# Why put pennies on Big Ben's pendulum?

08 May 2011

## Question

If the period of a pendulum is dictated by its length, not the weight of the bob, why do they put pennies on Big Ben's?

Dave - The period of the pendulum is entirely to do with the distance from where it is swinging to the centre of mass. The trick is where they put the pennies. They don't put them on the middle of the pendulum bob, they put them on the top. If you put a penny on the top of the pendulum bob, its adding some mass to the top of the pendulum bob so it effectively makes the pendulum slightly shorter, so it runs slightly quicker. If you put some weight on the bottom it effectively makes it slightly longer so it runs slightly slower.