Article

Bounce of an oval shaped football

Sports Technology 08/2010; 3(3):168-180. DOI: 10.1080/19346182.2011.564283

ABSTRACT

Oval shape footballs used in American and Canadian football are similar in size, shape and weight to those used in Rugby League, Rugby Union and Australian Rules, all being about 28 cm long, 60 cm in circumference and weighing about 410 g. A generic football fitting that description was filmed with a video camera at 100 fps (frames per second) to determine its bounce properties. Compared with a spherical ball, the bounce of an oval shaped football is less predictable since the normal reaction force can act ahead of or behind the centre of the ball, depending on its alignment on impact with the ground. Projected at an oblique angle without spin or with backspin, a football usually bounces backward if the top end points backward on impact. Projected with topspin, a football usually bounces forward, but it can sometimes bounce to a much larger height than usual, or roll for a short distance before it bounces. The coefficient of restitution was found to be greater than unity in some cases. Another surprise was that the horizontal speed after the bounce was sometimes larger than that before the bounce. The latter effect was due to a reversal in the direction of the friction force during the bounce, resulting in acceleration of the ball in the horizontal direction. These effects were found to be consistent with a simple theoretical bounce model. The forward bounce speed is maximised when the ball is inclined forward at 45° on impact, and the backward bounce speed is maximised when the ball is inclined backward at 45° on impact. Force plate measurements of the normal reaction and horizontal friction forces acting on the ball are also presented. In some cases, the friction force reversed several times during the bounce.

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