- This video is sponsored by Kiwi Co
More about them at the end of the video
- When me and Mehdi were making the chain fountain videos,
I got sent this clip at quite a lot,
and I can understand why,
because the behavior of the loop of string,
as it's pushed round by the rollers,
shares a lot with the behavior of a chain,
as it leaves the pot.
I'll get to those comparisons in a minute.
But my first thought was,
well,
I have to have one of these.
Because I was actually already familiar with a toy
called String Thing that does a similar thing.
And ever since I heard about String Thing,
I wanted to get ahold of one,
but they don't make them anymore.
So I decided to email Steve and Fazio,
who made the clip,
and say,
can I buy it
or pay you to make one for me?
And amazingly,
they emailed me first saying,
would you like one? I want to be clear at this point,
they're not paying me at all for this video.
In fact,
I insisted on paying them for the prototype.
So this isn't an advert,
but I will leave a link in the description
where you can pre-order one,
it's called a ZipString.
Before I get into just how weirdly
this loop of string behaves,
it's worth mentioning I'm not the first person
to show this on YouTube.
In particular,
I'd like you to go and watch Bruce Yeany's video
after you've finished watching this one,
I'll leave a link in the description.
His video is great.
He built his own string shooters,
actually his channel is great,
and he did it years ago.
One big difference between Bruce Yeany's string shooters
and this one,
is that this one is handheld.
And as a result,
you very quickly notice
some really counter-intuitive behavior.
The way the loop of string moves when you move,
look,
it's shooting out about two meters.
And as I move side to side,
look,
that far point of the loop moves with me.
It's a bit like when you put on a virtual reality headset
and the head tracking's broken.
So as you move your head,
the world moves with you.
Maybe that's a bit of a niche reference,
but it feels weird in that same way.
Like,
I'm moving my body,
and two meters away there's something moving
exactly in sync with me.
It's freaky.
But obviously it makes sense.
When I move my body to the left,
I'm moving the string shooter as well
and the string is now coming from that new position.
And because it's moving so quickly,
it reaches that far distance quite quickly.
And the delay is not discernible by my weak brain,
so everything seems to be moving in sync.
By the way,
if you enjoy this sort of stuff,
consider subscribing to my channel if you haven't already.
The second unusual behavior
is noticed when you move side to side repeatedly
like this.
You get waves traveling down the string,
but you want to get waves traveling down
the bottom part of the string,
not the top part.
And that makes sense.
If you think about it in terms of
imparting a wave on a string.
The string is under high tension
because of the dynamics of the thing.
So when I impart a wave here,
that wave shoots down the string at high speed,
but because the string is coming towards me at high speed,
the apparent speed of the wave is much slower.
Specifically in this case,
the wave speed is a little faster than the string speed,
so it wins,
and we see it propagates away from the handle.
On the top part of the string,
the wave is shooting out at high speed
and the string is shooting away at high speed.
So you don't even see it. You can actually think about it
in terms of the Doppler effect.
Like instead of thinking about string,
moving through rollers,
think about the rollers,
moving along the string.
And so when the rollers impart a wave on the string
that travel in the same direction
as the rollers are traveling,
you'll get a shorter wavelength,
in the same way that
when an ambulance is traveling towards you,
the siren sounds higher in pitch.
Similarly,
when the rollers impart a wave onto the string
in the opposite direction
to the direction that the rollers are traveling
down the string,
you'll get a longer wavelength.
That's like how,
when an ambulance is traveling away from you,
the siren sounds lower in pitch.
In our case,
the wavelength is so long on the top string,
you can't even see it.
You can also impart a wave in the string by tapping it.
If the wave speed and the string speed were equal,
then the wave wouldn't move at all.
And in fact,
that's what we see in the chain fountain.
It falls out of the mathematics
that the wave speed and the chain speed are the same.
So any kink introduced to the chain
seems to stay fixed in space
as the chain moves through it.
The fact that that chain moves through
any shape it's given when in steady state
is something I've talked about a lot
in the chain fountain videos,
but it comes from the fact
that wave speed and chain speed
are equal in this case,
or at least that's one way of getting to that conclusion.
By the way,
for those of you who've been following
the chain fountain dispute
with me and Mehdi,
it's finally over.
If you haven't seen Mehdi's most recent video
on the subject,
the link is in the description.
It's really good.
I'm kind of sad that it's over to be honest.
My understanding of the chain fountain
is so much deeper because of Mehdi's videos.
It's just a really great way to
get to the bottom of something.
I'm just gonna have to start beef with someone else,
aren't I?
The absolute weirdest thing for me with the string shooter
is when you change direction.
If I turn the string shooter through 180 degrees,
the far end of the loop doesn't follow me instantaneously.
Contrast that with the side to side motion
I showed you earlier,
where the far end of the loop does seem
to follow me instantaneously.
Obviously not exactly instantaneously,
but as far as I could perceive it is.
