When knights and men at arms traveled to war, one of the most challenging obstacles they faced was finding supplies and provisions. When they traveled, they were not able to transport larger items. Thus when they arrived for battle they had to live off of the land. This was true not only for provisions, but also for many of their larger weapons and siege engines, particularly the battering ram.
The concept behind the battering ram is simple – momentum coupled with mass. In other words, take a heavy object and hurl it repeatedly at a stationary object, such as a door or gate, to break through the object. Typically a large tree would be felled and the branches removed to allow those that remained to be used as grips. A group of soldiers would lift the tree trunk and, after a running start to build momentum, ram the trunk into whatever they wanted to break through, generally a castle gate. If this was done enough times, the door would break open.
Sometimes the tree trunk was affixed to a support system that was then positioned in front of the door or gate and swung like a pendulum. This allowed larger rams to be used, operated by fewer men. The support system could also be covered to shield and protect those operating the ram from attack.
The battering ram, while basic in function and design, remains a popular weapon today seen in use in both military and police settings.