A hand on a stopwatch is late by about two tenths of a second every time. Time a single swing and that error is a big slice of the reading. Time many swings and divide, and the same two tenths is shared out across them all, so the uncertainty in one period shrinks. Choose how many swings to time and drive the percentage uncertainty below the target.
MissionMeasure the period to within 2% uncertainty.Round 1Streak 0Best 0
Timing more swings makes the percentage uncertainty:
total time for N swings1.4 s
period T = total ÷ N1.40 s
percentage uncertainty14.3%
the 0.2 s timing error is spread over N swings: uncertainty = 0.2 ÷ total × 100.