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History of the drawbridge

Drawbridges have evolved over many centuries, its purpose changing over the centuries.  In medieval times the drawbridge's main purpose was defense.  Cities or castles built inside walls could have moors or chasms surrounding the protective walls could use a drawbridge to limit the traffic in and out of the city or castle.  Today the draw bridge is most commonly used as a convenient ways to allow large ships to pass through urbanized waterways.  Despite this change in purpose, the physics behind a drawbridge has not changed.
The Medieval Drawbridge

There is evidence that the Ancient Egyptians used drawbridges 4,000 years ago, but the widespread use of the drawbridge did not start until the medieval ages. The drawbridge, also known as the bascule bridge that consisted of two counterweighted leaves or spans which pivot upward on a horizontal axis to allow boat traffic to pass underneath., was used to protect castles.  The advantage of having a drawbridge was the control of traffic into the castle.  Unlike a bridge that allows passage to anybody, a drawbridge was an added level of security.  

The modern draw bridge

The medieval drawbridge used a one-leaf system.  This means that the one side of the drawbridge is pulled up.  The modern two-leaf system opens the drawbridge in the center. 
Most modern drawbridges work through counterbalance, though the mechanism is condense to take up as little space as possible. Underneath drawbridges, there is a counterbalance of very heavy material, usually concrete, which is lowered to move the bridge up. The room is moved by a motor which moves gears that shift the room down. To move the bridge down, the gears shift the room back up in a smooth, slow speed. The drawbridge also requires careful traffic directing through signals to allow for both boats and cars to use the same transportation area.