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Comment by ISL | Hacker News Reader

A cool article, laid out to be readily understandable for everyone.

Missing from the discussion, though, is a description of the change in the moment of inertia. If the ball's mass distribution has shifted radially, the moment of inertia can change significantly.

Spin matters very much to the aerodynamics of baseballs. A change in the moment of inertia could have a substantial impact (positive or negative) on the flight characteristics as the ball reaches the batter, the mechanics of the ball/bat interaction (particularly as relates to the spin of the ball leaving the bat), and the outbound trajectory. A lower-moment ball will spin faster initially, but slow its rotation rate more quickly, making any prediction of the overall effect of a change in moment of inertia difficult indeed.

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It's funny, because based off the header image on that page [0], that's what I expected the discussion to be about. The center of gravity in those four samples is hugely off.

[0]: https://espnfivethirtyeight.files.wordpress.com/2018/03/juic...

If you scroll down further in the article, you'll see that those are cropped images of the cores. The cropping is off-center, not the cores. Otherwise, they would wobble terribly when thrown or hit.

No, I understand that. But look closely at the x-ray/CT and you'll notice that the rubber is nowhere near even around the core.

The background images appear to actually be a video or something, but here's a screenshot I took:

https://share.neosmart.net/View/Index/nfgSDo.png

The two balls in the first column (so one old and one new) have a pretty symmetric distribution, but look closely at the others. Some are ridiculously uneven.

Yeah, but that doesn't matter here because it's present in both the newer era balls and the older era balls. We also know that seven of these are used baseballs, meaning they could very well be deformed or off center from use. There's a reason that they only use a baseball for about 8 pitches. (https://www.foxsports.com/other/story/major-league-baseballs...)

In cricket the same ball is used for 480 balls (depending of format). Initially the new hard ball is seen as an advantage to the fielding side as a bowler gets more bounce and deviation from the hard seam, but a batsman can also hit it further. There is also the phenomenon of swing (a bit like the curve ball in baseball) which is accentuated by the fielders polishing one side of the ball. As the ball ages and softens it's bounce becomes more true and therefore it favours the batting side. Often towards the end of its life the ball develops 'reverse-swing' where it curves the opposite way. They have to keep the 'rough side' of the ball completely dry to do this, so all fielders have to keep their hands dry. Penalties have been applied to players trying to alter the condition of the ball. Different brands of ball are polished differently and swing at different times. Some even have a lacquer which takes a while to come off and start swinging at all

why I think this is relevant

1) The aerodynamic effects of the kind polish applied dramatically affect performance

2) Since wear on the ball is critical to the long format games, the fans don't get to take one home and sell it on ebay, even if they catch a six (ball hit into the crowd) :(

The significant change in cricket hitting has been massively heavier bats, which used to be made of compressed willow to preserve their life (a whole season for a pro), but are now made of unpressed willow and considered disposable. The unpressed bats work much like corked baseball bats are supposed to, and have transformed the game. Slow-bowlers, like spin bowling where the bowler spins the ball to alter its flight and make it move when bouncing, have to be exceptional now or they get smashed into the car park!

The most significant change in cricket has been covered wickets when it rains. If you have ever had the fun of playing on an uncovered wet wicket you will understand how elite teams were able to be bowled out for under 20 runs on wet wickets.

I also think more body protection has played a massive role in more aggressive batting. When I was young I used to play with nothing more than pads, gloves and a box and had to face bowlers who could bowl over 140km/hr on very unpredictable wickets. I got out more than once getting out of the way of some bouncer that stayed low as I was looking to avoid getting hit. I have been hit enough times with a cricket ball to have learned self-preservation is the better part of valour.

^This.

As a teenager growing up in the UK I played cricket for my school and a local team, and when I was about 14 I had the "pleasure" of facing roughly four balls bowled by the Legendary West Indies cricketer Viv Richards[1] when he visited my school. I somehow managed to get the bat onto the first ball and score a 4, at which I foolishly celebrated shouting something along the lines of "in your face, I thought you were a professional cricketer!" What a stupid, stupid thing to do.

There then followed a three ball lesson in bodyline bowling until I literally lost my bottle and refused to carry on leaving the wicket with what little dignity I had left. Seriously, seeing that giant six-foot tall man taking a run-up and then bowling a cricket ball at my head at what seemed like supersonic speed literally scared the crap out of me.

[1]: https://en.wikipedia.org/wiki/Viv_Richards

You are right about the covered wickets, although when I started watching cricket in the 1990's they were already universal in the elite game. I should correct my statement and say that heavy bats have been the most significant thing in the last 20 years of the elite game with regards to big hitting.

I also agree about protection, being hit by a cricket ball is something you don't forget, and can indeed be lethal[0]

[0]:https://en.wikipedia.org/wiki/Phillip_Hughes

I had no idea that sixes were returned to play[0]. Thanks for that info as well as the rest of your comment :)

0: minus the occasional Chris Gayle blast out of reach of anyone, I imagine

It is possible that a ball can be hit out of a stadium and not be returned (but it is usually very rare due to stadium sizes and layouts).

If that happens they have a tray of prepared balls of different states, then they'll select one that most closely matched the previous one's condition/age etc.

The moment of inertia can change without a change in the center of gravity. Think ring vs. disc. I suspect this is what the GP was talking about.

Moment of inertia changes will also wildly affect a slightly off-center hit, as well as how hard it is to catch the resulting spinning ball.

I really appreciate your spin on this article.

sorry

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