Why does a football arc through the air and not tumble? | Hacker News Reader

Why does a football arc through the air and not tumble?

vox.com

28 pointsgumby48 comments

Imagine a football made of steel, and rotating very fast, e.g. at 1M rpm.

My gut instinct tells me that it will keep its orientation in space regardless of the trajectory and regardless of any air pushing against it.

Or a football in a vacuum

A spherical football in a vacuum

Your gut instinct is correct. That's because, in this example, you are scaling the mass of the object (giving it huge inertia), while keeping its size constant, hence leaving unchanged the magnitude of the forces exerted on it by air friction. As a very rough estimation, the work performed by the forces that lead to a change in angle will be identical. But due to the larger inertia, that work will result in a much smaller change in orientation.

What would your gut tell you if now that same steel football rotating at 1M rpm was also propelled at an initial speed of mach 1.5, yielding 3 or 4 minutes of flight?

my gut tells me I really don't want to be a wide receiver in this game.

I think any part of your body that touches that football will tell you that...

I think that mass does not even matter, i.e. even with the same mass, with 1M rpm rotation, and propelled at the original (slow, subsonic) speed, the football would have enough angular momentum to keep its orientation, regardless of trajectory or air.

1M rpm, circumference about ½ m ⇒ ≈500,000 m/minute = 30,000 km/hour ≈ Mach 25 (at its widest point)

That’s very fast, indeed.

It wouldn’t surprise me if that ball would partly melt, if it wouldn’t be torn apart by centrifugal forces first.

About 1970, the Washington Redskins (now the Commanders) had two quarterbacks, Sonny Jurgensen and Billy Kilmer. Jurgensen was a better passer, and said that Kilmer's passes offered the advantage that a receiver could choose which end to catch. So even at the pro level, sometimes footballs tumble.

gyroscope made of a football, when thrown in a spiral?

If it acted just like a gyroscope, wouldn't it remain tip-up when the receiver catches it?

i'm not sure, but that's a great question. Precession?

The answer presented in the article (er.. video) is that the drag from the air on the ball's trajectory is the stronger force, and it changes the attitude of the ball.

It tumbles when I throw it. Ba-da-da clang. I'll see myself out.

My guess is because of the rotational forces from spinning. Like a rifle spins a bullet. Seems pretty obvious. Not clicking because Vox.

> Not clicking because Vox.

Do they have an enormous number of trackers not screened out by ad blocking?

The paper, written by a physicist, deals with the non-obvious behavior by which the ball launches with one end pointing up and lands with that same end pointing down, which does not happen in other cases like (literally) a loaf of bread or a box of girls scout cookies.

Can we add the word "(American)" before the word football please?

"Gridiron" is more universally recognized IMHO.

Isn't "Handegg" the official term?

Football games aren’t called football because you kick the ball with your foot, but because they are played on foot rather than on horseback.

I now have a very strange mental image mixing polo, ice hockey, tennis, and anthropomorphic horse cheerleaders with pompoms.

I want to play this game.

They're not exactly the same thing. Gridiron football is a whole category of football games, including the substantially similar US amateur and pro games, Canadian football, Arena football, etc. But all use the same type of ball and have forward passing, and so I agree that gridiron football probably is the more accurate term for this article. On the other hand, I don't feel like it's often used in real conversation.

It's not recognized in the US at all. The only time the phrase "gridiron football" is used ever in the world is during conversations like this.

There's something ironic in rewriting the names of a US sport in a title to make it understandable, but in a way that is unintelligible to people from the US.

Oh, thanks. I got here wondering why a ball would tumble.

Americans don’t call it American Football though and this is an American centric article.

But this is not an only America centric website and hence the OP’s request. Never harms to have an accurate title when international audience is involved.

> Never harms to have an accurate title when international audience is involved.

But this does not _only_ apply to "American" Football balls but also to Canadian Football balls...

https://en.wikipedia.org/wiki/Comparison_of_American_and_Can...

But it would only be accurate for non-Americans. That’d be like asking a UK article to use soccer instead of football because other international audiences don’t call it football. There isn’t an accurate title when different populations use different vocabulary for the same thing.

Complain when titles are changed.

Complain when titles are printed verbatim as-is.

Complain when an article from Vox (an explicitly American website) is written from an American perspective.

Say what you want, but I can't imagine an American being lame enough to request that a football article from "theguardian.com" be renamed to specify "soccer".

Look on the bright side - we've moved beyond the "I think you mean handegg" meme when people complain about American football vs rest-of-the-world football.

