The physics of that ‘kickalicious’ kick

http://www.empiricalzeal.com/2012/12/31/the-physics-of-that-kickalicious-kick/?src=search

by llambda • 14 years ago

247 33 14 years ago

aatish3• 14 years ago

Hi- I'm the author of the piece (on twitter at @aatishb) and am happy to field questions on the post. Do people accept that the video isn't faked? The data isn't perfect, but it was good enough to convince me that the video is real.

Here's a follow up question, that I haven't thought much about. Assume that Rugland launches the first ball at 64 degrees and at 14.4 m/s (about 32 mph). 1.5 seconds later, and 1.5 meters ahead, he kicks the second ball. (So far these are the numbers I get from the data). My question is, given the size of a football, how accurate does he need to be in the launch speed and angle of the second ball, in order to be able to strike the first ball?

You might need to assume a reasonable range for the launch speed to work this out.

TheAmazingIdiot• 14 years ago

That is an awesome analysis of video and working through the physics.

Complete aside: Mind exploded from unit mismatch. Miles per hour then meters? Gah....

Just please dont use gradians for angle measurement :)

aatish3• 14 years ago

haha - the 32mph was originally 14.4 m/s. Given that the top story here today is a white house petition to adopt the metric system in the US, I should know my audience better :)

abcd_f• 14 years ago

Analyze this - https://www.youtube.com/watch?v=ZQX8FakToj0 :)

T-hawk• 14 years ago

Here's a first-order approximation. I'll assume the angle is perfect and work out the speed range. The first ball decelerated to maybe 8 m/s given its altitude at collision and some loss to air friction. If the ball is 25 cm wide, it crosses its own diameter in 1/32 second.

Assume the distance of the collision is 10m from launch and occurs 1.7 seconds after the second kick. The margin of error on the arrival time is 1/32 / 1.7 = 1.84%. Using your launch speed centered around your value of 38 mph = 16.9875 m/s, that means the possible range was 16.675 to 17.300 m/s. Interestingly, that's a difference of 1.4 miles per hour, which is also roughly the precision of velocity that baseball pitchers can produce on demand.

By the way, there's more variables on the inputs to the second ball. The timing of the kick is important and controllable too, creating a three-dimensional map of inputs. This could be transformed into and visualized as a 3D graph, showing all the combinations of speed/angle/timing that will result in a collision.

Also, the left-to-right angle must be on target too or the balls won't be in the same vertical plane for any collision at all.

Someone• 14 years ago

About the vertical angle that you assumed to be perfect: I don't think that is critical. If you hit the second ball a bit higher/lower than intended, it crosses the trajectory of the first ball higher/lower than intended, so it must be at the intersection point earlier/later. Luckily, you get some of that for free, as the intersection point will be closer by/farther away.

All else being equal, the harder you can kick, the easier this gets. The lower the arc of the first ball, the smaller the angle of intersection between the two trajectories, and the easier the hit. You can get a lower arc by giving the ball more horizontal speed, keeping vertical speed the same (just decreasing vertical speed won't help; the first ball would hit the floor before you can hit it with the second ball)

Of course, all else isnt equal; directional errors likely are some function increasing with both distance and ball speed.

rm445• 14 years ago

I think your assumption that good physics == not fake does not hold for certain classes of faking, i.e. simulating the collision first and modifying the ball positions (or creating one or both balls wholesale) to match.

I mention this not to cast any doubt on the kicking video, but because there are a lot of good simulation tools nowadays and faking something up by working out the physics first is only going to get easier.

aatish3• 14 years ago

Well, if fakers make the effort to do the math (and physics), more power to them. It just makes this kind of video forensics a more interesting challenge. :)

Here's another example of a physics based take-down (by Rhett Allain) of that eagle lifting a baby video that was doing the rounds. http://www.wired.com/wiredscience/2012/12/eagle-picks-up-a-k...

borlak• 14 years ago

Nice work, but these types of videos always annoy me. Anyone with the basics of athletic skill can perform these stunts with enough tries -- which the guy admitted to taking several tries. Some of the pure-power moves are out of the question for a lot of people, and those are definitely cool, but kicking balls into trashcans and basketball hoops is not.

The exception would be doing several difficult moves in a row without fail.

sophacles• 14 years ago

Yes, I could get a video of me doing a lot of these stunts. Great. Could I get all of them to happen in an afternoon of trying? Probably not. It would take a long time and a lot of luck to get any one of them.

An interesting, and far more skill showing (imo) version of this video would be to show how close they were on each of the tries - If for example he was within a couple of meters of collision each attempt, I would call that very high skill. But if the other attempts were dozens of meters apart, the skill would be much lower.

fla• 14 years ago

Human are capable of amazing things. Rémi Gaillard has for example been known for this very specific skill for years.

One example can be viewed at http://www.youtube.com/watch?feature=player_detailpage&v...

jschulenklopper• 14 years ago

And capable of funny things as well. This is a 2009 video of that same guy who roams the streets of Montpellier with apparently some soccer balls to spare: http://www.youtube.com/watch?v=QfKFIzX7jSY

ChuckMcM• 14 years ago

I think soccer bowling has a real shot at becoming an Olympic sport! :-)

evan_• 14 years ago

First of all- I totally buy that this video is legit, it would probably simply be easier to actually make all of these kicks than it would be to fake the videos. Plus, he's not saying he can make that kick every time, he freely admits that it took him many tries, so while it's impressive it's not so superhumanly accurate that it's impossible to imagine.

