Practical Investigation - Ballista Project - page 1
Keywords: Physics Practical Investigation Ballista Crossbow
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 1 of 4 pages: 1 2 3 4
Practical Investigation – Ballista Project
Introduction
I am investigating how the distance travelled by a bolt fired by a ballista is related to the force applied. To do this I will build a (simplified and scaled-down) ballista and experiment with it.
There are several variables I could consider in these experiments: the force used to pull back the bungee cord; the initial tension in the cord (could be changed by moving the bosses) and the size, shape and weight of the bolts fired.
History of Ballista’s
The ballista (Latin, from Greek ballistēs, from ballein 'to throw', plural ballistae) was a powerful ancient weapon which resembled a large crossbow. The early ballistae in Ancient Greece were developed from two weapons known as oxybeles and gastraphetes. The gastraphetes ('belly-bow') was a hand held crossbow. It had a composite prod and was spanned with both hands, with a ratchet preventing it from shooting while loading. The power available was not sufficient to be used successfully against hoplites (heavy infantry men) and phalangites (infantry men deployed in a closed formation). Bigger and heavier constructions, the oxybeles employed a winch and were mounted on a tripod. They had a lower rate of fire and were used as siege engines.
My ballista is roughly somewhere between a simplified gastraphetes and a scaled-down oxybeles. It is similar to the gastraphetes regarding its fairly simple layout and firing mechanism, while resembling the oxybeles in terms of being mounted on a stand and fairly difficult to manoeuvre quickly.
Planning and Construction
• I am using metal stands and stand poles to build the structure of my ballista and to reinforce it; they are strong but have the disadvantage that they make the ballista difficult to manoeuvre.
• I’m using a large wooden block for the stock (main section) as it is strong and can also be cut into to make a firing slot. I will attach rulers to the stock to measure how far the string is pulled back when firing.
• I will use G-clamps to attach the front of the stock to the pole which is acting as the prod (cross piece) securely and an ordinary clamp to attach the back of the stock as it is more convenient to move when adjusting the angle of the stock to get it level. I will use bosses to attach the poles to each other securely.
• Choosing the right material for the string is important as it must be quite elastic
Last 5 comments…
There have been no comments posted for this article, but you need to register if you want to be the first!