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Trebuchet problem help >.<!

  1. Dec 29, 2009 #1
    SORRY if its the wrong place for the topic but im new >.<
    hi umm ... im grade 11 and i got this trebuchet project for school, i built the thing
    and now i gotta do the calculations , i KNOW that mgh( for the counter weight) = 1/2 mv^2 ( for the projectile) but i dont know how to explain this, like WHY? is it so.. i know that according to the law of conservation of energy, that energy can neither be created nor destroyed, but i was thinking, as the counter weight drops down, its potential energy is transfered into it moving and as its a see-saw (or however u spell that) shape then the projectile gets the energy transfered too moving up (based on the axle) but if i wanna calculate the velocity of the projectile then i have to be exact, like umm...the sling increases the range of the trebuchet, but with that above equation i dont see anything that includes the sling in it, where the sling does really have a big role in increasing the velocity and acceleration so.first how do i include the sling in there, second is how do i know the angle of departure of the projectile..its aprox 45 but idk...anyways heres the data:

    projectile = wiffle ball, 47 grams
    counter weight = 20 kg
    distance travelled by proj. = 14 m
    drop of counter weight = 66.5 cm
    time...no idea..but after being calculated with an "attempt" i think its 0.74 sec..but i dont think thats right cuz it took longer than that while i t was airborne
    attempted calculation initial velocity of proj = 26.9 m/s
    so yah.....any help? please
  2. jcsd
  3. Dec 29, 2009 #2


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    Science Advisor
    Gold Member

    Unfortunately, I don't think a trebuchet can be effectively modeled using conservation of energy between the ball and the counterweight. It's really a dynamics problem, where the counterweight accelerates the arm, which in turn accelerates the sling, which accelerates the ball.

    Did you teacher tell you to make any special assumptions when analyzing the projectile or trebuchet as a whole? The calculations can be done, but it will take more than PE = KE...
  4. Dec 29, 2009 #3

    Welcome to PF!

    If you have access to a DiffE solver, (ie; Mathematica) this is a really good treatment of the principals involved in trebuchet design;

    http://www.algobeautytreb.com/trebmath35.pdf [Broken]

    At HS math level, the above may be beyond you... but I recommend you have a look anyway. You will gain an appreciation of all the variables involved and how they relate.
    Last edited by a moderator: May 4, 2017
  5. Dec 29, 2009 #4

    jim mcnamara

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    Staff: Mentor

    If a wiffle ball has holes in it - like the ones I saw a looong time ago - then
    you have a HUGE air resistance factor to cope with. Since it is a school project, you probably cannot re-run your test with a small mass solid object that is not like a wiffle ball.

    Wiffle balls decelerate very fast, and gollow an very unpredictable path.

  6. Dec 30, 2009 #5
    I helped my son and his buddies build one for senior year competition to shoot a waterballon. the score was 1/3 of the physics grade
    school record was 15 meters. we reset it to 56 meters.
    according to the rules of contest, they kids should have gotten 375 points
    I don't think the school was as impressed as we were
    (we shot water ballons all over the neighborhood that summer, and my wife was as impressed as the physics teacher)
    why don't they have contests between companies for us grownups like that
    good for 1/3 my annual pay raise

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