Measurement in sport – the long and the short of it
Box 4 | Rackets and balls
Elasticity of balls
On impact with a racket or the ground, a ball flattens or compresses, regaining its original shape as it pushes against the surface and rebounds. The property of a ball that causes it to regain its original shape is called its elasticity.
Energy changes on impact
A falling ball has kinetic energy (energy of motion). On impact, some of the kinetic energy is stored in the ball as elastic potential energy. As the ball returns to its original shape and starts to rebound, the elastic potential energy is converted back into kinetic energy. (Some of the original kinetic energy is lost on impact, being converted into heat and sound. This means that the rebound height is always less than the starting height.)
The greater a ball's elasticity, the faster it will return to its original shape and the farther it will rebound when it is hit or kicked. In tennis, changes in the design of tennis balls have increased their elasticity, significantly speeding up the game – the ball travels faster when hit, and the tennis players must travel faster to get near it. Squash balls are made with different elasticities. They are colour-coded as fast or slow balls.
Rackets
Modern squash and tennis rackets have enabled players to increase ball speed. The technology comes from aerospace research and new manufacturing techniques.
Tennis racket frames were first made of solid wood. A stronger racket was produced when the frame was made of laminated wood – thin layers of wood steamed, glued and pressed together. Later, metal rackets were made of steel, titanium, magnesium and aluminium. The strength of the metal meant that frames became narrower and lighter and could travel faster through the air.
Today, no top-class player uses wooden rackets, and very few use metal ones. Instead, the frames are made of composite materials. These consist of fibres – such as carbon fibre, glass fibre, boron, Kevlar and ceramics – bonded together using a resin. These composite materials are stronger, lighter and stiffer than wood. A stiff racket frame absorbs less of the ball's energy, thereby increasing the power available to the player.
Ball speed
Tennis has become a faster game as a result of new designs in rackets and balls. Top players can serve at up to 220 kilometres per hour, making the judgement of line-ball decisions extremely difficult. Line judges are now able to rely on an electronic eye to determine if a ball is out.
Boxes
Box 1. Olympic track and pool facilities
Box 2. Perfect timing: Timing devices and reaction time
Box 3. Physics in sport: Forces on an athlete
Related site
Energy basics (Nova: Science in the news, Australian Academy of Science)
Posted November 1998.