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How do Newton's Cradles connect to cancer treatments? Let's unpack the physics of some of our favorite desk toys, from dippy birds to perpetual motion machines, and explore how these scientific principles can be used beyond an office desk.

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Getting through a dull work  day is not always great.

So why not find a little solace with some  mesmerizing desk toys…or two, or four? We all probably have a favorite.

So let’s pick a few and explore not  just the science behind how they work, but how scientists use those same  principles to help run the modern world. [♪ INTRO] Who doesn’t love the steady metallic  clacks of a Newton’s cradle? And it’s so easy to play with these balls. You just pick your choice of  balls, lift ‘em up, let ‘em go, let the physics take it from there!

That’s not what I would have  expected to have happen! Let’s start out though, with just one ball. When it swings back to its starting  point, it collides with a stationary ball and that collision exerts a  force that gets transferred all the way down the line to  the opposite end of the cradle.

And since the last ball has nothing to run  into, it gets to swing out to basically the same height that you lifted the first one to, only to smack back down into its  neighbor when gravity pulls it back. Altogether, this little toy is a go-to demo  for a variety of basic physics concepts, from Newton’s laws of motion,  to conservation of energy. But we’re going to zoom in and take a closer look.

It turns out that while steel is  pretty solid, even small collisions cause the balls to flex a tiny bit,  making them behave like tiny springs. So even though a human eye might see  the end ball kick upwards at basically the same time as the starting  ball collides with its neighbor, the transfer of force is not instantaneous. Instead, the first ball compresses on impact,  and then expands to restore its shape, imparting a force on the adjacent ball.

And so on and so forth. And the fact that you have this  series of compressions and expansions moving through space means that you’ve  basically got yourself an acoustic wave. Yeah, your Newton’s cradle  isn’t just making a series of clicky clack sounds when each  end ball hits its neighbor.

Everything that happens  between all of the balls is basically the same as a sound wave. Now, what would happen if we tried sending an acoustic wave through our Newton’s Cradle? Well, it would transmit right through,  but it would also get a bit distorted… similar to how sound passing  through a wall gets muffled.

That’s because springs tend to vibrate  at a specific natural frequency, also called the fundamental or resonant frequency. And on the whole, they’re better at  transmitting that frequency more than others. So any sound waves that a spring is worse at transmitting will die off a lot more quickly.

But a Newton’s Cradle doesn’t  just muffle certain sounds. It also creates extra ones. Because a Newton’s Cradle doesn’t  just behave like a spring.

It also behaves like a really weird spring. Maybe that was a bit rude of me. Scientists don’t actually call any springs weird.

The technical term is non-linear. It basically means you need  more complicated math equations to describe how they work compared  to so-called linear springs. But that is where we can connect our  little desk toy to the real world, because nonlinear springs can have  some pretty cool applications.

For example, you can use  them to focus acoustic waves, like a magnifying glass does with light waves. And back in 2010, some scientists at  Caltech made a sort of 2-dimensional Newton’s cradle that could focus and  fire what they called “sound bullets.” That sounds a bit dangerous, given the name, but they were actually interested  in improving people’s health. Their set-up was tunable, meaning they could create different kinds of sound  bullets for different applications.

And they highlighted two in particular. One was that sound bullets could act  kind of like a suped-up sonogram, because a more focused sound wave would let a doctor get a crisper view  of the inside of your body. Alternatively, sound bullets could  be fired into the body in such a way that they could destroy a cancerous tumor  without having to cut a person open.

That sounds good. Also scary though. So the next time your coworker neighbor  gets annoyed at all the little clicky-clacks coming from your desk, tell them you’re working on the next breakthrough in  medical acoustic technology.

Next up we have the gyroscope. Another pretty straightforward, mechanical toy that operates on classical mechanics principles. All we need to do to get it going  is spin the center rotor disc.

Because once it’s spinning, no matter  how I move the housing around it, the disc seems to stay pointing in  the same direction. It’s like magic! But it’s not magic.

