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!

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Dennis Veasley

100 thoughts on “How SMOOTHNESS of a SOCCER BALL affects curve!”

  1. Very nice vid, Physics Girl! I'm making a science inquiry project about why it can be so hard when passing, curve/bending or even placing the ball on target when shooting it. My conclusion would be that its the way the ball is being kicked. But: for bending or anything that causes the ball to spin, I've come along the fact (from sources like you)that it's the Magnus Effect, or fluid dynamics.(and physical adjustment like how you turn your hips, lock your ankle, etc.)Thanks to your video I now know, but I don't want to copy information from you or any other sources i get information from. Any help?

  2. Can you provide a link to the "Tracker" application? The name is so generic that looking it up gives you contextless stuff.

  3. @Physics Girl: This has been quietly nagging at my mind for months.
    Doesn't your analysis imply, by the Intermediate Value Theorem, that there is some surface which will NOT experience either Magnus or reverse Magnus effect?
    I would really love to see you show a ball that never curves. This sounds crazy to me, but also seems implicit in your explanation.

  4. you make a regular soccer ball make weird movemnts we call it a Knuckleball its when the ball has no spin or very very little spin that the aero dynamics will react to it.

  5. Wait… that background painting or puzzle, whatever it is.. Thats the place the new csgo map canals is based from, right!?

  6. yeah i noticed that..i didn't know why when i was yonger the ball whent the wrong way but then i tried the same ball and it spined the wrong way.

  7. Need help: i was writing the italian subtitles for this video but i can't understand what "plates" mean in the sentence at 5:07 (more or less)… Can someone help me in figuring this out? maybe with an image.

    Sorry but i cannot understand it and the translation i thought doesn't fit in that context.

    Thanks and have a good day.

  8. Diana, Thanks for this analysis – great work! I'm using Tracker with my students too 😉
    One question: Is the 2:39 shot showing the opposite effect, not the one described seconds before 😉 (should be the orange ball, deflecting towards dashed line – isn't it?)

  9. What were the wind direction and temperature at the time of the experiment? Did these parameters change between kicks?

  10. LOL Eurocucks getting triggered by the word "soccer". USA has owned you atheist trash since WW2, so get over it and start worrying about the muslims you're being overrun by, they're all obsessed with girly soccer too, what you think USA is going to save you this time? #LOL

  11. It also can happen if the mass of the object is lighter the air is transferred in one direction while other is on the other direction.

  12. Great! Thanks for the video.

    I have been trying to understand how Magnus effect works on 'paper flying tubes'- when thrust forward with clockwise spin from throwers point of view, it gets lift as well as lateral turn, ofyen to left and sometimes to right. I cant figure out how the forward going spinning tube deflects air to get such a trajectory. Can you do a video on that please?!!

  13. Awesome! As a lifelong soccer player and official I've always wondered about this… Your explanation nailed it, and was easy to understand. Thanks!

  14. What caused the oscillations between normal and reverse imho is that it sometimes rotates parallel to the rubber band, that is, the rubber band is like a rotating disk, and some other times, its rotating parallel to the rubber band circular surface, like a rotating coin. The first one is laminar, the second is turbulent

  15. is this similar to what is going on when you curve a bowling ball? Or is the friction of contact to the surface of the alley going to change whats going on?

  16. Next idea. Table tennis with a bouncy ball. It will reverse the spin every bounce, so you can make it go zigzag and if you give it back spin, it will bounce straight up and then when the other player moves in, it shoots forward (sometimes in the face)

  17. I gues i am too late, bat heare some examples of courve, side courve, top speen, all together…

  18. There's Cricket, another sport where both Laminar and turbulent flows are used at the same time. Consider digging deeper in that, it's much intriguing!

  19. As usual.. explanation is misleading since people do not talk about forces. Rotation causes difference in pressure.: in other words LIFT FORCE. And iinverse Magnus effect is STALL

  20. i've been playing with soft balls at home and I WOULD GET FRUSTRATED every time the ball acted differently. THANKS to this video now i know why this has happened

  21. For the anti-"soccer" purists: the name football was used as far back as the Middle Ages for what is now called "mob football," which had no rules and involved two large groups of people trying to move the ball into goals a long distance apart. Nothing prevented you from using your hands and indeed carrying the ball (other than the fact that you might get trampled by a mob of opponents). The "foot" part refers to the fact that you played on your feet and not on horseback – a very important distinction at that time, as it marked football as a game for the masses and not the equestrian elite. So, no, "football" does not actually mean a game dominated by kicking. It just worked out that way in Association football/FIFA football/soccer – a happy verbal accident.

  22. But that doesn't happen witha regular football irrespective pf how smooth it is…..the 2nd ball here is so light weight

  23. Cricket ball swing has complexities due to the relative weights and smoothness of the sides of the ball

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