Published online 27 October 2006 | Nature | doi:10.1038/news061023-16

News

# Hoops, sweat and tears

Physicists help a US basketball team get to grips with its new ball.

Slippery when wet. Synthetic basketballs are also less bouncy than leather.© Getty

This year, the US National Basketball Association (NBA) is switching from using a leather to a synthetic, microfibre ball. The animal-welfare group People for the Ethical Treatment of Animals is thrilled, but basketball players complain that the new ball is slippery and doesn't handle as well as the old one.

Texas NBA team the Dallas Mavericks called in a team of physicists to clarify the differences between the two balls. The project's leader, Kaushik De of the University of Texas, Arlington, talked to news@nature.com about what they found. The key, it seems, is sweat.

## Where did you begin?

The very first test that we did was to just drop the two balls from the same height again, and again, and again, and see how high they bounce. Leather balls turned out to bounce 5-8% higher than synthetic balls, depending on the kind of floor.

## What other tests did you do?

The new balls also seem to be a little more erratic in how they bounce. After a few bounces, the synthetic ball would no longer be within the range of the measuring device. But the leather ball came back to roughly the same position.

We also measured the deviation. The circle of deviation for the leather ball was 15 millimetres, but for the synthetic ball it was 22 millimetres, although these are very preliminary measurements.

At that point we started looking at the surface of the ball. One basic difference is the logo. On the leather ball the logo does not change the surface much, but on the new ball, the logo is embossed, creating a groove on the surface of the ball. These grooves are half a millimetre deep, and cover 20% of the ball's surface. We think maybe that's what's causing the difference in the bounce.

The next thing that we tackled was friction. Our first question was how to simulate human skin. In a medical journal, we found that silicon has a similar friction coefficient as the human palm, so we used sheets of silicon to test friction.

We found that the synthetic ball has a friction coefficient twice that of the leather ball when dry. Even someone with a small hand like me can pick up the synthetic ball with my palm. I can't do that with a leather ball.

So then we said, 'How do we simulate sweat?' We decided teardrops should be somewhat similar. I had some eye drops, so we used those.

When the new ball is dry, it's twice as sticky as the leather ball. When we put one drop on it, the leather ball's friction coefficient stays the same— but for the synthetic ball, it drops by half. And as the leather ball gets wetter, it actually becomes stickier. The synthetic ball does exactly the opposite.

Those are the most important tests. We are also setting up a wind-tunnel test to measure drag or aerodynamic difference.

I've never played basketball. I've watched the playoffs, but I never play.