If U.S. Open fans notice an extra bounce in the tournament's tennis balls, it's not just their mood. If you ask a physicist, there's a perfectly logical explanation:
Call it the "coefficient of restitution."
Confused? Don't be. That technical phrase is just a scientist's way of saying "bounciness." As reported by Andrew Beaton in The Wall Street Journal, that bounciness is increased by high summer heats that warm the air and the playing surface of tennis courts.
When tennis balls are exposed, their coefficient rises.
Physics professor Michael Lisa conducted research at Ohio State University testing the bounciness of tennis balls at three different temperatures: 68 degrees, 100 degrees, and 28 degrees.
Compared to the room-temperature ball drop, the 100-degree ball bounced 16.5 percent higher. The cold ball was the worst performer: it only bounced 12.2 inches off the drop, compared with 57 inches at room temperature.
"It's a rough experiment," Lisa told the WSJ. "The results were consistent with what I expected."
The ITF, for what it's worth, does have bounciness standards for its regulation tennis balls -- and it does account for changes in temperature. With air temperatures at the U.S. Open topping 90 degrees, and playing-court surfaces stretching even higher, tennis balls are bouncing much more than they would in other conditions.
And given the hard playing surface -- compared with the grass used at Wimbledon and other tournaments -- U.S. Open players are competing in one of the bounciest situations they will face all year.
Here's hoping they consider the coefficient of restitution when bracing for the opponent's serve.