Did You Know? #2 (by DWL)

A measure of how bouncy a ball is, is the ratio (R) of the maximum height to which  a ball bounces to that from which it is initially dropped. According to the specifications for a “double yellow” squash ball, R=0.12 @ 23 degrees C (cold ball, i.e. room temperature), and 0.26-0.33 @ 45 degrees C (hot ball), when dropped from an initial height of 100 inches (254 cm). Basically, what this means is that a cold ball loses all but 12% of its kinetic energy (energy of motion) when it strikes the floor (this is also a good approximation as to how much energy is lost when the ball strikes a wall) . No wonder it is generally best to hit the ball before it strikes the back wall….the loss of kinetic energy means the ball will not bounce out very far!

The lost kinetic energy is mostly due to a production of thermal energy (heat) which warms up the ball (and the contacted floor, wall or racket). As the ball warms up, the rubber material of the ball becomes more “pliable”. The more pliable the ball becomes, the less kinetic energy it loses while being squished (i.e. deformation while striking a wall, floor, etc.). This is why R increases with temperature!

A “blue dot” ball, which is recommended for beginner players, is designed to be more bouncy(R=0.17 for a cold ball, and R=0.38 for a hot ball). This translates into a significantly higher bounce for easier play.

A tennis ball by comparison is much more bouncy. Dropped from a height of 100 inches, a tennis ball will bounce to a height of 53-58 inches (i.e. R=0.53-0.58). I can recall (to my embarassment) an incident when I first started playing squash. At the time, I was also playing tennis. So, one day, after playing squash, I went out onto a tennis court. I leaned over to bounce the tennis ball in preparation to serve. Not being used to the extra bounciness of the tennis ball, I threw down the ball with such force that it bounced up and hit me in the head!

It should be noted that, regardless of the ball temperature, the faster the ball is traveling when it strikes the floor, wall etc., the more it squishes, and hence the larger the fraction of kinetic energy lost (e.g. if a cold double yellow ball is dropped from 200 inches, the resulting measured R will be smaller than 0.12)

The fact that a pre-match warm up involves warming up the ball as well as the players is one of the unique features of squash as a racket sport.