Building a Trebuchet
Building a Trebuchet
For this project, I am building a trebuchet. The goal is to launch a tennis ball at least 20 feet. I did this project because I thought it would be a good way to incorporate physics, designing, prototyping, and engineering into one project. I started by researching how a trebuchet works.
Initial hypothesis: The tennis ball will not launch past 20 feet.
Hypothesis after manual testing: The tennis ball will launch past 20 feet.
The trebuchet uses gravity to generate energy. For gravity to take hold, the counterweight must be much heavier than the payload (what is being thrown). This is because of moments and leverage. Moments is the tendency for something to rotate about a specific point. In this case, it is the axis that goes through the beam. Because there is more distance, therefore area, on the throwing arm, the counterweight needs to be heavier than the opposing pull of gravity, the payload, and the leverage of a longer arm. When the trebuchet is released, it swings up in a circular motion. Because the sling is unrestrained, it experiences centripetal acceleration. This results in a massive increase in linear velocity. The payload is released when a certain angle of a is reached. This angle can be changed by changing the angle δ.
Next, I researched the history of trebuchets.
They were one of the main siege weapons used throughout the medieval ages. The design allowed for an arcing trajectory that made it easy to fire over castles walls from below. They could also fire farther than an average catapult.
The first version of the trebuchet, the traction trebuchet, was invented in ancient China 300 BC. The traction trebuchet used people as the counterweight. A large group of men would pull down on the counterweight arm to propel the payload.
Nowadays, there is no use for a trebuchet because we have much more powerful weapons such as cannons, mortars, and bombs. However, they are still used in some science classes as a fun example to demonstrate physics.
The trebuchet made its way to Europe in the early medieval ages. The design was then revised to the counterpoise trebuchet (the one I built) which replaced people power with the force of gravity and weight.
Finally I started designing. I looked into some sources and found some important ratios to help me start building my trebuchet. I also looked up different trebuchet designs to help me draw inspiration.
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Length of payload arm is 3.75 times longer than counterweight arm
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Sling is the same length of payload arm
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Increasing payload to counterweight ratio will increase velocity but also increase counterweight
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Counterweight is 133 times weight of payload
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The weight of the counterweight is limited by free fall velocity (10m/s^2)
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Side view
Top View
Side View
Top View
Design Number 2
Design Number 1
Built base
Changes from Design 1 to 2
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Metal tube instead of pivot
Why: A proper pivot would require bearings (like inside a bike). This would overcomplicate the structure when the goal is just to make something spin around a point. Also, I could not find a pivot that long.
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Using steel weights instead of water for a counterweight
Why: Initially, I thought bringing gallon containers would be easier to transport and then fill up with water. However, after talking with my dad, we concluded that the amount of water needed to reach approximately eight kilograms will take up too much space. A gallon of water is roughly 3.75 kilograms or 8.25 pounds. It would take about 2.25 gallons to equal the counterweight. This takes up a lot of volume. On the other hand, 7.5-10 pounds of steel weights will only take up the space of about a quarter gallon. This is because the density of steel is about 7700 kg/m^3 compared to water which has a density of 997 kg/m^3.
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Adding a central support beam
Why: The main reason is that it would be easier to mount a metal rod through a piece of wood which would not have been possible if there were two pieces joined together like the first design. Also, it would provide more lateral support.
I decided to start building based on my second design.
Attached side supports and bar
Built side supports
Hook Design
Side View
Front View
Prototype 1
Side View
Front View
Prototype 2
Front View
Side View
Changes from Prototype 1 to 2
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Steel Bar with PVC Vs. Aluminum Bar
Why: The density of steel is around 8,000 kg/m^3. This is almost three times denser than aluminum which is around 2,700 kg/m^3. In addition, the aluminum bar was hollow whereas the steel bar is solid. I chose to put PVC around the steel because metal bends but does not snap whereas PVC snaps but does not bend.
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Caribiner Attachment Instead of Tie
Why: In the first model, I tied the counterweights straight to the throwing beam. This made it very difficult to move the throwing beam around because the counterweights were dangling on one end. In my second prototype, I tied a piece of rope to the original hole where the counterweights hung from. I then tied another rope around the weights. I used a carabiner to attach the two together. Now, whenever I need to move them around, I can detach the counterweights and move them separately.
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Testing with Velcro means that there was a little Velcro piece in the pouch that stuck to the tennis ball
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Testing with angle means that the hook was set to an angle of 135°
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The sidewalk is 20 feet
Learning & Reflections:
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Learned the physics of a trebuchet
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Learned importance of density in weights
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Real world example of centripetal acceleration and leverage
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Bisecting lines and creating angles in real world context
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Hands on building
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If I were to do this again, I would create a timeline so I can stay consistent with building and testing.