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
secrity table here I got this from a
company called Sterling engines. co.uk
it's also where I got my Sterling engine
which will recognize from my video
explaining entropy Link in the
description for that as well um Sterling
engines. 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 10 seg table like
this works by making a 2d version of it
but first let's clarify what's the
criteria for a 10 seg 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
aluminium they're really good at that
and they've got these wires here that
are intention 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
intention so this is an example of a
tense secrity structure as well you've
got these six uh 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
uh at all and I'll tell you a bit more
about this later on because there's some
really interesting stuff about this
particular structure but first just to
go back to the table like 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 um something
like that and of course in doing so
we're stretching this elastic band
similarly if we try 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 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 10 segy
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 leg 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 know 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 uh in
these you get a much wobbler
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 10 seg
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 see 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
cuz well this is probably aluminium and
aluminium is pretty good in t ion 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 you know it might squish a little
bit but if you put a payload in 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 tens seg like
arrangements and of course Architects
and Engineers have put tensity
structures to good use for example the
Clipper bridge in Brisbane Australia all
right final thing on the subject if you
take some blue Tac and squeeze it in One
Direction it gets wider in the other
direction which makes it 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 oretic
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
10 secrity structures I actually
showcased the 10 secrity structure
without even realizing it in a previous
video uh the first episode of five
interesting things so check that out if
you haven't already I've got a few more
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this video if you did don't forget to
hit subscribe and I'll see you next time
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