Callum Brown, Blake Evans and Nick Stoddart, of Balclutha Primary School, asks :-
What design features are important in a rugby ball? Does spinning a rugby ball when it is thrown affect its performance?
Chris Button, a sport scientist at the University of Otago, responded.
There are numerous factors including size, weight, shape, robustness, grip, valve placement and integration, as well of course as the aesthetic look of the ball.
When Richard Lindon and William Gilbert first started making balls in the 1823 for Rugby school (England) they were made out of hand stitched, four-panel brown leather casings with inflated pigs' bladders inside!
The early balls were larger and rounder in shape than today's rugby balls and looked more like soccer balls. Modern rugby balls have a rubber inner layer rather than a bladder, and waterproof synthetic materials are used for the casing panels.
Other new performance developments include a dimpled surface to improve the grip of the ball and innovative valve designs to deliver superior shape retention and spin rate. To impress your friends you can tell them that the scientific term for the shape of a modern rugby ball is called a prolate spheroid. The narrower, slimmer shape and more robust materials introduced over time have made the rugby ball more reliable in terms of flight characteristics.
Does spinning a rugby ball when it is thrown affect its performance? The ball is typically thrown with one of the pointed ends first to make a more aerodynamic shape as it flies and thus to reduce the amount of drag force acting on it. If the major (long)axis of the ball is parallel to the ground, the ball will have more velocity than if its major axis is perpendicular to the ground.
Players apply spin to the ball to make it fly with more stability (less wobble) and more predictably. The spiralling ball creates something known as a centrifugal effect which also reduces air drag and helps to align the ball as it travels through the air.
A similar effect happens when you ride a bicycle. The faster you spin the wheels the more stable the bike feels but as you slow down the gyroscopic effect is less and the bike begins to get wobbly. So rugby players spin the ball to make it fly further, faster and straighter which are key characteristics of accurate passing.
The rugby ball shape is also handy when kicking the ball towards the opposition. The bounce of the ball is unpredictable, depending on how it lands.
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