NARRATOR: You've all seen the shot.

Kick the ball with some spin, and it'll curve, if you're lucky, right into the goal.

This is known as curving the ball, or bending the ball, or if you're from the 19th century, as a screwball.

And in fluid dynamics, it's known as the magnus effect.

And supposedly the magnus effect can be reversed.

That is, kick the ball in the exact same way with the exact same spin, and it'll bend in the opposite direction.

So-- Is it recording?

Yes, it is recording.

Just to have the drone hover, like, right about there?

This is awesome.

Look at this sweet drone.

We were attempting to film the magnus effect from above to try and capture the curve, and then hopefully get the reverse, OK?

How does that sound to you?

That sounds great.

NARRATOR: First we try the regular magnus effect with the soccer ball.

That was perfect.

The ball was kicked on the right, and therefore spinning counterclockwise, and it curves to the left.

And now, same kick, with a smooth bouncy ball.

That was amazing.

Nailed that one.

Yes.

The ball so clearly curves to the right this time, overlaying the two kicks you can see just how strange this is.

What's going on?

Everyone knows which way to make the ball spin to get it to bend in a given direction if you're a soccer player.

I remember, I was in Nice with my friend, and we were playing pickup soccer, and the ball just bent the wrong way, and it just completely missed.

I know the magnus effect, I've been lecturing about this for five years, and the bloody thing's going the wrong way, right?

So what could make this extremely common effect suddenly reverse?

Well, first, why does the regular Magnus effect happen?

It's all about fluid dynamics, in this case the dynamics of air.

And scientists consider gases like air to be fluids.

Now you know.

So when you kick the ball on the side just right, it'll start spinning.

As the spinning ball moves through the air, you can also think of air flowing past the ball.

Right near the ball there's a thin layer of air that essentially stays right with the ball as it spins, because of friction between the surface of the ball and the air molecules.

So on the bottom of the ball, the airflow further out opposes the motion of the spinning ball.

That makes the airflow leave the ball here, and pretty much travel straight back.

The air moving over the top is flowing with the spin of the ball, so it's pulled along the curve of the ball and deflects downward.

Overall, more air is deflected downward, and by conservation of momentum, when the air goes down, the ball must go up.

Add that movement to the forward motion of the ball, and it'll look like it's curving.

Now, what is it about this smooth, bouncy ball that causes the magnus effect to flip?

For this explanation, I'm going to seek help from aerospace engineer Nicole Sharp from FYFD.

Thanks for joining me, Nicole.

Thanks for having me, Diana.

OK, so what is it that causes the reverse magnus effect?

So the key to the reverse magnus effect is in the thin layer of air right next to the surface of the ball, or what we call the boundary layer.

So the boundary layer can come in two basic varieties.

You can have a laminar boundary layer, or you can have a turbulent boundary layer.

So a laminar flow is smooth and orderly, and it's like what you get when you first turn the water faucet on.

And turbulent flow is what you get when you turn the faucet on higher, and it gets all crazy and chaotic.

And on a soccer ball that's rough, it's typically turbulent?

Yes.

But if you had a really smooth ball instead, that boundary layer might switch from being turbulent to being laminar.

So looking at our spinning ball again, we can see that air flowing over the top is moving in the same direction as the spin of the ball.

That means the velocity difference between the air at the surface of the ball and the air a little ways away is going to be very small.

So our boundary layer here on top is going to become laminar sooner than the boundary layer on the bottom, where the air is moving against the spin.

That laminar boundary layer on the top is not as good at sticking to the ball, and it's actually going to separate right here at the top.

On the bottom of the ball, the air is still moving fast relative to the surface of the ball, so that boundary layer is going to stay turbulent.

Turbulent boundary layers are better at sticking to the curve of the ball, so it's going to follow the curve of the ball around, and be deflected upward.

Since the overall deflection of air around the ball is now upward, that means the ball is going to move downward.

Which is the exact opposite of what we saw with the regular magnus effect.

Yep.

It's the reverse magnus effect.

So the reverse magnus effect happens because of this super sensitive boundary layer transition from turbulent to laminar.

It's so sensitive that even little changes can affect the ball's flight.

We did some experiments here at MIT, a grad student, she looked at the influence of roughness on the magnus effect.

So we basically took a beach ball, and it indeed bent the wrong way.

And then if you put an elastic band around it, then you get-- it basically reverses the sign of the force.

So of course we had to try this.

And with the rubber band we saw some unusual behaviors.

Dan Walsh, our drone operator, who's also a grad student at UCSD in physics, analyzed the trajectories of these balls using a program called Tracker.

For one of the plates with the rubber band we got this path.

So the tiny rubber band is enough to cause the magnus effect to flip back and forth.

He also found some interesting things, like the acceleration due to the magnus effect on some of these kicks was comparable to gravity for the lighter ball.

So thank you so much for watching, and-- Happy physicsing.

MAN: Watch out!

Ah!

Incoming!