Stopwatch reading, 0.01 sEffective length to the bob's centreReadings table and L–T² graphHang the bob, measure the effective length (thread + hook + radius), pull it aside by a small angle and let go. Start the stopwatch as the bob passes the mean position, count 20 oscillations, and divide by 20 for the time period T. Longer pendulums swing more slowly: T = 2π√(L/g), so L against T² is a straight line, its slope is g/4π², and at T² = 4 s² it gives the seconds pendulum (99.3 cm). A bob of the same size but a third of the mass swings in the same time. g = 9.8 m/s².