- They call this an atomic trampoline,
and for a brief period in the late naughties,
you can actually buy one of these things.
But I didn't know about them back then
and I've been trying to get hold of one ever since.
This one is on loan from Grand Illusions,
linked to them in the description.
They're not paying me to say that.
I'm just really grateful.
The thing that's remarkable about the atomic trampoline,
of course, is how long the ball bearing bounces for.
A quick note on language, this is a ball bearing.
The balls inside are also called ball bearings.
I know that makes some people angry.
Those people want those balls inside to be called
bearing balls, not ball bearings.
And I can understand why, but most people call the balls
inside a ball bearing ball bearings.
So I'm gonna call them ball bearings.
But the point is look at it bounce.
That last one was 37 seconds.
Compare that into how long a ball bearing bounces
on just a regular slab of steel.
The reason the one on the left bounces for so much longer
is because there's a thin layer of amorphous metal on top.
Amorphous just means that the atoms in the metal
don't form a regular lattice arrangement.
They don't form a crystal structure.
Instead the arrangement of atoms just looks random.
I'll get to why amorphous metal is bouncy in a minute,
but first I want to see if I can significantly
improve how long this thing bounces for
by tweaking a few things.
So how might I be able to increase the bounce of this thing?
Well, this is the ball bearing that came with it,
but is it the best ball bearing for the job?
Ball bearings are made of different materials,
stainless steel, hardened steel, chrome-coated steel,
glass ball bearings, ceramic ball bearings.
I couldn't get hold of a diamond ball bearing,
but I did get a ruby ball bearing.
Here are a few different ball bearings compared together.
(Balls bouncing)
You'll notice I was able to remove the clear plastic tube
from some of them, but not all of them.
That's because they either stopped bouncing
before I got the chance,
or because they kept bouncing side to side
and hitting the walls of the tube.
So by testing these different ball bearings,
I'm not just testing the material,
but I'm also testing the tolerance.
The balls that are bouncing side to side,
they're just not spherical enough
and that's what's causing the lateral motion.
The ruby in particular is disappointing,
'cause that costs a lot of money.
(balls bouncing)
So the ball bearings that outperformed the original
were that sodium lime glass.
The AI SI 52 100 steel,
and a type of ceramic called zirconium oxide.
So, armed with that information,
I bought even more ball bearings.
Basically those ball bearings but in a few different sizes
because maybe that's important.
I didn't go too big or too heavy,
because I didn't wanna risk damaging that surface,
especially as as it's on loan.
So it looks as though, if the AISI 52 100 steel ball bearing
is too big or too small, that affects the bounce.
If it's too big, that's probably
because the ball is managing to transfer
a lot of the energy
through that thin layer of amorphous metal,
and then into whatever that piece of metal is sat upon.
Just anecdotally you can really feel the vibrations
in the table every time a large ball bearing hits the plate
which is why I placed the assembly on this breeze block.
It's heavy and it's rigid.
It's hopefully gonna be less dissipation of energy
through that surface,
and that's certainly what I've noticed experimentally.
So the optimum diameter for this ball bearing
seems to be five millimeters.
These are the zirconium dioxide ball bearings, and again,
there seems to be a sweet spot in the middle,
but why are the smaller ball bearings
not performing as well?
I think it's probably for two reasons.
The first is air resistance disproportionately
affects the smaller ball bearings.
That's covered quite nicely in Veritasium's recent video
about dropping pennies
from the Empire State Building,
but in short it's because of the square cube law,
which I won't go into in this video.
The other thing to consider
is what happens at the very end of the bounce.
Like if the height of the ball is going down
by a fixed fraction every time,
then it should never stop bouncing.
The height should just get closer and closer to zero
without actually reaching it.
But of course in reality,
at some point there will be intermolecular forces
between the two surfaces that overpower the kinetic energy
in the bounce and that will disproportionately
affect smaller balls, again, because of the square cube law.
So we should expect smaller balls to stop bouncing sooner.
So the best of the zirconium dioxide ball bearings
is the seven millimeter.
That's actually the first of that type that I bought.
The best of the different sized sodium lime glass
ball bearings was actually the one we saw
in the original test,
which means the best overall
is the seven millimeter zirconium oxide ball bearing
at 42.7 seconds.
Compare that to the original ball bearing
that bounced for 34.7 seconds under the same conditions.
That's an improvement of 23%, which isn't bad at all,
but I also wanted to know how many bounces there were,
not just how long it bounced for.
