How Physicists Build a Bridge

Imagine the following, you have a room, twenty stories high and 50 feet across. You want to hang a Foucault Pendulum in this room. Here’s the catch: the ceiling is made of glass and can’t support the pendulum. The only way to hang it is with a cable that traverses the gap between the walls. Now we do have a little something to work with, on the other side of the walls are banks of offices from floors 1 to 20. Furthermore these offices have windows opening on to the room.

This is the situation physicists at Fermilab found themselves in when they wanted to hang a pendulum in Wilson Hall [ picture ]. Now they could call in a construction crew, build scaffolding up twenty stories and just connect the cable like that. Not pretty but it would get the job done. However they wanted something pretty. Firstly, things are a little easier. We don’t actually need to get the heavy duty cable across to begin with, we can start with a lightweight string, and use that to pull across increasingly heavy cables. The Niagara Falls Bridge for example started when a young boy flew a kite across the falls.

Unfortunately ¬†kites don’t work inside. Well maybe we could have someone throw a baseball across the gap with the string pinned to it. Maybe but the physicists were more inspired by the 0 brawn approach: two people go up to 20th story offices, each with a ball of string. They drop the balls (holding on to an end) all the way down to the ground 20 stories below.¬† A third guy ties the ends together. Wind the string up. voila, we have our bridge.

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