A trebuchet is a medieval engine of war with a sling for hurling missiles. As Halloween comes around, wouldn’t you like to spend your weekend building your very own pumpkin-launching trebuchet?
Trebuchets have been around since the middle ages and were often used to throw objects at the enemy’s fortress. Usually it was large rocks, but often dead and rotting livestock were thrown. Depending on the size of the trebuchet you build, you will probably be throwing smaller objects, like tennis balls. Or pumpkins….
There is a difference between a trebuchet and a catapult. A trebuchet uses a sling and has a counter-weight which, as it is dropped, forces the long arm up to pull the sling and the projectile along a slide at the base. The counter-weight uses the pull of gravity to provide the force necessary for the arm to swing upwards. The sling increases the length of the arm and the performance. The catapult uses a leaf spring mechanism to release the long arm. A rope is wrapped around a rotating drum and when the spring mechanism is released, so is the arm and the projectile. A catapult has a cup at the end rather than a sling.
In 15 Dangerously Mad Projects for the Evil Genius, author Simon Monk says, “The trebuchet takes its energy from the weight that falls as the arm swings. The ‘potential’ energy is transferred to the arm and sling of the trebuchet and is released as kinetic energy in the tennis ball.”
When you know the energy stored in the weight and how far the projectile can be thrown, the energy going into the system and the energy released can be measured. Input energy can be calculated using the formula: E=mgh where ‘m’ is the mass of the weight, ‘g’ is the gravitational acceleration on Earth (9.8) and ‘h’ is the height. You can also calculate the amount of energy transferred to the tennis ball using the distance it traveled and its weight. E=1|2 mv2 where d=v2|g v2=dg. You can then calculate the efficiency of the catapult by dividing the energy transferred by the energy input. From this, you are then able to calculate the efficiency of your trebuchet!
Sometime in the 1480s, Leonardo da Vinci drafted two designs that would improve on the catapult. There were drawbacks to each of the designs and included the considerable cost to produce the steel mechanism and the swing arm tension would have been tremendous. It is believed that neither of the catapults were built during his lifetime. However, if you’d like to see a life-size working model of his catapult check out the DVD Doing da Vinci from our DVD collection. In this 2-DVD collection, 4 builders and engineers tackle building some of da Vinci’s never-before-built designs.
Monk says, “It’s a simple design that should only take a few hours to construct and needs little in the way of special tools or equipment.” A list of materials needed is listed and instructions are clearly laid out, including how to make the weight from a plastic cereal container filled with wet sand. The sling is made with rope and a patch of square cloth. Monk also says, “A reasonably strong material like denim is ideal. The Evil Genius’ minions can often be found wearing jeans with large patches of cloth removed. The Evil Genius tells them that this is the latest fashion and the minions are pleased.” (pg. 20).
If you prefer a smaller, desk-top version, you can make one using popsicle sticks.
You have a couple of weeks to build and fine-tune your trebuchet before Halloween! Have a fun (and safe) one!
Video from dangerouslymad.com
Resources:
Doing da Vinci. 2010. [Silver Spring, MD : Discovery Channel : Louisville, CO : Gaiam Americas. Engineering Circulation Desk Video record 37148 DVD.
Monk, Simon. 2011. 15 dangerously mad projects for the evil genius. New York : McGraw Hill. Engineering Library TK9965 .M66 2011
Leonardo Da Vinci’s Life. Da Vinci’s Life. Date accessed, October 14, 2015.
Trebuchet. dangerouslymad.com Date accessed Oct. 14, 2015
How to Build a Catapult – a Popsicle Stick Catapult. Stormthecaslte.com. Date accessed Oct. 15, 2015
Other Resources:
Medieval Siege. 2004 [Boston] : WBBH Boston Video. Engineering Circulation Desk Video Record 39034 DVD
How to Build a Catapult. Storm the Castle.com. Date accessed Oct. 15, 2015.