One way to do that is to record the sound
that it makes and then analyze the sound wave file.
You can see there's these peaks every time
it strikes the plate.
I thought about writing some software to analyze the file
and spit out the number of bounces,
but I figured that although that would be really fun
it would probably take longer to do that
than just to count the peaks manually.
Look at this bit at the end,
you can see it's an audible frequency.
Can you hear that?
Reminds me of the space crickets toy, actually.
(balls vibrating)
If you can hit something repeatedly
with a high enough frequency, it becomes an audible tone.
But anyway, there were 259 bounces in that particular run.
That's amazing, isn't it?
But is it the best I can do?
Like one of the ways that the ball bearing loses energy
is through air resistance,
but how much of an effect
does that actually have?
To find out, I borrowed this vacuum chamber
from the Royal Institution.
But first, why is this metal amorphous
and why does that make it so bouncy?
Well, technically you should be able to make
any metal amorphous, so long as
you can cool it down quick enough.
But if your metal isn't an alloy,
then all the atoms are the same size
and it's just too easy to fall into
a regular lattice pattern.
By creating an alloy of metals with different sized atoms,
it's harder for them to find that regular repeating pattern,
and so you don't have to cool it down so quickly.
The earliest examples of amorphous metals
that were created required cooling rates
of a million Kelvin per second.
I mean just crazy rates of cooling.
And so those samples of amorphous metal
had to be incredibly thin.
They needed a large surface area
from which to lose that heat quickly.
But by the year 2000, Caltech scientists
had come up with an alloy of zirconium, beryllium,
titanium, copper and nickel.
And in just the right proportions,
that alloy could be cooled as slowly
as one Kelvin per second,
and it will still form an amorphous metal,
which means you can make these nice thick samples
of metallic glass.
It's a funny use of the word glass,
but in chemistry, glass just means amorphous.
So this metal is actually glass, but why is this so bouncy?
Well, let's formalize that question
by talking about the coefficient of restitution.
The coefficient of restitution is just the ratio
of the speed of the ball coming into the impact
and the speed of the ball leaving the impact.
And because some energy is always lost,
it's always slower coming out than it is going in.
If it was a perfect bounce and no speed was lost at all,
then the coefficient of restitution would be one.
At the other extreme,
if the ball just stopped dead as soon as it hit the floor
that would be a coefficient of restitution of zero.
Practically speaking, it's quite difficult
to measure the speed of the ball
before and after the collision.
It's much easier just to measure the height
that the ball reaches after the collision,
and the height that it was dropped from.
And there's this relationship between speed of collision
and height of ball and there's a square root in it.
So the coefficient of restitution in terms of height
is the square root of the height after the bounce
divided by the height before the bounce,
and in our case calculated by measuring
the number of pixels from screen grabs,
we have a coefficient of restitution of 0.990.
And that's incredible because, like I say,
you can never have a coefficient of restitution
of exactly one because energy is always lost somehow.
One of the main ways that energy can be lost
is through plastic deformation.
Plastic deformation is like the opposite
of elastic deformation.
If you deform something elastically,
and then remove the deforming force
then the thing will return to its original shape.
But if you deform something plastically,
then when you remove the deforming force,
the thing doesn't come back to its original shape.
When you deform something elastically,
the energy you put into the deformation is stored.
And in principle that stored energy can be returned
to the thing that deformed it.
In other words, you can bounce a ball
off a stretched rubber sheet for example.
But if you deform a surface plastically,
then the energy that went into making that deformation
is now lost.
The surface isn't going to push back
on the thing that made the deformation.
So for some reason amorphous metal
resists plastic deformation.
Why is that?
Well, normal metals can be deformed
because of their imperfections.
In my video about the heat treating of metals,
I showed how crystalline substances have imperfections
like vacancies and dislocations.
You can see from this animation
that much less force is required
to slide one region of crystal
against another region of crystal
by doing it one atom at a time,
versus having to shift a whole load of bonds in one go.
Metals are full of these dislocations,
and that's how you're able to bend metals into new shapes.
Dislocations are sometimes called the carrier of plasticity.
In metals, when you permanently bend a metal object
you are causing dislocations to glide
through the body of the metal.
And actually, if you look up close at this normal metal,
there are loads of little pits
from where the ball bearing has struck it and left a dent.
But in an amorphous metal, the atoms are arranged randomly.