Or if it  is, it’s magic that can be completely explained by the laws of physics. For one thing, everything’s set up  so there isn’t too much friction between the individual pieces. That lets this axel here, and  this gimbal here rotate basically on their own as you move the outer structure.

As for the disc appearing to mind  its own directional business, it can only change direction if a  torque, or twisting force, gets applied, that will change the disc’s angular momentum. And for anyone who didn’t just  have a high school physics lesson pop back up into their memory (because  honestly I didn’t - that’s too far gone), angular momentum is like regular  momentum, but for rotating masses. And it involves both a rotational  speed and a direction of that rotation!

But because both the gimbal and the  axel in this toy have very low friction, it’s super difficult to transmit a force  from the outer structure to the rotor. And that means that you can’t apply a torque, and the direction of the  disc’s rotation never changes. And because it doesn’t change,  that makes gyroscopes not just a mesmerizing desk toy.

You can also stick one inside a larger  object and use it to keep track of direction! Like, imagine you’re a pilot doing a bunch  of fun aerial maneuvers in an airshow. No matter where you are in one  of your many loop-de-loops, a spinning gyroscope will  always be able to tell you which way’s the wild blue yonder,  and which way’s the ground.

Or maybe you’re a submarine pilot, and your boss gives you the simple order to head North. You might want to pull out a compass,  hoping to detect Earth’s magnetic field. But because you’re sitting inside a giant  metal tube with a bunch of electronics everywhere, that signal can get distorted,  and point you in the wrong direction.

Plus, the Earth’s magnetic poles  aren’t in the exact same places as its geographic poles, so you also  might not have the right heading if you forget to compensate for that. So instead, you can make use of a  technology called a gyrocompass. Although to find North, those  have to account for the fact that the Earth is spinning all the time, too.

Let’s pretend this tiny desk toy could  keep spinning for a full 24 hours. I spin it up and set it here so that  the disk is parallel to the table. But it will not stay that way.

See, the disk has a different  perspective on directions than we do. It does not care where the ceiling or floor is. It only cares about maintaining  its alignment with respect to the greater universe…which  the Earth is moving inside of.

The only places where the disk  wouldn’t appear to move over the course of the day would be the  geographic north and south poles. So if I’m literally anywhere  else, a simple gyroscope won’t always point North if it  is set to start off that way. Plus, the Earth is also moving through  space because it goes around the Sun, so after one full day, the gyroscope won’t even be pointing exactly back at the  north geographic pole, either.

Luckily, there’s an easy  solution: unlike my gyroscope, a gyrocompass actually wants  to apply a bit of friction. That means the gyrocompass  can now feel the Earth move. And as the Earth rotates, it exerts  a relatively constant torque, helping the gyrocompass point  toward the North/South direction defined by the Earth’s spin axis.

Who would have thought that  playing around with toys like a gyroscope could give  you a little direction in life? Pardonne moi si vous plait. Now, if you don’t know what I just said,  you could find out with the help of Babbel.

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But don’t worry, I will get there  soon, because the time commitment is just 10 minutes per lesson. So even with my busy schedule  hosting SciShow videos and making SciShow videos, I can start  speaking French in just three weeks. And, sure, I might not have  perfected my accent just yet.

But that’s something I can work on. And Babbel is a no risk commitment, with a money back guarantee in the first 20 days. So you don’t lose anything for trying.

And you can get up to 60% off when you sign up using the link in the description down below. Thank you to Babbel for supporting  this video, and let’s get back to it! Hopefully your head isn’t spinning too much yet, because this next toy is a  personal favorite of mine!

The drinking bird might seem just  as mechanical as the last two toys, but it actually keeps moving  thanks to thermodynamics. Liquid inside of the desk toy is something  volatile like methylene chloride. And that might sound like it’s super dangerous  or it’s inappropriate for a desk toy.

But volatile just means that even applying a little bit of heat leads  to a lot of evaporation. By dipping the bird’s beak  into a glass of cool water, you create a temperature difference  between the head and the tail bulb. And if you have a temperature difference, that means you also have  different evaporation rates, and different amounts of gas pressure.