And, to be fair, I am an American and am certainly lame enough to complain when I see something Britishly done that I don't like. For example, when a BBC article is posted about UNICEF, their capitalization rules have them bizarrely spell it "Unicef".

> Look on the bright side - we've moved beyond the "I think you mean handegg" meme when people complain about American football vs rest-of-the-world football.

Sure?

https://news.ycombinator.com/item?id=33194609

Especially since "soccer" is actually English and not American slang. I'm not clear on the details on account of English slang delights in its incomprehensibility to outsiders, but apparently it's short for "association (football)."

Also used in Australia where four kinds of football are played: AFL, Rugby Union, Rugby League and Soccer.

"Soccer" is not really used in the UK unless you are a) posh or b) need an alliteration (such as the TV show Soccer Saturday).

> Complain when titles are changed.

> Complain when titles are printed verbatim as-is.

Unless you have in mind a specific instance of StevePerkins's doing both of these, you're not pointing out an inconsistency, just that you can't make everyone happy—which isn't a good reason on its own to write off any particular request.

> Complain when an article from Vox (an explicitly American website) is written from an American perspective.

I didn't see any complaint about anything, and even the request was made to HN, which, to my understanding, is not an explicitly American website, or at least welcomes an international audience. I did not see even an implicit suggestion that Vox should change their headline.

You should assume good intent. Unless you specifically know that the specific user you replied to have complained about both you should not put words in their mouth.

The article is about American football, but the headline is non-specific and I thought this website (and YC) was global?

I struggled with this when submitting it (particularly as I played AFL -- poorly -- as a kid). But decided it would look too strange as people don't say "American Football" in the US, any more than they say "rugby football" elsewhere.

I did not use the verbatim headline from the article because it was clickbait that was even less informative than what I wrote. So I was already doing some editorializing. I should have added the word "Physics" though.

I did say put it in brackets. Maybe square brackets would make it clearer it's an addition.

If this is what troubles you, the proliferation of the ambiguous pronoun must be hell.

It made the headline harder to understand than necessary. I was confused at first.

TL;DR: Footballs follow ballistic trajectories thanks to their spin imparted by spiral throws and punts.

But they also can tumble, particularly on kickoffs, to make their trajectory harder to predict

A while back I was working on a VR football game, and had been looking at these kinds of articles about the physics of the arc.

We added a fake aerodynamic force that nudged the ball towards it's direction of travel if it was reasonably closely aligned, and scaled the force with the rotational velocity of the ball around it's the "spin" axis.

It worked quite well and felt pretty good. The ball would still tumble if released poorly, but a well thrown ball would arc nicely.

Interesting. How did the dynamics of a VR football game work? I've always considered American Football a dynamic sport - the excitement of the game is that everyone on the field is in a tightly choreographed dance. It seems to lose its luster if you were just sitting/standing in a room with a VR headset, unable to run around.

We pretty much just implemented the quarterback and passing game.

So, you could drop back in the pocket, step side to side a bit and throw the ball.

There's a version of football called "7 on 7" which is basically just the Quarterback, Receivers, and Pass Defenders. It's common as a scrimmage practice for the passing skill positions, and it's a sport in its own right (see: https://www.youtube.com/watch?v=X2cckOaEOWM for an example).

So that's basically what we implemented. We also had, basically, minigames, which would focus on one receiver and throwing the ball to them in the right spot of the route.

Between play selection, pre-snap defensive reads, post-snap defensive reads, needing to release the ball within a few seconds, and having to release the ball with good form at the right time, to the right position on the field, there's a ton going on at the quarterback position, which is why we started there.

Once you released the ball, your job was basically done, and you watched the rest of the play unfold. It worked pretty well, it was a bit of downtime, and there was some randomness in the result (players could miss a tackle, for example) so it was kind of engaging. One thing that was very satisfying was hitting a wide open receiver perfectly in stride and not even bothering to watch them run into the end-zone because you knew they were going to score.

Was that really Rodgers-Cromartie playing in that game?

[edit]

Yes, https://giantswire.usatoday.com/2021/09/02/ex-new-york-giant...

Spin acting against the force of air, which acts as a force against the end of the ball just as gravity acts as force against the bottom of a spinning top. In other words, it should not tip over if thrown in a vacuum.

We've known that spin prevents an object from tumbling thru the air since at least the invention of rifling (a little before the American Civil War). It's the tipping over that was unexplained.

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