|MadSci Network: Physics|
Dear No Name Entered, You probably know squash was invented in England in the 19th Century and initially was restricted to school and university play. I myself learn to play at the University of Pennsylvania. With the instructional prompting of my postdoctoral adviser I became an adequate player, as she was faculty champion. And I have also heard the sound you described, although I cannot recall the specific conditions that generated it. So, I’m giving you a mission! Squash balls are made from two pieces of highly durable rubber compound glued together. Different type balls are provided for the varying conditions and standards of play: beginning players are able to use balls that are bouncier and larger than those used by more experienced players. Colored dots on the ball indicate the level of elasticity, or “speed”. A bouncier ball is said to be "fast" whereas a less bouncy ball is said to be "slow". The ball becomes more bouncy as the temperature of the ball increases. Pro players hit harder and have longer rallies, and so play with a ball at a much hotter temperature than amateurs. The "faster" balls actually allow amateurs to play the game with the same amount of ball bounce as the pros. Many club players end up playing dead ball squash because they use a ball which isn't suited for their level of play. The color code for squash balls is: Double Yellow - Extra Super Slow Yellow - Super Slow Green or White - Slow Red - Medium Blue – Fast The yellow dot you describe is a super slow ball that previous to the year 2000 was considered only for championship tourneys. What does this super slow designation mean thermodynamically? It means that about 75% of the kinetic energy impacted to the ball by your raquet on a hit is eliminated and lost through heat and sound dissipation. Roughfully, only 25% of the kinetic energy is store as potential energy through ball strain for use on the rebound. As the ball heats up over the course of play, the elasticity of the ball increases, allowing more energy to be stored as ball strain. It begins to respond more like a green or red dot ball and rebound further. An interesting study was done comparing a Double Yellow and Yellow dot ball. The URL is here… => http://www.amstat.org/publications/jse/v2n1/mackisack.supp.html According to this investigation, the room temperature (unplayed) balls rebound distances off the wall are as expected. However, the balls hit at “playing temperature” rebounded further…the Double Yellow actually rebounding further than the Yellow dot at comparable temperatures… Playing Temperature Room Temperature Double-yellow 664.50 cm 475.25 cm Yellow-dot 624.25 cm 531.25 cm So, the yellow dot is a particular slow ball, even when hot. Thermodynamically, we could say it is a particularly good dissipation sink for kinetic energy, producing alot of heat and alot of sound. So, ok. Why does it make the high pitch whine you only hear in sci-fi movies? It does this because under some circumstances, heat dissipation is not fast enough, or efficient enough, during the volley to remove the excess kinetic energy given the squash ball. The only avenue left for the little yellow dot guy is to produce high frequency sound. What circumstances are those? It may be a couple of things, which you can investigate. So, your mission is to answer these questions: What are the condition of your squash courts? Are they dry or humid? Are the wall and air temperatures generally cool or warm? For another, what move did you make when the ball “sounds off”? Did you just put some weird spin on the ball, placing it a mode more conducive to sound dissipation than heat dissipation? And after it “sounds off”, is it still as “fast” on the next rebound, or has it perceptively become slower with less bounce? In other words, does the heat build up on the yellow dot squash ball to the point it must release energy as sound? Does it then cool down and become slower…less bouncy? Does it feel cooler after emitting sound? Sorry I have no easy answers for you No Name Entered, but perhaps you can find them for us! Hope this helps in your investigation, ---* Dr. Ken Beck
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