- Okay, you know that sketchy rolling carnival that comes to town once a year?

Then you definitely know this masterpiece, the Gravitron.

A big wheel furiously spinning, with you and all the other cool kids glued against the outer wall.

It's like the salad spinner from hell, and you are the rapidly wilting lettuce.

According to most descriptions of this ride, it works because of centrifugal force, which is a bit odd, because according to physicists, centrifugal force isn't real.

So why do we keep talking about it, and how does that ride actually work?

First, let's talk about what is real.

In physics, real forces come from actual physical interactions between objects.

Things like friction, gravity, and magnetism are all real forces that affect how things move.

And back in the 1600s, Isaac Newton came up with three laws that describe how forces affect motion.

According to the first law of motion, objects have inertia.

That means something that's not moving will keep not moving, and something that's moving in a straight line will keep moving in a straight line until they're acted on by a force.

So when you're at the carnival riding the Gravitron, your body wants to move in a straight line.

But the ride moves your body in a circle.

And to do that, there needs to be a constant force working against inertia to move the ride and the people on it in a circle instead of a straight line.

That force is called centripetal force, which is whatever real force or combination of forces keeps acting against inertia to move something in a circle.

For example, look at this yo-yo.

If I hold the string and swing the yo-yo in a circle, the string provides the centripetal force that keeps yo-yo moving in a circle.

But say the string breaks, the yo-yo won't keep moving in a circle.

Instead, it will fly off in a straight line until some other force stops it, like gravity, which will turn that straight line into an arc that curves downward.

And there are all kinds of things that act as a centripetal force.

Gravity acts as a centripetal force that keeps earth orbiting the sun.

You have friction between your car's tires and the road that provides a centripetal force that allows your car to turn a corner.

And the walls of the Gravitron provides centripetal force to keep you moving on the ride.

If those forces magically vanished, Earth would drift off into space, your car wouldn't make that turn, and a spinning ride would send you flying across the carnival.

So centripetal force is the real force that makes circular motion happen, and that makes the Gravitron ride work.

In that case, you'll experience dizziness, nausea, and centrifugal force, which is the outward push you feel when you're experiencing circular motion.

And that feeling is a real phenomenon, but it's not a real force.

Instead, centrifugal force is what's known as a fictitious force, a pseudo force, or an apparent force.

That means there's no physical source generating it, like there is with real forces.

And while real forces can be observed from any frame of reference, fictitious forces can only be observed from the perspective of rotating, or non-inertial reference frames.

In other words, you can only see centrifugal force if you're experiencing it, like when you're inside a turning car, or on the Gravitron.

If you're on the outside looking in, you're in a non-rotating or inertial reference frame, and you'll more easily be able to see that the people on the ride are experiencing centripetal force.

That's because there's nothing actually pushing them outward.

Instead, they're feeling the interaction between their inertia and the rotating frame of reference surrounding them.

Their bodies wanna keep moving in a straight line, but the walls of the ride are pushing them into the circle.

In other words, what they're really experiencing in a physics sense is centripetal force.

But if that's the case, why talk about fictitious forces at all?

Why not just say the Gravitron uses centripetal force, and forget about centrifugal force altogether?

Well, fictitious forces can still be really helpful for analyzing and describing motion in non-inertial reference frames.

For example, say you're building a rollercoaster.

You'll use equations to make sure you can build a loop-the-loop that's thrilling and safe.

And it might be helpful to think about that circular motion from the perspective of the people on the ride, not just the people safely on the ground watching it.

Or say you need to calculate the motion of the oceans, or atmosphere on Earth.

We live in a non-inertial reference frame on our rotating Earth.

So fictitious forces make it easier to write down equations that describe what we actually see on Earth, rather than describing everything from an abstract perspective somewhere out in space.

And there are all kinds of ways for us to use fictitious forces.

For example, our washing machines, they spin to get water out of our clothes.

And there's a process called spin casting, which evenly disperses metal, or wax, or plastic into a rotating product mold.

And then you have centrifuges in labs, which spin to separate samples into layers of different densities so that we can analyze them individually.

And someday, a space version of the Gravitron could be used to simulate gravity in space.

It would keep astronauts stuck to the floor, and make long haul space travel more comfortable for our gravity-adapted bodies.

It all works because of centripetal force.

But centrifugal force helps us explain what's happening to the stuff inside the spinning object.

So the difference between real and fictitious forces, and what we decide to do with them all comes down to perspective.

Fictitious forces may not be real physical forces, but the way they feel is, and that feeling can serve us in the lab, the laundry room, the carnival, and the vast reaches of outer space.

(calming music)