# Talk:Bouncing ball

## Related projects on commons and wikiversity

• File:Happy_vs._Sad_Ball.webm is a video illustrating how an elastic collision to a heavy object delivers more momentum than does an inelastic collision. Please consider either a link to this video, or perhaps placing it in this article. A strong reason for doing so involves using Wikimedia as a vehicle for allowing young people to establish reputations via Creative Commons works. This video was produced by an undergraduate student user:13hartc.
• v:How_things_work_college_course/Momentum_transfer_under_elastic_and_inelastic_collisions is rather sloppy and disorganized effort conducted with a couple of classes to model this theoretically. We could not get a good match between theory and experiment. This should probably be linked as a Wikiversity sister link. If you wish, I will make that link for you.
• v:Advanced Classical Mechanics/Compound ballistic pendulum with spinning ball is a calculation that I did, with a bit of student input described as follows: I posted the calculation on Wikiversity and conducted a lab for calculus-based physics majors that worked as follows: Each pair of students was assigned a portion of the calculation. They were to assume that all preceding formulas were correct, and only check the algebra up to the end of their assigned portion. I'm not sure how it worked...I probably should have assigned it later in the course.--Guy vandegrift (talk) 14:28, 16 February 2017 (UTC)
Hi , thanks for the links. I've given them a quick review, and... well my initial impression are as follow.
Concerning the video, I feel it a bit of a bad demonstration because it is unfocused on what exactly it wants to highlight. On a technical level, the interesting aspect of this video would be what happens at the impact for each of these balls, but we don't really see what's happening with great clarity. Two ball roll down and bounce. If you pay attention, you'll notice the ball falls in the gap and makes the block wiggle, while the second one bounces back and makes the block fall. My suggestion for your student (if you're the one supervising them) is to
• Shoot the video to have a side view of the ramp and put the block flush against the ramp in both cases. The gap between the ramp and the block is a distraction and prevents an apple-to-apple comparison of the two situations. The sideview highlight the 2D-nature of the situation, and should be shown accordingly. Having a sideview will clearly show the recoil of the block in both cases, as well as bouncing distance.
• Use a tripod/fixed camera
• Start with the happy ball, as this is what most people would expect a ball to behave like.
• Put a ruler on the side of the ramp, this way we can have sense of scale and see how far back the balls bounce.
If that student isn't around anymore, making such a video could be project for another student. But as the video stands, I'm very ambivalent about including it in the article. I feel it would muddy the waters more than it would clarify them. There is an argument that we don't link to any free (libre) videos, and I'm partial to that, but I'm not convinced the quality is high enough to be useful here.
Concerning the Wikiversity stuff, I'm not quite sure what the policy on linking to materials from Wikiversity is. It certainly doesn't pass as a WP:RS, and we'd be linking to material such as " The simplest theory says that happy is fully elastic and sad is fully inelastice, and the sad/happy pathlength ratio is 4." which (on top of the poor grammar/spelling), stands out as an very unclear and unsubstantiated claim. And the later analysis completely neglects the rotational energy gain by the ball as it goes down the ramp, which cannot be neglected here, and the energy need to knock the block will depend on the geometry of the block. But if linking to such material is done in other articles / if Wikiversity is treated like other sister projects (e.g. Commons, WikiSpecies, etc.), I'm not objected to have the usual linking templates added to the see also/external link sections. 15:33, 16 February 2017 (UTC)

## GAN, deflategate, bouncing and gauge pressure

I read this article and considered reviewing it for the GAN, but I do not have enough technical expertise in mechanics to feel competent to verify most of the content. I also feel it would be a better process for me to nominate an article and learn from the feedback provided by a reviewer. If it goes another week lacking a reviewer, though, I could probably be persuaded to just jump into the deep end, maybe with a mentor?

I do have a question, though, about the last paragraph, first sentence: "The bouncing of a gridiron football was at the center of the deflategate controversy." Bouncing is not mentioned in either of the references provided, but the Well report does quote Tom Brady on page 40, commenting on the inflation level of the balls during an earlier game:

According to Brady, Jastremski and other Patriots personnel, during the October 2014 Jets game, Brady complained angrily about the feel and inflation level of the game balls. He told Jastremski between drives that the balls felt "like bricks" and were heavier and harder to grip than they had been when he approved them prior to the game. Although Jastremski believed during the game that Brady was simply being competitive, he tested the air pressure of the game balls the next morning, discovered that many of them were over-inflated, and told Brady that he was right. [emphasis mine].

