Medieval Physics - Siege Engines

I don't think a blog series titled Medieval Physics would be complete without a discussion of siege engines. Siege engines were the machines that made possible attacks on castles and highly defensible fortifications. It is the simple physics behind these machines that make them what they are. Let's take a look at a few examples.

The Battering Ram

When an invading force had access to the castle gate, the battering ram was the siege weapon of choice for breaching the fortress. Battering rams allowed attackers to hit doors and gates with massive weights repeatedly, in order to break or force them open.

Battering rams typically incorporated a frame which allowed for the suspension of a large wooden log. Operators inside the structure would stand on either side of the suspended log and swing it back and forth into the target structure. Some more rudimentary rams with wheels were intended to be pushed by the operators (at speed) into the door or gate. The ram housing was usually covered to protect its occupants from arrows, stones, and other methods of bombardment from above.

The basic principle of the ram is Newton's first law, which states that an object in motion (the ram) will stay in motion unless a force (the door/gate) is acted upon it. The door must repeatedly take the force of the heavy log, weakening the door each time. When the door is too weak to stop the inertia of the ram, it will break open. Newton's second law, represented by F=ma, is demonstrated in that a larger (heavier) battery ram traveling at a higher speed will hit the door with more force.

The Onager

Onagers (also sometimes called Mangonels) are catapults that were used to bombard fortresses in Ancient Rome and the Middle Ages. They hurled large stones (sometimes laced with an combustible material and set aflame) across great distances. They could also be used as a defensive weapon against siege towers and the like.

Onagers worked using the principle of torsion. To fire, an operator would force down the firing arm held in tension by twisted ropes or a similar spring mechanism (see left) wound on a windlass (a winch). At full extension, the bucket or sling of the arm would be loaded with the projectile. When released from tension, the arm would swing forward, hitting a padded stop as the projectile was released. Firing distance and power were based solely on the construction of the onager, with variables including size and length of the arm and tension of the spring mechanism.

The Trebuchet

The trebuchet was the most fearsome of catapult technology used in ancient and medieval history. Trebuchets utilized the principle of counterweights to propel extremely large objects hundreds to thousands of feet at high speeds. The effect was devastating to enemy defenses and forces.

The mechanics of the trebuchet are different than the onager but equally straightforward - just imagine a see-saw (a really really big see-saw) with a menacing object sitting in a sling at one end. When a heavyweight (or force) is applied at the other end, the arm with the object will swing up at speed. As it hits a stopping point, the menacing object will be released from the sling, remaining in motion until impact with its target. The momentum transferred into the sling adds to the force of the release. Positioning the fulcrum closer to the side applying the force allows the object to be projected farther, but requires more force application and puts more strain on the lever.

If you want a very good visual example of the mechanics of these machines, check out the battle of Minas Tirith in the fantasy film The Lord of the Rings: The Return of the King.

In fact, that movie battle includes the use of not just of trebuchets, but of onagers, ladders, siege towers, and a battering ram. In the Middle Ages, some attackers would also use fiery weapons to melt the cement holding the stone construction together, or they would mine underneath the structure to destroy its foundation… the possibilities were many. Looking at the engineering and physics behind these things gives me a better understanding and appreciation of the scale and complexity of Medieval technology.

Sources:

stormthecastle.com

real-world-physics-problems.com