by Justyn Myers

A while back I was given the chance to participate and compete in a local science fair.  When presented with the idea, the inner kid in me got real excited as images of trifold presentation boards and short sleeved collared shirts ran through my mind.  What kind of project would have the most impact, would be the most fun to build, and is something I could use after it was done?  I had figured the volcano was played out, and robotics would be too complicated given the short 2-week time frame.  I had always wanted to make a trebuchet and this was the perfect time to do so.  Now I have the opportunity to explore how it was made by going through those motions and get a chance to really understand how to maximize its potential (energy).   I’m going to run through the steps of how to make one and explain my conclusion and explore what I would do differently when I build the next one. 

The trebuchet is an ancient siege engine used for throwing large rocks. It was invented in China in about the 4th century BC, came to Europe in the 6th century AD, and did not become obsolete until the 16th century, well after the introduction of gunpowder.  Trebuchets were so popular because an army would show up to invade a castle and use the surrounding trees to build the catapult, thus relieving the army of having to carry this large thing around when travelling.

A classical trebuchet involves a falling counterweight which accelerates the throwing arm and the sling attached to it. A small speed on the counterweight provides a large speed on the end of the throwing arm and an even larger speed on the projectile in the sling because the arm and projectile have a much larger radius from the fulcrum than the counterweight does. Ideally, the sling will release when the projectile is traveling at a 45-degree angle to the ground, and the counterweight should impart all of its energy to the projectile. I decided to make a trebuchet that would use a case of soda as a counterweight, and would throw a single can of soda as far as possible.

treb 1.jpg
treb 2.jpg

Aside from the basic components of the trebuchet there are other factors to consider when building a trebuchet.  After building the initial small-scale model I made changes and added these items:

WHEELS: As the counterweight swings down, the trebuchet rolls forwards and then backwards. The forward motion adds to the velocity of the projectile, much as the forward motion of a baseball pitcher add to the velocity of a ball. The forwards motion of the trebuchet also helps to smooth out the motion of the swinging beam, adding to the control of the projectile.

COUNTERWEIGHTS WERE OF TWO TYPES: fixed or hinged. Fixed counterweights were easier to design and build. However, in an effort to harness the full energy of the falling mass, hinged counterweight trebuchets were built.  I used both and found the projectile went further with a hinged counterweight.  The hinge allowed the counterweight to fall at maximum velocity without any horizontal drag.

There are a couple things I would change when I tackle this project again.  One thing would be to increase the amount of counter weight used from (1) case to (2) cases of soda because generally in a trebuchet as the weight increases the distance increases as well.  We should have cut the hole in our arm further so that the arm could reach the trigger without additional string. This would make our trebuchet more efficient.  And lastly, I would create an adjustable pin mechanism to adjust firing angle on the battle field.

This trebuchet model was an enjoyable and surprising project. The design and construction were both challenging and interesting, and this small project allows you to exercise both your mind and your hands. It was surprising because the mathematical model corresponded so well with the range and actual performance in the field. I think it can be said with certainty that the more soda you drink out of the can to be thrown (making it lighter), and the more soda you add to the counterweight, the distance travelled by the projectile increases. In addition, the greater the height above the ground the counterweight is held, and the greater the angle the projectile turns through, the greater the displacement of the projectile. For an object that is 36" tall and works on only gravity to fire an object 33 feet (as mine did) attests to the beauty and pragmatism of the trebuchet design.