Over the years there have been various articles in the Gazette [1, 2, 3] concerning the running time of cassette tapes, but none of them uses the property described here, although it is relevant to all of them.
Let us assume that the revolution counter on a cassette or video recorder is operated by the run off spool. Let a be the radius of the full spool and r the radius when an angle θ radians has been unwound. Rather as in [2] it is clear that a reasonable continuous model of this situation satisfies the equation
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