But, if I were going to fake the video, I'd have my kicker kick the first ball for real and mime kicking the second (or if that looks fake, really kick a second ball but paint it out)- and then use the same video physics package this guy used to predict where the balls should be at time t, painting the second ball (and both balls, after they "collide") in, frame-by-frame, on the path it charts out.

nhebb• 14 years ago

And here we have empirical (zeal) evidence that should make you reconsider calling someone a "dumb jock". Rugland calculated in his head what a PhD in Physics worked out on paper. We call it coordination. I look at it as another facet of intelligence.

philh• 14 years ago

> Rugland calculated in his head what a PhD in Physics worked out on paper.

Besides that the problem doesn't begin to approach PhD-level...

You could probably say the same thing of me when I'm running or cycling. But whatever calculations my brain performs, the results are not accessible to myself or to anyone else; and I can't tell my brain what calculations to perform, so I can't tell anyone else what angle to lean at, and if I go to the moon I will have to let it work out for itself that gravity is not a universal constant.

You could also say the same thing when a cat twists in midair to land on its feet, "using" the principles of angular momentum. (Scare quotes because the cat has no idea what angular momentum is.)

Rugland's feat is certainly impressive. (And in a different class to my examples, in that he doesn't get constant feedback; if my balance is wrong I can adjust, if he kicks wrong the ball misses. But you could describe my feedback-integration using complicated physics as well, so I don't think that invalidates my point. I'm pretty sure "knife throwing" and "blindfold juggling" would have been suitable examples, but I don't personally possess those skills.) And I'm certainly not saying he's stupid.

But to call this intelligence, seems to be extending the word far beyond its usual boundaries.

gadders• 14 years ago

I wonder if there is a way to make the results of those calculations visible? i.e. you can't calculate an intersection of two arcs, but if a VR environment showed you a ball and said "Throw another ball to hit it", could you then get a good approximation? Just thinking out loud.

oh_sigh• 14 years ago

Perhaps you should contemplate the phrase and its usage. The term "dumb jock" is always referring to dumbness, or a lack of care for, academic and intellectual pursuits. That's it. Your pedantism is confounding.

gnaritas• 14 years ago

Being able to kick a ball, no matter how accurate, is a feat of coordination and physical dexterity, not brains.

SideburnsOfDoom• 14 years ago

Moravec's paradox: http://en.wikipedia.org/wiki/Moravecs_paradox

> The main lesson of thirty-five years of AI research is that the hard problems are easy and the easy problems are hard. The mental abilities of a four-year-old that we take for granted – recognizing a face, lifting a pencil, walking across a room, answering a question – in fact solve some of the hardest engineering problems ever conceived. ... the most difficult human skills to reverse engineer are those that are unconscious.

sixothree• 14 years ago

It seems to me that most of these kicks are better suited for a different sport.

mahmud• 14 years ago

Soccer. In fact, the acrobatic "freestyle soccer" sub-category.

chmod775• 14 years ago

Reminds me of Rémi Gaillard

http://www.youtube.com/watch?v=Ps6s45Pg0To

yk• 14 years ago

Nice video and analysis, but the big animation programs all contain simulation packages, which will calculate precisely this, it may even be easier to simulate the collision than to animate by hand. ( And what really put me off in the video is the use of time lapse in the repeated kicks, precisely at the moment where one could hide a cut.)

On the other hand a NFL level kicker is roughly 50% for 50+ yard field goals. [1] Therefore the 5 50 yard field goals should be possible for an NFL kicker in something like 2^5/2=16 tries. ( Similar for the 60, 50 etc sequence, using again the stats,

1/3.* (24./47)* (96./141.)* (120./136)* (119/121.) = .1

it should require something like 10 tries. ( Assuming a probability of 1/3. for the 60 yrd.) So I think the video is genuine, but only because I wonder why anyone would spoil an impressive video by just one special effects shot.) To really convince me, one would need to calculate the kick of the second ball, since this is the moment where animation and reality have to match.

[1] e.g. David Ackers 24 - 47 for 50+ http://sports.yahoo.com/nfl/players/4587/career

jstclair• 14 years ago

Actually, for 2012, kickers are 61% for 50+ yard field goals [1], including an incredible 10/10 performance.

[1] http://sports.yahoo.com/nfl/stats/byposition?pos=K

aptwebapps• 14 years ago

I bet they do better in practice on an empty field than in a game where they have a limited amount of time and a bunch of big guys headed their way quick.

hardik9882• 14 years ago

I'm more of a soccer fan, and there's this video of a famous soccer player Ronaldinho doing the "crossbar challenge" - from about 30-40 yards out, he repeatedly hits the crossbar with a soccer ball, only to have it come back to him. The precision required to pull of such a feat is just mind boggling. http://youtu.be/KNwLn85I75Y?t=1m25s

kayoone• 14 years ago

Sorry but that Ronaldinho video is a Nike ad and is obviously fake!

hardik9882• 14 years ago

I knew it was an ad. But, I wouldn't be surprised if a professional soccer player could pull that off. Also, could you tell me why you think it is "obviously fake"?

kghose• 14 years ago

I remember a section from Feynman's "Surely you are joking..." where he says he went over a textbook chapter for Newton's laws and found a graph, purportedly from an experiment where they rolled a ball down an inclined plane. He says the graph followed the equations perfectly, with a little Gaussian noise sprinkled in, but they had neglected to account for the rotational inertia of the ball, and so he concluded that the 'experiment' was faked.

The physics of that ‘kickalicious’ kick

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