So my question is: are you using "bouncing" as a synonym for "inflation level" in this context? It is true that gauge pressure is correlated with bounciness, and gauge pressure is what is regulated by the NFL, but bounciness was not really the issue — it was whether Brady gained an illegal advantage being able to grip and throw a softer, lighter ball. It's misleading/inaccurate to say that the bouncing of the gridiron football was the issue at the center of the controversy; it would be more accurate to say, "Gauge pressure was at the center of the deflategate controversy." Cheers! — Grand'mere Eugene (talk) 22:05, 19 February 2017 (UTC)

It's... I suppose it depends on how picky you want to be. In general if I throw something at you, and you want to catch it, you can consider the ball as making an impact with your hand. If it's deflated, it will have a lower coefficient of restitution, thus bounce less, and thus be easier to catch. If you inflate it more it will be bouncier, and thus be harder to catch. If you want to say this is about the gripping properties you wouldn't be wrong either, with the subtle difference that there is no impact (thus bounce) at when you throw a ball, only when you catch it, and the gripping properties also cover other things like friction. So deflate gate is partially, but not purely, about the bounce of a gridiron football.
If this is something you think needs to be clarified, we could go with "The pressure of a gridiron football was at the center of the deflategate controversy." instead. 00:39, 20 February 2017 (UTC)
Thanks for your explanation. The revision you propose is obviously supported by the sources, and would be better, especially if the article is read by fans of American football! Cheers! — Grand'mere Eugene (talk) 01:07, 20 February 2017 (UTC)
Done. 01:11, 20 February 2017 (UTC)
Isn't "Deflategate" treated as a proper noun (and hence capitalized)? —Quondum 02:20, 20 February 2017 (UTC)
NYT and ESPN both capitalize it, but NPR does not. Grand'mere Eugene (talk) 02:28, 20 February 2017 (UTC)

## magnitudes and signs ...

From [note 1]: "Here, v and u are not just the magnitude of velocities, but include also their direction (sign)."

This is confusing to anyone with a bit of background of Euclidean vectors (adding a sign to a magnitude is undefined), so clarification might be appropriate. Should it be "Here, v and u are the single component of the velocities, including sign, where all motion is one-dimensional."? —Quondum 18:12, 20 February 2017 (UTC)

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## GA Review

Reviewer: Reyk (talk · contribs) 12:28, 30 October 2017 (UTC)

I'll start on this in the next day or so. First impression is very positive. Reyk YO! 12:28, 30 October 2017 (UTC)

OK, so I've had a careful read through the article and it is very close to being passed as a GA. The most obvious criteria are fulfilled already: a high standard of writing, complete and accurate sourcing, no copyright issues that I can detect, and a sensible selection of images. My major concerns are that it's a bit math-heavy, and that it lacks a discussion on the history of the scientific investigation of bouncing balls. I'd also suggest moving a few of the images to the left to break up the monotony of them all being on the right side. In the next few days I'll go through the article thoroughly, section by section, and make a few specific suggestions, but I don't see any reason why this can't be passed within a week or so. Nice article! Reyk YO! 18:51, 31 October 2017 (UTC)

Well written and representative of the rest of the content, if perhaps a little brief. Adding a few words on the historic importance of investigating the physics of bouncing balls would help in this regard.

Well, historically speaking, it's not a very important topic. The physics of bouncing balls has been mostly used as a teaching tool, since in the simplified version it illustrates projectile motion rather well, and you can talk about energy losses at impact and so on. This is done at the high-school level and first year university level. In more advanced courses (2nd/3rd year physics), you'll simplify less, adding air resistance into the picture, sometimes using the bouncing ball as a problem to be dealt with numerically. The real importance is in sports engineering, but there's little historical aspect to it. The coefficient of restitution has a lot more history, but bouncing balls themselves have very little "prior research" on them. I could try to dig really old examples (e.g. Mesoamerican rubber balls), but we'd veer into WP:SYNTH territory very soon. There's plenty of history available on balls in general, but very little on their physics. Headbomb {t · c · p · b} 21:16, 9 November 2017 (UTC)
Added a bit. Headbomb {t · c · p · b} 17:02, 19 January 2018 (UTC)

### Forces during flight and effect on motion

This looks OK.

