why is this structure stable?
Well, any way that you try to collapse the structure
it turns out that at least one
of the elastic bands ends up being stretched.
And when you stretch an elastic band, it pulls back.
So for example, when I try and collapse the structure
in this direction, this elastic band stretches,
and it restores the structure
back to its equilibrium position.
And we can look at individual elastic bands.
For example, if we try to keep this elastic band
and this elastic band the same length,
and try to collapse the structure, what would happen?
Well, we need to sort of do this, somewhat like that.
And of course in doing so
we're stretching this elastic band.
Similarly, if we trying to keep these two elastic bands
the same length, you know,
we're trying to collapse the structure.
This one is stretching.
Or if we try to keep these two the same length
we'd end up doing something like that,
wouldn't we, or something like that.
And so the middle elastic band stretches.
So hopefully, just by inspection,
you can see that no matter how you try
and distort this tensegrity structure,
the stretching of the elastic bands will restore it
back to its equilibrium position.
Just to hammer the point home,
I've replaced two of the elastic bands with string.
So we really can't stretch these ones now.
And look, we don't have that much freedom of movement.
I can do that and I can do that.
And either direction that I go in
the elastic band ends up stretching.
A 2D tensegrity structure actually isn't stable
in three dimensions, which is why I've got it
leaning on this incline here,
that keeps the whole thing from falling apart.
So a 2D tensegrity table requires
two outer cables to remain stable.
But for it to be stable in three dimensions
you need to add a third cable, like in this one here.
And that makes sense intuitively.
It's the same reason why a stool
requires three legs to be stable.
It's that old saying, two points fix a line,
three points fix a plane.
So it doesn't matter what direction you try
and push the top of the table in,
it will stretch the wires in such a way
as to restore the table back to equilibrium.
It does mean that actually there is
a bit of motion laterally like that
because there is some flexibility in the wires.
And it's a really strange sensation actually,
because you can move it laterally like that,
but it's really strong in the up and down position.
It's very strange.
You can even set up a bit of simple harmonic motion
in the top, isn't that cool?
Actually with this design,
you can tune the stiffness of the wires.
So if I let out a bit of the stiffness in these,
you get a much wobblier table.
Having said all that, it is actually possible
to create a stable three-dimensional tensegrity table
with just two outer cables.
This one's made of Lego.
And you can see, because the center of mass is here
the whole top section wants,
in inverted commas, to tilt this way.
But it can't because of these cables in tension.
And it's only stable for as long as
you don't push it in this direction.
It's not an official Lego product,
but if you search for tensegrity Lego
you'll find one if you want to make one for yourself.
Strictly speaking, for a structure to be
classed as tensegrity it needs to follow an extra criteria,
which is that all the parts that are in compression
need to be only in compression.
And that's actually not true here.
So you've got that bend there.
So this part is experiencing a bending force.
So there'll be a part on the inside that's in compression,
part on the outside that's in tension.
Which is okay, 'cause well this is probably aluminum,
and aluminum is pretty good in tension.
But this structure is a tensegrity structure
even under the strict criteria.
All the bits in compression are straight.
All the bits in tension are straight as well.
And so the bits in compression only feel compression.
The bits in tension only feel tension.
So by building the structure this way
you can make it much stronger
because the parts that are good at compression
are only experiencing compression.
The bits that are good in tension
are only experiencing tension.
You could also make it lighter as well.
You can use less material to get the same strength.
Like this basswood is good in compression,
but if I bend it like this,
it experiences tension at the top.
And it snaps.
Which is why NASA are designing a planetary lander
based on this exact configuration,
because it's incredibly robust.
You know, you can bounce it around,
you can land it on a planet.
It might squish a little bit.
But if you put a payload in the middle,
maybe it will survive.
And by changing the length of the cables in tension
you can actually move the structure around robotically.
Because tensegrity structures optimize
for strength and lightness,
and because evolution often optimizes
for things like strength and lightness,
it wouldn't be unreasonable to expect
to find tensegrity structures in nature.
And in fact, if you look at the bones in your body
that are under compression, and the muscles,
tendons, and ligaments in your body that are under tension,
you sometimes find them in tensegrity-like arrangements.
And of course, architects and engineers have put
tensegrity structures to good use.
For example, the Kurilpa Bridge in Brisbane, Australia.
All right, final thing on the subject.
If you take some Blu-Tack
and squeeze it in one direction,
it gets wider in the other direction,
which makes intuitive sense.
And most things behave that way.
But not all structures are like that.
Some structures when you squeeze them
in one direction will actually contract
in the other direction.
That's called an auxetic structure.
And some tensegrity structures behave that way as well.
For example, the NASA lander structure.
See if I pull these two rods apart,
perhaps you can see very slightly
that the green and red rods come apart as well.
So there you go, tensegrity structures.
I actually showcase the tensegrity structure
without even realizing it in a previous video.
The first episode of Five Interesting Things.
So check that out if you haven't already.
I've got a few more Skillshare
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I hope you enjoyed this video.
If you did, don't forget to hit subscribe.
And I'll see ya next time.
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