It’s that extra pressure that  forces the liquid in the tail up this little tube toward the head. And as that liquid rises, the center of  gravity changes, causing the bird to tip. But as the bird tips, the liquid  shifts just enough for the pressure to equalize across the whole bird, making  the liquid flow back out of the head.

And everything starts over. We can see from our bird  friend here that it is possible to seemingly create motion from nothing! But in reality, it is a temperature  differential across the bird that is creating the motion, converting  thermal energy into kinetic energy.

And to take it a step further, that  kinetic energy can be converted into useful electric energy, creating  what we all know as a generator! For example, in 2024, a team of  researchers published a paper wherein they built a small  proof-of-concept electric generator that relied on their version of a  drinking bird to get things going. It only managed to output a small  amount of power, 40 microWatts, but that was 13 times better  than previous experiments had managed with a similar set-up.

So while this bird might be drinking on the job, he’s also working hard to inspire  some valuable engineering. Last but not least is possibly the most  perplexing toy: the perpetual motion device! I mean it’s a fake perpetual motion device.

Perpetual motion devices are not real. The laws of thermodynamics simply forbid it. Every loop of a theoretical perpetual  motion machine must lose some energy, so if you want to keep this toy going forever, you’ve actually got to keep putting  energy into the system somehow.

Let’s take a look at how one toy does this. Someone drops a ball down a ramp. And as it's going down, it seems  to just maintain its momentum.

And then it flies up the  other side and is shot back up into a tray above the ramp with a hole in it. The ball falls through a hole to start  the cycle over. And over.

And over. Until the heat death of the universe, or  I get tired of the sound that it makes. Now in an ideal world, conservation  of energy would allow the ball to make it back to its starting location.

But in the real world, friction  always steals a bit of energy. So instead, this desk toy cheats  with a little help from magnets. Also with help from the  fact that the ball is metal.

Hidden in the base of the toy is a  battery-powered circuit that activates a little electromagnet to pull  on the ball for just a moment as it approaches the lowest point in its path. This gives the ball a little speed boost. So long as the electromagnet turns off just  before the ball begins its upward climb, the ball will continue to  lose energy from friction, but still have enough leftover to  make it back to where it started.

And to know exactly when the electromagnet  needs to do this little pulse, the circuit emits a separate  pulse to give the ball its own small electromagnetic  field that a sensor can pick up. But these electromagnetic  pulses are not just used in toys that pretend to violate the laws of physics. They’re also used in a bunch of  important technologies that you can find everywhere from under a super  speedy train to inside your wallet!

It all comes down to the relationship between electrical currents and magnetic fields. A changing magnetic field can create  currents in another metal object, which can, in turn, create  even more magnetic fields. This is called electromagnetic induction, and it’s how we create magnets out of  seemingly uninteresting hunks of metal.

And for technology like magnetic  levitation trains, electricity is used to both create and modulate the magnetic  fields that levitate the train. But you know what else uses it? Assuming  it has an RFID chip, your credit card.

Let’s say you want to pay by tapping  your card against the little scanner instead of doing the whole insert. And then it beeps at you, or you  swipe it and it doesn't work. When you hold your card up to the reader, the machine sends an electromagnetic  pulse to the chip inside the card.

Then thanks to induction, a circuit  inside that chip gets enough energy from the pulse to emit a response  pulse with a very specific pattern. It’s basically an encoded version  of your credit card number that can only be read in a very  close proximity to the scanner, and only when an electromagnetic  pulse is emitted first to the chip.   Then, a sensor inside the scanner will confirm the pattern to make sure it's unique  to your card before charging you. It may seem like magic that we can make  trains levitate or read credit cards just by waving them near a scanner, or  have a tiny toy defy the laws of nature.

But it’s all just electromagnetism at work. Between the laws of motion, of  thermodynamics and of electromagnetism, these simple desk toys are not so simple. And at least to me, that makes  them all the more mesmerizing.

I should probably get a newton’s cradle that doesn’t have one of them broken…we  lost a ball is all I’m sayin. [♪ OUTRO]