This looks OK.

#### Drag

This is all pretty much OK, but it would perhaps benefit from an example of a kind of ball with low Reynolds number where the Stokes Drag effect dominates.

Any ball moving at a very low speed. For a ping pong ball in air, we'd be talking ~1 mm/s to get Re = 1, less for larger balls, which is something that would never be encountered in actual sports. A 5mm ball bearing dropped in a viscous liquid such as oil would have a much better chance of having low Reynolds number (Re = ~1 if v = ~5.4 cm/s), since the combination of buoyant forces and drag will be much much larger, and the ball will be moving much slower (terminal velocities ~1.5 cm/s). I could expand this. Headbomb {t · c · p · b} 21:34, 9 November 2017 (UTC)
I feel this is adequately covered by "In reality, flow is never inviscid, and the Magnus lift is better described by..." Headbomb {t · c · p · b} 17:13, 19 January 2018 (UTC)

#### Magnus effect

I don't understand why the paragraph on ball tampering is in this section. Did these instances of ball tampering involve changing the spin characteristics of the ball? If so, that should be clearer.

• On a related note I seem to remember other Cricket-related controversies where the pitch was tampered with (by running on it unnecessarily, or poking at cracks), to change the way the ball bounces off it. That might be relevant later in the Impact section?
I don't know if the 2006 instance in particular involved with messing with aerodynamic properties, but in cricket, that's generally why you do it. Not really sure what is unclear here, but I'm open to suggestions. Headbomb {t · c · p · b} 21:38, 9 November 2017 (UTC)

All fine.

### Impact

This is all OK, but perhaps a few words on how impact velocity affects the COR.

I can add that. e slightly increases with the impact velocity generally speaking, as the ball will deform more, thus heat up more. Headbomb {t · c · p · b} 21:40, 9 November 2017 (UTC)
Added "In general, the ball will deform more at higher impact velocities will lose more energy, decreasing its COR." Headbomb {t · c · p · b} 17:13, 19 January 2018 (UTC)

#### Spin and angle of impact

This is all good, well written and informative.

#### Non-spherical balls

Again, this is all good. Earlier, the article says the COR can exceed 1 in the case of, eg, a spring-loaded impact surface. It might be worth briefly mentioning there that rotational velocity can influence this too.

That's covered by "Because it is possible to transfer some rotational kinetic energy into translational kinetic energy..." Headbomb {t · c · p · b} 17:14, 19 January 2018 (UTC)

#### Multiple stacked balls

This is well sourced and interesting, particularly the applications to astrophysics.

### Sport regulations

Also fine. I would suggest adding Australian Rules Football, because that is the most significant oval-ball sport in which bouncing the ball is a major part of the game: [1]

Added. Headbomb {t · c · p · b} 17:28, 19 January 2018 (UTC)

### Status query

Reyk, Headbomb, it's been over two months since the article was last edited, and over six weeks since the last post to this page. Where does the review stand, and how much work remains to be done on the article? Many thanks. BlueMoonset (talk) 02:48, 14 January 2018 (UTC)

• I'm done on my end. Once the suggested changes are done I am happy to promote this. Cheers! Reyk YO! 08:08, 14 January 2018 (UTC)
• Right, I forgot about this with work flaring up and other projects. I'll get to this by the end of the month. Headbomb {t · c · p · b} 12:54, 14 January 2018 (UTC)
I added a couple of things, let me know if there's anything more you need. Headbomb {t · c · p · b} 17:30, 19 January 2018 (UTC)
It's not up to me, Headbomb, it's up to Reyk, who's the reviewer; we'll see what Reyk says. BlueMoonset (talk) 20:02, 19 January 2018 (UTC)
doh, my bad. Mixed up who was the reviewer after so long! Headbomb {t · c · p · b} 22:46, 19 January 2018 (UTC)

### All done

OK, I had another read through the article and I think it's good to go. The bit about the ball tampering in cricket still is a bit lacking in context but that's not a dealbreaker, so I'm going ahead and promoting this article. Thanks for improving the encyclopedia. Reyk YO! 11:22, 21 January 2018 (UTC)