Purpose To provide the Physics 30 student with hands-on application of the principles of physics and an opportunity for creativity.
History of the Trebuchet The belief is that the trebuchet began in China about 300 B.C.
In the 12th century, the Crusaders depended on the trebuchet as a weapon against their enemies though Western Europe. The trebuchet combines manpower with weight, counterweight and a series of pulley systems and could generate enough power to fling stones as big as 300 pounds to a distance up to 300 yards.
In turn, the development and design of the trebuchet caught the attention of inventors that demonstrated a practical application of physics. It contributed to further understanding of how movement could be used to create power.
Applicable Physics Principles
1. Newton’s Laws
Newton’s 1st law - An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force
Newton’s 2nd law –The acceleration of an object is dependent upon two variables – the net force acting upon the object and the mass of the object
Forces - In physics, the concept of force is used to describe how a massive body is affected by acceleration or mechanical stress
Newton’s 3rd law - According to Newton, whenever objects A and B interact with each other, they exert forces upon each other
2. Projectile Motion
With no air resistance, all objects fall with the same uniform acceleration. This means that two objects of different sizes and weights, dropped from the same height, will hit the ground at the same time.
An object is controlled by two separate motions. So an object projected horizontally will reach the ground in the same time as an object dropped vertically. No matter how large the horizontal speed is, the downward pull of gravity is always the same.
Kinematics is the branch of classical mechanics that describes the motion of objects without consideration of the causes leading to the motion.
4. Centrifugal force
Centrifugal force represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation.
The trebuchet is actually a lever. Energy is used to raise a heavy weight. This energy is stored and is released later as gravitational energy. Once the trigger is released on the trebuchet, the weight drops and pulls the short end of the lever (or arm) of the trebuchet. The long end of the arm travels at a greater distance in the same amount of time as the short end of the arm. This means that the long end travels at a greater speed, pulling the sling around, which gives it even more speed. Some of the energy is transferred to the projectile and some remains in the arm. It is centrifugal force that keeps the projectile in the sling until it reaches the point of release.
Cut all the wood pieces according to specifications (the base).
I drilled a hole near the top of each of the wood pieces (b) (the tower).
I used a jigsaw to cut the center out of the pieces of wood (a).
Using a drill, I screwed the two pieces of wood (b) perpendicularly to (a).
I used a jigsaw to cut the centers out of the piece of wood (c).
I then glued and screwed (c), which were triangular pieces, to the sides of (a) and (b).
I drilled a hole in the center of the piece of board (d) which was the bottom of the arm.
I also drilled another hole near the bottom of (d) to attach the counterweight.
I drilled a hole in the end of piece (d) to fit the wood dowel into.
I threaded the copper pipe through (d) and fit it into the holes in the each side of the tower.
Placed cotter pins on either sides of the pieces of wood to hold the arm and copper pipe in place.
I glued the dowel into the end of piece (de) (arm).
I fastened a nail near the top of the dowel at an angle.
I cut a piece of cloth (felt) about the size of about 2 “ x 5”.
I threaded one string through one end of the felt and another through the other end.
I attached one end of the string to the end of the dowel
I attached the other string to a small metal ring.
I then attached a shoelace through the end hold in (d) and through the middle of the counterweight so that it would swing freely through tower.
At first, I tested the trebuchet indoors, but found that there wasn’t enough room. The projectile kept hitting the ceiling. I took the tebuchet outside to test it, but also found that the cold, dense air inhibited the distance of the projectile. I finally took the trebuchet over to my uncle’s garage and tested it a number of times. I used several different types of projectiles such as: a small block of wood and several different sizes of Christmas balls.
Arm Length (m)
Analysis of Changes The first attempt I made at building the trebuchet was clumsy and heavy. Trials showed that the projectile would only travel a distance of 3 or 4 meters. Rather than try to modify the first model, I decided to start fresh and build a second model.
Initially, this model had a longer, more solid base. In order to lighten up the trebuchet, the base was shortened and the middle was removed. I also cut the centers of the two triangular pieces that were attached to the sides of the model. By making these cuts, I was able to cut the weight by 2 – 3 pounds.
The most troublesome part of the project was finding out at what angle the small nail at the end of the arm should be. At first, I had the nail at the tip of the dowel, but the sling kept falling off before the arm was able to reach 45 degrees. This meant the projectile did not shoot any distance at all. Once I determined that the nail should be on the upper part of the dowel, the projectile went much further.
Conclusion I found this project to be more difficult than I initially thought. It proved to be an interesting project and it was fun to see the project develop and to see a notable difference when changes were made. Overall, I found it rather satisfying.