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.
That's amazing.
It's all about those boundary layers.
So that was the reverse magnus effect.
Thank you so much to Nicole for joining me to help explain it.
That was brilliant.
Thank you so much for having me, Diana.
It was a pleasure.
So check out Nicole's channel, FYFD.
We did another video a little bit more about boundary layers
over there.
What are we going to learn about in that video?
So you're going to see roughness and boundary layer
transition there too, but this time it's
going to be in the context of the space shuttle.
And it's an epic story.
Check that out.
So thank you so much for watching, and--
Happy physicsing.
MAN: Watch out!
Ah!
Incoming!