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One of us is 7, so obviously we made a trebuchet. Actually we have made several in the last year or so – the current version goes sufficiently high and far to only be useable outside.
We used FischerTechnik, which allowed us to easily experiment with the design ourselves by adjusting each component.
This website was helpful in detailing what to aim for in the design.
The design features we found useful were:
- The main frame needs to be quite sturdy – we used quite a bit of bracing
- The optimum ratio for the length of the weight arm to the projectile arm is 1:3.75
- The arm needs to be strong but light
- The length of the sling (we used linen thread) should be about the same length as the projectile arm
- The counterweight needs to swing freely
- The mass of the weight should be about 100x the mass of the projectile
- The angle of release (affected by the pin angle) should be about 45o
- There needs to be a smooth surface for the projectile to travel along before it is launched (we used some cardboard to smooth the transition over the base)
- Solid ground underneath helps (e.g. not carpet)
It is interesting to adjust various parts and see the effect. For instance,
- small weight + 60o pin = 4.1 m range
- heavier weight + 60o pin = 3.7 m range
- small weight + 30o pin = 8.3 m range
- medium weight + 30o pin = 8.7 m range
The projectile is difficult to see in a video, so we fired it at dusk with LEDs attached to the projectile:
And here is a video:
It would be nice to be able to provide a general description of the physics of how a trebuchet works, but it turns out to be rather more complicated than it initially appears. There is an explanation by Donald Siano here if you really want to get into it.
Physics explanations aside, they are quite satisfying to make and to fire. I expect our next (better, bigger) version will require trips to a park so as to be able to launch projectiles without losing them into the neighbours place.