Okay today I'm going to be putting a Newton's Cradle in my vacuum chamber.
So I've had a ton of requests to do this. I finally bought myself a Newton's Cradle.
I'm gonna put it in the vacuum chamber and see how long it lasts when it's under vacuum versus when it's in air.
But before I do that, I'm want to show you the Newton's Cradle and how it works
and I want to explain something that you probably hadn't been taught about the Newton's Cradle.
It can not be explained how it works simply by using the Law of Conservation of Momentum.
Let me show you what I mean.
So a typical Newton's Cradle looks like this.
It has five balls that are suspended on both sides by some strings.
And they're suspended so they're hanging directly in the middle of those strings.
Now watch what happens when I lift one of these balls up and let it drop and hit the others.
So you can see that when I give one ball some initial energy
it transfers that energy all the way to the end ball and the end ball pops off
and then that one swings up and comes back and hits the other one
and it just goes back and forth and back and forth.
But now watch what happens when I let two of them go at the same time.
So the middle one just kind of stays in the middle and now it knocks two off the other end
and they come back two off the other end.
And you can continue that pattern and do three balls...
and then even four balls.
But here's the really interesting thing about Newton's Cradle,
you probably always had Newton's Cradle explained to you that the
reason how it works is through conservation of momentum.
So when you lift one ball up and let it go momentum is conserved so that when one incoming ball
is going at a certain speed, and it stops, then an outgoing ball has to continue in this direction
in order to conserve momentum.
But this actually does not explain how a Newton's Cradle works at all.
That's because, for example, momentum would still be conserved if I let a ball go
and it hits here and then instead of one flying off the end, two fly off the end but at lower speed,
at half the velocity.
So if you're just teaching it through conservation of momentum
then there's nothing to say that when you release one ball and it hits
then two balls don't fly off the end with half the speed.
Or three balls come off the end with a third of the speed.
But here's how a Newton's Cradle really works.
So you can think of a Newton's Cradle as masses that are connected through tiny little springs.
And the reason they have to be modeled like that is because actually when they hit,
they elastically hit each other, meaning they compress just a tiny little bit.
So basically it's like there's tiny little springs connecting them.
And because these can be modeled as masses with little tiny springs in between them,
that means that they have a specific frequency that they resonate at.
Kinda like when you shake a Slinky it can only oscillate at specific frequencies.
So when you release these first two balls and they collide,
what happens at the point of collision is it sends a shock wave in both directions.
And that shock wave is going to propagate the length of at least five balls.
And so the shock wave that goes in this direction, it goes one, two, three
and it reflects off the end and comes back one, two, and it ends up right here.
And then the shock wave that went this direction, it goes one, two this direction, reflects off the end
and goes one, two, three, and then it ends up back here.
So this is now the point of separation.
So where the shock waves meet, that is now the direction that continues the momentum in that direction.
So that's how the system knows where to separate.
That's how it knows to only send two balls off of the end.
It's due to the propagation of shock waves reflecting off the ends of the balls.
So it knows to only release two in this case.
And it knows to release one in this case.
Okay so first we're gonna try this in air without a vacuum,
so we can see how long it lasts in air,
and then compare it to one in a vacuum with no air.
Okay, three, two, one...
Okay it already stopped bumping back and forth,
so now let's compare it to what it looks like in a vacuum.
Okay now let's vacuum out the chamber.
Three, two, one...
Okay we are at full vacuum now, let's close our valve, turn off the vacuum pump,
and do our experiment.
Three, two, one...
So the first thing you'll notice it's completely silent.
So that looked very similar to the one in air.
That's because a ball swinging through air at these low speeds
actually has very little air friction already.
And so air friction plays a minimal role in the reason why these balls stop.
As I showed in a previous video with swinging a pendulum in my vacuum chamber,
I showed that the majority of the stopping motion
is simply due to the friction in the string.
So the string itself is wiggling and turning and all that wiggling and turning
is taking swinging motion away from it and turning it into heat.
In this case there's movement on the strings that gets turned into heat
and so these balls heat up slightly and they lose a lot of their energy to sound.
And so eventually all of that initial energy that you put into it gets lost to entropy
or gets lost to sound or heat in the room.
Hey everybody, thanks for watching another episode of The Action Lab I hope you enjoyed it.
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