Whereas with it changing direction of the shooter,
the far end of the loop seems almost reluctant
to fall into the new position.
Because of the way the of string behaved
with side to side motion,
I expected that when I changed
the direction of the string shooter,
it would be like waving a light saber around.
You know,
the far end would just respond instantaneously.
But it clearly doesn't do that.
My first thought was that it was related to gyroscopes.
Because the loop of string is spinning
then maybe it has some gyroscopic stability,
but I'm not sure that that's what's going on here.
With a gyroscope,
for example,
if you try to change the direction of the axis of rotation,
you end up tilting the gyroscope
in a direction that is perpendicular to the applied force,
which is pretty weird.
And I'm not really seeing that here with the loop of string.
You also get a thing called gyroscopic procession.
When you apply a torque force to a gyroscope,
and you don't see that with the string shooter either.
So what's going on?
Well,
I think there might be a clue
in trying to get a wave to propagate
along the top part of the string loop.
It's weird that a wave traveling
along the bottom of the loop
never makes it around the bend
to the top part of the loop.
Surely we should be able to
at least start a wave here at the top
by tapping the string,
and then have that wave move along the top string
to the handle.
Because we've already worked out that the wave speed
is faster than the string speed,
but that doesn't happen.
Even weirder,
is that if you start a wave here,
it moves away from the handle.
That's in the opposite direction around the loop
compared to the wave at the bottom.
And that seems counter-intuitive,
but it can be explained.
If we take a freeze frame
and imagine plucking the string here,
a wave would travel out in this direction
and in this direction.
This one we would never see
because it's whisked away at high speed by the string.
But the wave going in this direction,
well,
if it's traveling slower than the string,
we'll see it as a wave traveling in this direction.
So the behavior we see
implies that the wave speed is slower on top
than it is on the bottom.
Which makes sense actually,
because wave speed is proportional to tension.
And the bottom string is being pulled by the rollers,
increasing tension.
But the top string is being pushed by the rollers,
which decreases tension.
This also explains why waves don't propagate
all the way around the loop.
This vertical part of string is a bit like
a boundary between two media with different impedances.
I've made a couple of videos
about impedance mismatch actually.
I'll leave a link to those in the description.
But anyway,
all this means that
if you were to apply a perturbation
to the top part of the string
and the bottom part of the string at the same time,
by,
for example,
changing the direction of the string shooter,
then those two perturbations will
travel together slowly away from the handle,
which is exactly what we see.
What I can't explain
is why we don't see waves on the top string
when we move string shooter side to side.
Surely if the wave traveling towards the handle
is going slower than the speed of the string,
we should see it propagate away from the handle.
We just don't.
If you've got any ideas,
let me know in the comments.
You can also discuss this video
on the Steve Mould subreddit.
I'll leave a link to that in the description as well.
Attaching the string shooter to an electric screwdriver
turned out to be a really fun modification.
You can't spin it too fast,
otherwise it gets tangled up with itself
and it ends up popping off the rollers.
What I discovered though,
was that you can spin the string shooter quite quickly
so long as you start off slow
and bring the speed up gradually.
And you end up with loads and loads of twists
in the loop of string.
Isn't that cool?
(cheerful music)
thanks again,
Steven Fazio for sending me the ZipString prototype.
Link in the description for pre-orders.
And thanks to Professor Noel Perkins
for guiding my thinking on this video.
(cheerful music)
A common thing you'll hear in my household is,
when's the next Kiwi Crate coming?
Which is annoying,
cause I've already explained that they come once a month
and it's only been a week since the last one came.
Kiwi Crates are these projects.
They come in the post,
everything you need for the project is in the box.
There's eight different subscription lines
for every possible age group.
My kids jump on them
as soon as they fall through the letterbox.
I'm always on the lookout for
ways to spend time with my kids that's enriching.
You know,
we go out a lot,
but as it's getting colder,
we're spending more time indoors,
and it's an absolute godsend
having these projects to do with them.
I can talk endlessly about
the importance of STEM education,
but from a purely emotional point of view,
watching my kids get excited about building something
just puts a massive smile on my face.
And it's not just about the build.
There's always a STEM angle as well.
For example,
maybe you're building a paint spinning art machine,
but you're also learning about how motors and switches work.
Every Crate comes with a magazine
with additional activities and things to do.
So each Crate ends up lasting for days and days.
You can get the first month of a KiwiCo subscription
for free if you use my special URL,
Kiwico.com/SteveMould.
The link is also in the description,
so check out KiwiCo today.
I hope you enjoyed this video.
If you did,
don't forget to hit subscribe,
and the algorithm thinks you'll enjoy this video next.
- [Child] Squeeze this,
the air goes through here.
- [Steve] Yeah.
- [Child] And then...
(child laughs)
And then it connects to this.
- [Steve] Yeah.
- [Child] So it goes so hard
that it pushes this off.
- [Steve] Oh I see.
- [Child] Look.
- [Steve] Go on. (dart pops off)
- [Steve] Ah!