I timed the period of revolution of the larger and lower of the two radar arms as the ferry docked at Kennacraig. I timed 10 periods each time because that reduces the percentage uncertainty in the final time period. The readings were 22.87s, 22.42s and 22.69s. The mean is 22.66s. Uncertainty = 1/2range = +/- 0.225s. Percentage uncertainty = 0.225/22.66 x 100 = 0.99%. This justifies using 3 sig figs when I calculate the time period. Time period = 22.66/10 = 2.27 seconds.
