We rang a quarter peal on Wigton's bells on Sunday night. For the uninitiated, that means ringing non-stop for about 45 minutes whilst changing the order in which the bells sound every time you pull. Congratulations to our youngest ringer for achieving a first quarter peal.
It got me thinking about the physics behind the bells. Here's a picture of the bells that I took at Easter.
You'll notice that the bells are upside down. This is necessary because it allows us to control quite precisely when the bell sounds. It also makes the bells unstable. This is because it raises the centre of gravity whilst reducing the area at the bottom. My GCSE classes know that when the line of action downwards from the centre of gravity falls outside the base area, an object will fall.
You'll also notice that bells are designed to move in circles. In Physics, we say that the bell will have angular momentum. Now, linear momentum is a measure of how difficult it is to stop an object that is moving in a line. Linear momentum is calculated by inertial mass x speed. Angular momentum is how hard it is to stop something moving in a circle. Angular momentum = moment of inertia x angular speed. Moment of inertia is roughly a combination of how heavy and how far from the centre of turning. Think of ice skaters. They spin slowly with arms outstretched because they have mass further from their body. They have a large moment of inertia at this point. As they bring their arms in, they spin faster - in other words, a lower moment of inertia means that they have a higher angular speed. This is because they conserve angular momentum.
When I pull a bell, it swings in a circle until I stop the bell by holding the rope. The problem is that I can't stop the clapper, which still has angular momentum. It carries on until it hits the bell and stops.
I also thought you might like to see the view of Wigton from the top of the bell tower!
