What if we could create a machine that creates - not uses up - energy? Well, it's already been invented, and it works like this. Imagine a simple wooden wheel with coin-holder slots inside. As the wheel turns, the coins fall to the bottom of the slots to ensure that one side of the wheel is always heavier. This means the wheel would never stop turning, ever, and it does so without the need for any external power sources or even need a little push to start it up. If we scale this up, we could have fields of giant windmill-like structures that don't even need the wind to create power. Incredible, right?

Well, sorry to bust your bubble there, but this incredible perpetual motion machine isn't going to be solving our climate change woes any time soon. Why? Because despite the fact that it sure looks like it works in the latest episode of Physics Girl, it doesn't. The problem is the axel in the middle of the wheel - because of friction, this part would eventually wear down and stop working, making the perpetual motion machine not so perpetual. 

The other problem is gravity. While it might look like one side of the wheel is heavier because all of the coins are dropping to the bottom of their slots and pushing it down, the way gravity affects wheels interferes with this mechanism. In a regular wheel, its centre of mass is in the actual centre of the wheel. If you start adding coins to this wheel, they might push one side down a little, but instead of making the wheel rotate, they're just going to make it swing back and forth a bit according to its centre of mass.

"The wheel swings back and forth, slowing down until it stop where the centre of mass is at its lowest position - always," says Physics Girl. "So the unbalanced wheel will swing back and forth like a sad pendulum until friction wins."

Basically, the perpetual motion machine is an inventor's dream, because if we can figure out how to make it work, we can get something for nothing - we can generate energy without having to spend any energy.

For centuries, people have claimed to be the inventors of perpetual motion machines that actually work, but despite all the patents - the US Patent Office has stopped granting them unless you have a working prototype - we've still got nothing. Physics Girl calls them "the snake oil of physics", because despite seeming fairly simple, they're impossible, and always will be impossible, thanks to the laws of physics.

I'll let the video above explain why, but let's just say that cute little drinking bird toy you've got there on your desk isn't the perpetual motion machine you think it is.