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Variables in the ball's trajectory

As one whose maths degree puts him at around the same level in maths as Professor Marcus du Sautoy says he is in football (TES, July 9), it is with some trepidation that I write to correct the assertion in his article. But it is surely wrong to assert that "the trajectory of a football or tennis ball depends on equations".

First, he has it the wrong way round: equations are derived from the physical world, so nothing in the physical world "depends" on equations.

And, second, the trajectory of a ball, like everything else in the real world, is determined by myriad circumstances, most of which are assumed to have such small effects that they can be ignored in formulating equations to describe the trajectory. The equations describe an idealised world which, by definition, can at best be only an approximation.

David Terry 24 Alexander Avenue Droitwich, Worcestershire

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