There's no way for defects to travel around
over any long distances.
So without mobile defects,
plastic deformation becomes very hard indeed.
And so energy isn't dissipated through that process.
But to my mind, that can't be the whole story.
I feel like you should be able to tune the material
that the ball and the floor are made of.
Assuming there's no plastic deformation, right?
You've got these perfect materials that only
deform elastically, but they're different.
What would happen then?
Imagine the floor really strongly resists
elastic deformation.
In other words, it's really stiff,
but the ball isn't really stiff, it's squishy.
Well then when the ball bounces off the floor,
you can imagine as it's flying through the air
it's gonna be wobbling as a result of that bounce.
It's gonna be oscillating back and forth.
That oscillation would eventually dissipate
as heat and that represents lost energy.
Similarly, if the ball is really stiff,
like in the case of a ball bearing,
but the floor is not really stiff like this,
then after the collision,
well the floor is going to be oscillating
and that's gonna dissipate as heat and that's lost energy.
I suspect if you can get the stiffness of the ball
to match the stiffness of the floor,
that's when you're gonna get the best bounce
you can possibly get.
And I suspect that's what I've been doing
as I've been searching methodically
for the best ball bearing.
You might be thinking that the best way to
match stiffness would be to get a ball bearing
made of the same amorphous metal material as the plate.
And I would absolutely love to test that.
If I could get hold of some,
if anyone knows anyone who can help me out
sourcing an amorphous metal ball bearing
please let me know.
But actually the stiffness of an object
isn't just about the properties
of the material it's made from.
It's also about the geometry of the object.
So a ball bearing made
of the same material will have the same modulars
of elasticity, but it might not have the same stiffness.
But anyway, what we really wanna do now
is see if we can improve the bounce even more
by sucking out all the air.
Now here's the crazy thing.
Before sucking all the air out,
I thought it's a good idea to run a control test
in just atmospheric pressure.
And this run lasted much longer than any previous run
that I'd had, and I think it must be due to the setup.
Look, it's resting on a brick,
which is resting on this heavy metal slab.
The reason I have a brick in there at all
is because I needed to lift up the tube
so the top of the tube was near the top of the dome
so that I could use a magnet to release the ball.
But you'll remember that I mentioned earlier
how the surface you run the test on
seems to have an effect on the bounce time.
That's because if the surface is too flexible
when the ball bearing hits the slab,
the slab will move down into the surface
and energy will dissipate in that way.
So I guess this combination of brick and metal slab
is better than my breeze block from before,
which I thought was really good,
but clearly not good enough.
And perhaps if this metal slab could be fused
to a much larger, heavier metal slab,
we'd get even better results.
But look, one minute and one second.
I'm so happy with over a minute.
So can I get better than that
with the air sucked out?
Well, in truth, I ran the experiment loads of times,
with air and without air,
and there was a variability of around two or three seconds.
And the best time that I got, which was in a vacuum,
is only slightly outside of that range.
So you could argue that it's not statistically significant.
But anyway, I'm gonna show you the best run anyway
and it's this one you're watching now.
You might be wondering how you can hear the sound of it
because surely if I've sucked all the air out,
then sound can't travel. (ball bouncing)
But actually when the ball hits the plate,
the sound wave travels through the plate
and through the metal underneath,
through the brick and then through the base of the chamber.
So the sound that you're hearing
is coming through the base of the chamber.
(ball bouncing)
You might have noticed that the tube is broken at the top,
and I've super glued it back together.
Sorry, Grand Illusions, I promise I'll buy you a new one.
(ball bouncing)
There you go.
One minute and five seconds.
How amazing is that?
Kids, who wants to do KiwiCo this morning?
(kids screaming)
We've been getting a KiwiCo subscription for years,
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It's amazing to see how my kids have turned into
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When they get a new toy, for example,
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And they love to explain things to me,
and often they explain in terms of KiwiCo.
Like they'll say, "Look, the leg on this robot
moves in the same way as that lamp that we made that time,"
because that's the first time they saw
that particular coupling.
And they understand it because they made it themselves.
Like this box for example, it's a lock box.
I made it with my daughter 'cause this is
the 14 and up range, the Eureka range.
So it's a bit above her, but we work on it together
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There's a lot of replay value as well.
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We just spent ages afterwards tweaking the rockets,
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There's lots of additional activity suggestions
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so check out KiwiCo today.
I hope you enjoyed this video.
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