TREBUCHET
Background Statement Hypothesis Method Data Collection Results Discussion Additions Limitations Improvements Bibliography
Background
Trebuchets were first designed
as siege weapons in the middle ages, and were used to hurl projectiles into
masonry walls to destroy them or over them to create havoc during war. There are two types of
trebuchets; the first was the traction trebuchet, which used man muscle to pull
it instead of a
counterweight (Chevedden). The more common trebuchet and the type we will be
making is the counterpoise trebuchet. This trebuchet was created to be more
accurate and more powerful then its predecessor - the catapult. The way a
trebuchet works is it takes advantage of leverage by dropping a counterweight to
fling a projectile. The trebuchet has five basic parts: the frame, the
counterweight, the beam, the sling and the guide chute (Trebuchet.com).
One end of the sling is fixed to the end of the beam, while the other is
tied in a loop and slipped over a release pin extending from the end of the
beam. As the beam rotates, it pulls the sling, with its enclosed projectile,
down the guide chute. As the sling exits the chute, it accelerates in an arc
away from the beam, but because the beam is still pulling the sling behind, the
loop is held on the pin.
The sling continues accelerating through its arc
until it eventually swings ahead of the release pin. At this point, known as the
release angle,
the loop slips off the pin and the sling opens releasing the projectile (Trebuchet.com).
The first trebuchets (traction trebuchets) were first believed to be used in China, as early as 5th Century B.C. (Needham Joseph). The projectiles shot by trebuchets were normally large rocks, but human corpses were also used to spread terror. Prince Korybut, of Severian Novgorod in 1422, flung corpses and manure over enemy walls and managed to spread disease and sickness (Wheelis, Mark). This strategy is an early form of biological warfare. Trebuchets built today are used only as models, the largest of which towering 18 meters tall and weighing 22 tons.
The purpose of our experiment is to see how the counterweight’s mass affects the initial launch velocity.
If we change the mass of the counterweight, then the best weight to get optimum launch velocity would be a heavier weight, because a heavier weight will pull the sling down faster, flinging the projectile harder. We believe the graph of the velocities will have the shape of an upside down parabola.
Our independent variables are time the ball lands (in seconds) and mass of the counter weight (in kilograms), and our dependant variables are distance – how far the ball traveled (in meters) and velocity – how fast the ball traveled (in meters / second). We need time and distance in order to calculate velocity, which in turn is compared to weight.
Our Trebuchet
Trebuchet X was built using a kit that we bought from OMSI. It is a small, simple trebuchet, but it has everything a fine trebuchet has. It is equipped with four excellent performance wheels, a loose sling, a superior grade beam, a reliable release pin, and an easy to use counter weight box. And don’t forget the red clay used to morph into a cannonball, ready for battle.
Our projectile is a red clay ball, measuring about 2.5 cm in diameter and weighing about 7.8 grams. To throw this ball, it is placed on the sling. Then, we pull the beam down (the sling side) and insert the pin into three loopholes to hold the beam in place. When we are ready to collect our raw data, we release this pin, and the counter weight (with a varying mass) pulls its side of the beam down, using the force of gravity. This brings the other side of the beam back up, and the sling flies up. The ball is thrown.
We set up our experiment by laying down a tape measure, going as long as fifty meters. We put down Trebuchet X next to this tape measure and made sure the very front of Trebuchet X was on the zero meter mark.
Before releasing the pin, we all count to three, to insure accuracy by making sure the pin puller and the timer start at the exact same time. When the ball is released, our timer starts timing using a stopwatch.
Two scouts (including the timer) see and find the exact place where the ball lands. When it does land, the timer stops timing on his stopwatch, and we have the necessary variables to calculate the ball’s velocity. Someone jots down our data. Weight is then added to the counter weight by putting fish sinkers with varying masses inside the counter weight box. Repeat.
After many trials of using many different counter weights, we have all the data we need to carry on to our results. Looking at our promising results, we have hope that our hypothesis is correct.
Since we didn’t have velocity to compare with the mass of the counter weight, we needed to find that velocity. From our experiment, we used varying values for weight and in turn we gathered the data needed on the two important variables: distance and time. Now that we have obtained these two variables, we are able to calculate velocities of the different masses.
To do this, we used the very simple equation Velocity = Distance / Time. Velocity was calculated this way for each and every mass of the counter weight, using the distances and times associated with them.
After we found the velocities that corresponded to the different counter weights, we wanted to see this visually to find any trends. Using Microsoft Excel, we plotted the data and Excel did the graph for us. We noticed that the graph was exponential, and every time weight was added, velocity increased dramatically. We were satisfied with the results.
