How the ball bounces

Driving to work this morning, I noticed a tennis ball lying in the middle of the street. I began to imagine a child’s guessing game: It’s round (an orange?), greenish yellow (a Golden Delicious apple?), and it bounces. I wondered if that last clue would put me on the right track or if I’d be stuck thinking about fruit and trying to think of one that would bounce.

What makes a ball bounce, I wondered, while a ripe piece of fruit will more likely go splat. A ball is made of bouncy material, of course, and a fruit is not. But that doesn’t actually explain anything, it just identifies an object’s composition as the factor that determines bounciness.

But what actually happens, from the point of view of physics, when a round object hits the ground, that makes some of them bounce and not others? My first thought was that a ball can partially flatten where it hits, then that part recovers its normal shape and pushes itself off. A ripe piece of fruit flattens but can’t get its round shape back.

That didn’t seem to answer the question very well, though. A rubber ball flattens where it hits, but what about more rigid balls that still manage to bounce? I suppose there must be some slight flattening to a baseball or a golf ball, but it couldn’t be all that much, could it? I’ve even seen someone bounce a bowling ball, which I wouldn’t have thought possible, and it’s hard to imagine that a bowling ball can flatten much at all.

(My husband and I joined a bowling league early in our marriage. I had never bowled much, and Jon was out of practice, so we had low average score and therefore high handicaps. By the end of the season, we had improved a good deal, and between our raw scores plus the handicap, we ended up in the playoffs for the league championship. Even with the handicap, we were happily amazed when we actually won. The captain of the other team, arguably the best bowlers in the league, was also surprised, but not happily. He actually started dribbling his bowling ball, I suppose to take out his frustration on an inanimate object.)

So I started looking for a scientific explanation of bouncing balls, which was easy to find. This article explains the physics clearly and simply. It also describes how the shape of a ball deforms a good deal more than I had imagined.

Looking for more information on the work of Harold “Doc” Edgerton, whose photographs revealed just how greatly the ball deforms (in a fraction of a second too short for our eyes to see it), I found this article, which also has a link to a site devoted to Edgerton and his life and work. (I had seen his photos of water and milk, but I had never seen the sports-related pictures.) I couldn’t find a really clear picture there showing a ball being deformed as it hit, but I did find one here. Hard to believe that’s a baseball, isn’t it?

Finally, for a more thorough discussion of exactly what happens when the ball hits, including diagrams, take a look at this site.

Amazing what I end up learning just by noticing a ball lying in the street…

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