Physics Challenge – The Pendulum
The pendulum has been a cause of fascination and enquiry for thousands of years. The ancient Greeks used sundials to gauge time and water clocks to ‘measure’ it. There is evidence of a unit of time being adopted across ‘the ancient world’, it being the time of one swing of a pendulum of ‘agreed’ length. This may go back to circa 3000 BC and of Sumerian origin.
Pendulum clocks were invented in the 17th century. Useless at sea, they provided accurate time measurement regardless of the amplitude of swing. Such pendulum clocks are still common today, though not everybody can live with a loud ‘tick-tock’. When the New Year is celebrated in London we hear the chime of Big Ben, the popular name of the bell, and we observe one of the four clock faces of the enormous pendulum clock housed within the tower of Westminster. The clock’s accuracy is maintained by adding or removing old English pennies to the top of the 210 kg pendulum bob, the addition of one old penny removing 0.4 seconds in 24 hours.
The answer can be found on the ‘Easter revision’ page of the website.
The period of a pendulum is independent of the mass of the bob, so why is the period of the clock within Big Ben adjusted by adding or subtracting pennies to the top of the bob?