[music playing] What if I told you I built a machine that creates energy?

My ingenious design works like this.

As the wheel turns, the coins fall in the slots so that one side of the wheel is always heavier than the other, which would keep pulling that side down, and the wheel would never stop turning.

Without so much as a push, this wheel would spin forever.

Think about the possibilities.

I could build a giant fleet of wind turbines that turn without wind to power the entire Earth.

We wouldn't have to burn fossil fuels anymore.

This could solve climate change.

So let's see it in action, or not, because it doesn't actually work.

You may have seen this wheel before, and there is a reason it doesn't work-- well, physics, but also friction here on the axle, which will eventually stop any wheel from continuing to spin.

But then there's also gravity, so what I said earlier about one side of the wheel being heavier than the other was misleading because of the way gravity works on wheels.

Imagine a regular wheel.

Its center of mass is at the center of the wheel.

Now, imagine a wheel with a single coin in it.

It might swing back and forth a couple times, but it won't rotate.

Add a second coin.

Add a third coin.

The wheel swings back and forth, slowing down until it stops where the center of gravity is at its lowest position, always, so the unbalanced wheel will swing back and forth like a sad pendulum until friction wins.

This wheel is a perpetual motion machine, a device that is supposed to move without any energy.

NARRATOR: But by far the most persistent dream of inventors has been to get something for nothing.

That is to create a machine that will start itself, overcome all friction, and still have enough power left to do useful work-- in short, perpetual motion machines.

In countless instances in history, people have claimed that they've made a perpetual motion machine-- Bhaskara's unbalanced mercury wheel in the 1100s, Zimara's self-blowing windmill in the 1500s, the capillary bowl where capillary action forces the water upwards, the Oxford electric bell, which takes back and forth due to charge repulsion, and so on.

In fact, the US Patent Office stopped granting patents for perpetual motion machines without a working prototype.

The reason you've never heard of Cox's timepiece and you don't power your smartphone with a capillary bowl is that perpetual motion machines are the snake oil of physics.

They are impossible.

This is the classic drinking bird toy.

It tips itself into a glass of water and then rights itself and then tips again, over and over, looking forever thirsty.

Where's the source of energy?

At room temperature, inside the bird is part liquid and part vapor.

The bird contains a substance called dichloromethane, the same stuff in this little toy, which boils at a much lower temperature than water.

When the bird dips its beak into the water, the bird's top cools down.

Because of the ideal gas law, PV equals nkT, the pressure in the tube at the top of the bird is lower than the pressure in the bottom of the bird.

This pressure difference pushes the liquid into the top of the head.

The toy gets top heavy and tips over.

When the bird tips over, the bottom of the tube is exposed to the vapor, which rises up and pushes the liquid back into the bottom.

The bird stands up, and the cycle continues.

But it's not perpetual motion.

There is an energy source making this bird drink over and over, but it's hard to spot.

The energy source for the bird is the ambient heat in the room.

Bear with me.

The water cooled the bird's beak, creating a temperature difference between the top and bottom, and that caused the pressure difference that made the liquid flow to the top and tipped the bird over.

When the pressure equalized, the top and bottom of the bird became the same temperature, but slightly cooler than it started out.

Then the room warms the bird up again, and that's where we see our energy source.

This back and forth between pressure and temperature is what causes Earth's weather, and though it seems like weather and wind and storms never stop, they're powered by the sun's heat.

So even Earth's weather isn't a perpetual motion machine.

Now, there is a more fundamental reason why these machines are impossible, and it's called the law of conservation of energy, which states that energy can neither be created nor destroyed.

Energy never comes for free.

If you see a machine producing motion, heat, light, or another form of energy, keep looking for the energy source.

I repeat, keep looking for the energy source.

So now you know.

If anyone tells you they've built a perpetual motion machine, tell them to peddle their wares elsewhere.

Thank you so much for watching.

It's all very, and so are you if you think it'll work.

NARRATOR: You can't get something for nothing.

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