Are we right?
Our data did support our hypothesis: “If we change the mass of the counterweight, then the best weight to get optimum launch velocity would be a heavier weight because a heavier weight will pull the sling down faster, flinging the projectile harder.” Every time we increased the mass of the counter weight, our red ball flew further, and plus, time was lower or the same as the previous trial. This happens because since the counter weight was heavier, force was greater. A heavier mass yields stronger energy, because of the equation KE = (1/2)(mass)(velocity^2) [velocity could be calculated by using another equation: Velocity = g * time the mass fell, where g = 9.8]. When the pin was put in to secure the beam, there was a greater potential energy waiting to be released. Since the counter weight fell faster and stronger, the other side of the beam flew up faster and stronger, which gave way to a faster ball. The velocities were higher with every subsequent addition to counter weight.
However, the second half of our hypothesis is clearly wrong: “We believe the graph of the velocities will have the shape of an upside down parabola.” This is probably due to not thinking about the shape of the graph as much as the research in whole. With our thinking, we should have thought that the graph was exponential, which it turned out to be.
What could have added to our results?
The most powerful trebuchets were built with wheels. This is used to absorb excess kinetic energy and give it back to the ball (Wikipedia.org). Trebuchet X is equipped with wheels; therefore it is a powerful trebuchet for its size.
Of course, we don’t have the resources necessary to take away air friction or have a robot distance calculator. So, there was an evil force that discouraged accuracy.
Since we have tested Trebuchet X on asphalt, the floor was rough. This might have hindered the trebuchet from rolling backwards and forwards. In turn, some energy might have been transferred to the ground. Trebuchet X didn’t roll back to its original position after swinging back.
Sometimes, when we first used the trebuchet, the swing hit the pin puller. Other times, the ball traveled backwards (at the lowest possible counter weight). This is probably due to the occasional wind, which is also an unstoppable force that could also have slowed down the ball’s velocity. It might shatter our egos if we found out the wind blew the ball further than was capable of it.
Even though there were two scouts to find distance, we disagreed on some numbers. The ball just moves too fast for our eyes to capture the exact moment and place the ball landed. Using a video camera and then using LoggerPro to capture that optimal moment might have leaded us to more accurate results.
But, there was a reason why we couldn’t do the suggestion above. We have already tried it. The best place to capture both Trebuchet X and the ideal moment was maybe twenty feet away. Since the camera was too far, the tiny ball could not be seen in the video.
TOPHow do we make a better experiment?
To better measure velocity, we could have used a video after all. We could record a video right next to Trebuchet X, maybe five feet away so that we can see the ball, and use LoggerPro to calculate just the initial velocity. This would be a much better method and this would lead to one of the most accurate results ever.
If we repeated this exact same method though, we would change some things. Three scouts are better than two. Also, we would buy a very reliable and accurate stopwatch. An even better suggestion is to do it indoors, like in a gym. This would give us more accurate results since the floor is smooth and no wind is blowing. We could also do three trials of each weight and a calculated average. But we did do multiple trials on each weight to insure consistency (we just didn’t write them down).
Another thing we could have done was to add more mass to the counter weight. There might have been a peak at which the velocity would go slower when more weight was added. This just couldn’t be done with Trebuchet X because space was limited in the counter weight box, and because there might be a chance that the trebuchet will break if too much weight is added because of its small, light frame.
And finally, one of the best suggestions is to make a bigger trebuchet. This wouldn’t exactly change results, except by yielding faster velocities and requiring heavier weights, but it would be a more fun and motivating experiment to conduct since bigger is better.
Chevedden; et al. (July 1995). "The Trebuchet". Scientific American (Original Version), 66-71.
"Trebuchet." Wikipedia.
<http://en.wikipedia.org/wiki/Trebuchet#cite_note-FAT-2>.
"Trebuchet Plans." Red Stone Projects. <http://www.redstoneprojects.com/trebuchetstore/trebuchet_plans.html>
Wheelis, Mark. Biological Warfare Before 1914.
Needham, Joseph (2004). Science and Civilization in China. Cambridge University Press, 218.
* Intro picture taken from a search for “big trebuchet” in Google Images
Trebuchet - Anatomy of a trebuchet, and other info
Dan Becker's Trebuchet Page - How to build a big, quality trebuchet
Background History of Trebuchet - A brief history on what trebuchets were used for
Trebuchet.com - A Megasite, dedicated to trebuchets
Virtual Trebuchet Mk5 R2.3 - A trebuchet simulator, just in case you want to wonder about trebuchets but don't know how to build one
