Conduct the following experiment using either a VCR or a tape deck equipped with a digital tape-counter.  Put a tape in the unit, set the tapecounter to zero, and, as you start the tape playing, note the time.  Then record the time every 5 to 10 minutes along with the number on the tapecounter.  This data set was obtained in this way.  It shows the number of revolutions - the number on the tapecounter - (second column) as a function of time in seconds (first column).