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Homework Help: How can i find the efficiency of a trebuchet and/or catapult

  1. Sep 22, 2007 #1
    I'm doing something for a science fair experiment and i'm trying to compare a catapult to a trebuchet. i will be comparing range, accuracy, and efficiency. But i don't know the equation to find efficiency of each siege wepaon. both of these medieval wepaons will be tabletop size som they won't be very big.
  2. jcsd
  3. Sep 22, 2007 #2
    Efficiency is tied to the energy transfered into the projectile. To find this, about the only way I can think of would be to find its velocity, but I have no idea how you'd find that without something like a radar gun.

    I think theres some kind of measuring device that places a tick on a strip of paper at certain time intervals, which you could use to find the moving speed of the arm. However, I'm not sure how well this would work.
  4. Oct 9, 2007 #3
    My brother tells me that i need to find the force of my catapult/trebuchet for my science fair experiment. I know that F=M/A. i don't know how to find A though.
  5. Oct 9, 2007 #4
    F = ma, not m/a. m is the mass and a is the acceleration.
  6. Oct 9, 2007 #5
    How do i find the acceleration then
  7. Oct 10, 2007 #6


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    The applied force and therefore the acceleration will change during launch for both devices. So I do not think that trying to measure them will teach one much. Accellerometers can be stuck on the projectile (used by shipping companies to record the maximum g's - see Mythbusters site). This will most likely give you the deceleration upon impact with the earth. Once the projectile is in the air it will follow a parabolic path where you can apply the projectile theory (if one ignores drag). Measurements like the range and angle of launch could give you some indication of the initial speed of the object though. This will enable you to calculate the (kinetic) energy transferred to the projectile (which in theory can be used to calculate an average applied force).
  8. Oct 10, 2007 #7
    thank you for that long and almost incomprehensible paragraph (i'm only in eigth grade).
    Last edited: Oct 10, 2007
  9. Oct 10, 2007 #8
    In that case, you won't need to do much in the way of calculations, so just go for empirical data. i'd just do something along the lines of throwing something that has the same mas,, and the one that throwes it farther imparts greater energy.

    if I assume correctly that your brother is in a higher grade, he could help with some calculations. The force of the trebucket is easy to find (F=m*g), and I'de use a suspended pendulum to measure the energy imparted by the catapult. To do this, you'd suspend a weight in such a way that it will swing upwards when carried and released by the arm, and measure how high it goes. We or your brother could help you with the calculations once you find some data for us.

    Honestly thoug, you may have picked a slightly too hard project. It will be very hard to accurately measure anything with the basic suplies you'er likely to have.
  10. Oct 11, 2007 #9
    well you see, it's a school project and i don't think i will get counted down for knowing trigonometry and whatnot. right now i'm taking an online course teaching me the basics of trig and i also have to build my own small scale trebuchet, catapult, and ballista which wont' be very easy. i'm pretty sure that this will be a pretty impressive posterboard during the night i have to present all of this.

    On a different note, how could i find acceleration without accelerometers?
  11. Oct 11, 2007 #10
    i guess (if you're ignoring drag). You can always calculate the time it took from launch. Then calculate the angle (i'm not exactly sure from where you measure the angle). Also You'd need to calculate the exact distance from launch to landing (first bounce). Then use the formula deltax = vnot(t). Oh i also forgot that you should time it right from when it hits the ground and also find the distance right when it hits the ground. best results will be if you have some one help you. Then plug in the distance in meters for delta X and the time in seconds for T and solve for starting velocity for x. Then using trig you can find the starting velocity for the launch of the object. Then I really don't feel like going into finding acceleration (maybe cause i hit a dead end). Im also in a low level physics course as of now (still high school). I think that i gave you the right information but don't count on it. If some one else from this forum says i'm on the right track then I am, but otherwise don't do your project based on what i've said. So yeah, this was probably a pointless waste of time but whatever.
  12. Jan 6, 2008 #11
    how do i find the efficiency after i have found the velocity?
  13. May 30, 2010 #12
    An easy way to find trebuchet efficiency (all credit goes to Rom Toms http://www.trebuchet.com/): [Broken]
    Actual measured range in meters /theoretical range= efficiency

    theoretical range in meters= 2 * (counterweight in kg * counterweight drop in meters)/ weight of projectile in kg

    Just weigh the counterweight, measure how far it drops, measure how far the projectile shoots on the fly, and plug in and solve.
    Last edited by a moderator: May 4, 2017
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