NBA tribology

In fall 2006 NBA started using basketballs made with synthetic material instead of leather. They made the switch because they wanted every basketball they use to feel and bounce the same. Not all leather balls are exactly alike in weight or how they bounce, but the synthetic balls are. However, some players complained right away that the new balls bounced differently and were actually harder to control than the leather ones. Dallas Mavericks owner, Mark Cuban, asked for help from the Department of Physics at the University of Texas in Arlington, Texas.

What was the result of research conducted? One of major reasons for the differences observed was frictional behaviour.

Research summary (fragment):

1. The surface of the synthetic balls display a factor of two higher coefficient of static friction when both leather and synthetic balls are dry. This characteristic would make the synthetic balls easier to grip (stickier) than the leather balls, when dry.


2. However, with a few drops of added moisture on the surface of each type of ball, the new synthetic balls have a coefficient of friction which is at least 30% smaller than similarly moistened leather balls. We have also measured the moisture absorption rate, which confirms that the synthetic ball absorbs moisture at a much slower rate, leaving more of the moisture on the surface. Therefore, when wet, the synthethic balls are much harder to grip and handle (slippery). By contrast, our measurements indicate that the grip of the leather ball improves after similar moistening.

Details of Measurements (fragment):

1. The synthetic balls absorb water at a much slower rate, averaging 8.6 grams per minute. The conditioned leather balls will absorb water more rapidly, about 70 grams within a minute. After quasi-saturation at these water masses, the leather balls absorb water at a much slower rate than the synthetic balls, averaging 3.3 grams per minute.


2. Initial friction tests show a much higher coefficient of friction for the synthetic ball when dry. The coefficient of friction between the surface of the synthetic ball and a silicon surface (medical literature shows silicon to have a friction coefficient similar to the human palm) is about 3.2, for our experimental setup. The friction coefficient is 1.69 for the leather ball, using the same procedure.


3. Friction tests with liquids such as Visine (which has viscosity higher than water, similar to human tear drop, possibly closer to sweat) applied to the silicon (one drop per 2"x2" area) show that the coefficient of friction increases for the leather ball. After repeated application of drops, the coefficient increased gradually by at least 30% for the leather ball, thereby making it more "gripable". After quasi-saturation. adding drops reduced the coefficient by 20%, relative to a dry ball. However, for the synthetic ball, the coefficient of friction reduces immediately by 55% with the first drop of liquid. A larger reduction is seen with repeated application of liquid. In conclusion, the wet synthetic ball is significantly more slippery compared to wet leather balls.

In January, the NBA went back to using the traditional leather balls. They aren’t perfect, but for now, that’s just the way the ball bounces.

Fast Facts:

• It takes the hide of one whole cow to make four leather basketballs.
• Leather balls absorb moisture eight times faster than the synthetic ones.
• Because it absorbs so much sweat, a leather ball may increase its weight by 10 percent during a game, but a synthetic ball remains the same.

Sources:

National Geographic Kids
The University of Texas at Arlington