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Michael: Imagine being strapped into a big chunk of metal, raised over a hundred twenty meters into the sky, and then dropped, whizzing over hills and even looping upside down. When you put it that, way roller coasters can seem a little terrifying, but they're also exhilarating and involves some really interesting physics make all those speedy gut-dropping thrills work.

Most roller coasters start off with something called a lift hill, which mechanically lift to the top of the first and tallest hill, but other roller coasters start a little more... suddenly. The're rapidly propelled forward thanks the hydraulics, the branch of physics that deals with fluids and how different mechanical forces affect them. Hydraulic launch systems work by using a cable that's attached to two important pieces: a catch car which connects to the bottom of the roller coaster train, and a giant winch which is basically a huge spool that wins the cable when it's turned by some motors.

So to launch the train, this cable pulls it down the tracks like reeling in a fish really fast. After a few seconds the catch car releases the train so it can zoom up the big hill. It's kind of a complex mechanism, but we'll go over the basics.

A hydraulic fluid and some nitrogen gas are separated by a piston inside this chamber called hydraulic accumulator. And the key to these launch systems is the fact that liquids like hydraulic fluid or incompressible, so the hydraulic fluid is pumped into the accumulator and compresses the nitrogen gas so there's more gas molecules bouncing around in a smaller space, which increases the pressure. Nitrogen gas is cheap, easy to get, and won't react dangerously to heat and high pressure, basically no huge explosions. And once the pressure is high enough, a valve is opened, and the hydraulic fluid rushes out to power a bunch of motors, which turn the winch, which pulls the cable and accelerates the train. Just like that, within a couple seconds, you're speeding along at hundreds of kilometers per hour.

So now that the ride started you've made it to the top of the first hill and you start to head over the other side. You might feel something weird... like besides the adrenaline or other chemicals that give you that exciting rush. There's that sinking feeling in your stomach and you might feel weightless. That's because you're in freefall, meaning that gravity is the only force acting on your body.

Normally throughout the day you're able to feel your weight because something like the ground or a chair is pushing back on your body, from your feet to your bones and organs, but when you're in freefall you're not being supported by anything so there isn't a force pushing upward on you, you're just accelerating toward the earth at 9.8 meters per second squared like any other falling objects.

Now all good... or slightly terrifying things have to come to an end, so the roller coaster train needs to stop somehow. Traditionally roller coasters use breaks that coming contact with a speeding train, like something that skids along the bottom, or clamps that close round metal fins on the cars. These breaks rely on friction which causes the kinetic energy, the energy of motion, to be converted into heat energy and stops the train. And these brakes work, but they get worn out from all this contact. But there are some breaks that don't involve friction: they work by moving a conductor, like the metal pins sticking out from the train, through a magnetic field.

A magnetic field can be produced by electric currents, like the currents running through a wire, or the microscopic currents formed by electrons moving within atoms which is kinda how permanent magnets work. So on the tracks of some roller coasters there are rows of permanent magnets that create a magnetic field, and when the metal fins pass through the magnetic field, it induces a ring-like current called an eddy current.

This has to do with Lenz's law, which essentially tells us that conductors like these metal fins don't like change. So when the fins enter a magnetic field that wasn't there before, they'll create a current that makes its own magnetic field that opposes this change. The train will slow down because the kinetic energy of the moving train is dissipated as heat by the eddy currents flowing through the metal. And eventually the train comes to a stop, all without friction. So whether you love 'em or hate 'em, hopefully knowing some of the physics behind roller coasters can make you appreciate them a little more.

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