Balls Behaviour
Why does a squash ball bounce higher when it’s warm? To understand this we need to do a little physics. Ready? In order for a solid material to be deformed, work has to be done on it. For that work to be done, energy must be expended (in the case of a squash ball, it is hit by a racket). Some of this energy is dissipated (as heat, etc.), but some is stored in the deformed material and is released when the material relaxes. The extent to which a material stores energy under deformation is called ‘resilience’. Some materials (like sprung steel) store a lot of energy and are described as having high resilience; others (like putty) store very little and therefore have low resilience. Squash balls, being made of a rubber compound, are of fairly low resilience. Unfortunately, as we know, the lower the resilience of an object, the higher the proportion of the energy used in deforming it must be dissipated. When a squash ball hits the racket strings and the wall and floor of the court, some of this energy is transformed into heat in the strings, wall, floor, and surrounding air and some into sound, but most of it becomes heat in the ball itself. This has two effects: the air inside the ball (which was originally at normal atmospheric pressure) effectively becomes ‘pressurised’, and the rubber compound from which the ball is made becomes more resilient. For both these reasons, the ball bounces higher. Obviously, the ball does not continue indefinitely to heat up; eventually equilibrium is reached where heat loss to strings, wall, floor, and air is equal to heat gained from deformation. This point is normally at around 45oC, which is why the WSF’s rebound resilience specification is calculated at that temperature. It also explains why squash balls are designed to have too little resilience at room temperature and therefore why they need warming before play. But why have balls of different speeds and how are they made? The actual ball temperature reached in play varies according to two main factors: the temperature of the court and the ability of the players. The point at which the ball temperature reaches equilibrium is in fact an excess over the ambient temperature of the court. So if the court is at only 5oC, the ball may only reach 35oC. On the other hand, even on a warm court, if the players don’t hit the ball hard or often enough to raise its temperature to that optimum 45oC, the ball won’t perform as it should. To compensate for either factor, players will need to use a ball of higher basic resilience, i.e. a ‘faster’ ball. These are produced simply by making a different mixture of polymers. More elastic polymers create higher resilience; more viscous polymers lower resilience. So how can you have a ball with ‘instant bounce’? For a ball not to need warming, it must either lose as much heat as it gains during play and therefore remain at court temperature, or it must be made of a material whose resilience is the same at any temperature. It remains to be seen whether Dunlop’s new Max Progress and Max balls meet either of these criteria. As soon as we have them (they’re now due to be launched in June this year), we’ll be -*test*-('")ing them to find out...
Why does a squash ball bounce higher when it’s warm? To understand this we need to do a little physics. Ready? In order for a solid material to be deformed, work has to be done on it. For that work to be done, energy must be expended (in the case of a squash ball, it is hit by a racket). Some of this energy is dissipated (as heat, etc.), but some is stored in the deformed material and is released when the material relaxes. The extent to which a material stores energy under deformation is called ‘resilience’. Some materials (like sprung steel) store a lot of energy and are described as having high resilience; others (like putty) store very little and therefore have low resilience. Squash balls, being made of a rubber compound, are of fairly low resilience. Unfortunately, as we know, the lower the resilience of an object, the higher the proportion of the energy used in deforming it must be dissipated. When a squash ball hits the racket strings and the wall and floor of the court, some of this energy is transformed into heat in the strings, wall, floor, and surrounding air and some into sound, but most of it becomes heat in the ball itself. This has two effects: the air inside the ball (which was originally at normal atmospheric pressure) effectively becomes ‘pressurised’, and the rubber compound from which the ball is made becomes more resilient. For both these reasons, the ball bounces higher. Obviously, the ball does not continue indefinitely to heat up; eventually equilibrium is reached where heat loss to strings, wall, floor, and air is equal to heat gained from deformation. This point is normally at around 45oC, which is why the WSF’s rebound resilience specification is calculated at that temperature. It also explains why squash balls are designed to have too little resilience at room temperature and therefore why they need warming before play. But why have balls of different speeds and how are they made? The actual ball temperature reached in play varies according to two main factors: the temperature of the court and the ability of the players. The point at which the ball temperature reaches equilibrium is in fact an excess over the ambient temperature of the court. So if the court is at only 5oC, the ball may only reach 35oC. On the other hand, even on a warm court, if the players don’t hit the ball hard or often enough to raise its temperature to that optimum 45oC, the ball won’t perform as it should. To compensate for either factor, players will need to use a ball of higher basic resilience, i.e. a ‘faster’ ball. These are produced simply by making a different mixture of polymers. More elastic polymers create higher resilience; more viscous polymers lower resilience. So how can you have a ball with ‘instant bounce’? For a ball not to need warming, it must either lose as much heat as it gains during play and therefore remain at court temperature, or it must be made of a material whose resilience is the same at any temperature. It remains to be seen whether Dunlop’s new Max Progress and Max balls meet either of these criteria. As soon as we have them (they’re now due to be launched in June this year), we’ll be -*test*-('")ing them to find out...