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Building a Trebuchet

  1. Aug 2, 2006 #1
    I am building a Trebuchet That is made out of steel and is about 8 feet tall, with a 1000 pound weight. here is what i need to know.

    I want it to throw a 20 to 30 pound object 300 ft or more.

    What Does the length of the arm with the weight and the lenght of the trowing arm have to be. To achieve this.

    Its kinda urgent.
  2. jcsd
  3. Aug 2, 2006 #2


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    Laying seige to your neighbours? :biggrin:
    Sorry, I can't help with the math.
  4. Aug 2, 2006 #3


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    WARNING: Any machine capable of that is *very* dangerous.

    Now, assuming we can ignore air and the ground is flat, and we're launching for 300 meters ('cause I know all the constants in metric).

    You're going want a launch speed around 600 meters per second. Ignoring the mechanical losses that figures to a drop of around 8 meters (more than 24 feet) for the 1000 lb weight - so it's out of your reach.

    In addition, you'll need the long arm to be about 2.5 times as long as the short arm for that projectile. So for the 1000 - 20 ratio, the structure would start getting very large.

    In practice the angular moment of the arm is also an issue.


    For peak efficiency, the weight should be free swinging (common) and the trebuchet should be on rollers (uncommon).
  5. Aug 2, 2006 #4

    Andrew Mason

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    Have a look at this analysis of the physics.

  6. Aug 2, 2006 #5

    Well how far could we throw a 20 pound projectile with the height we already have? How long do the 2 arms have to be on the fulcrum?
  7. Aug 3, 2006 #6


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    From experiance you usually need at least a 80 - 1 CW projectile ratio. And for 20-30 pounds 8 feet isn't very big. Why don't you try throwing something smaller?
  8. Aug 3, 2006 #7
    i dont have an answer. but may i know why you are building this
    is it a physics project or something
  9. Aug 3, 2006 #8


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    The easiest way to find out is to go to the firing range.
  10. Aug 3, 2006 #9


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    It helps first to build a small model before going for the full blown one, I did that and it helped we make multiple improvements.
    Here you can see my treb (for physics project and for fun) fireing:
  11. Aug 3, 2006 #10
    Nice vid! I'll bet that sparked your creativity for optimization.
  12. Aug 5, 2006 #11
    I built a very small trebuchet during 9th grade in school; and I browsed the internet for information. I found a lot of sites dealing with trebuchet-physics. [I don't have the adresses, but a quick google search would give you the wanted results].

    Beware though that a model in a much smaller scale wouldn't work as a test-machine for your larger one as many parameters will change. Instead to a medium-sized model (if you're even going to make a model)
  13. Aug 7, 2006 #12
    I agree with myspip...trebuchets don't "scale up" like you think they should.
    However, there is a pretty nifty trebuchet simulation program out there for free - you specify several parameters (weight of the counterweight, arm lengths, sling lengths, mass of projectile, etc.) and it will calculate the theoretical distance the projectile will travel. The math is not trivial for this system (several coupled differential equations), but I used this simulation program when building a trebuchet in college, and it was fairly accurate - albeit at the small scale we were using (we were limited to a 2ft x 2ft x 2ft cube, and threw golf balls approx 125 ft).
    Try googling it (sorry no link).
    Hope this helps...
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