Making a Catapult

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lu6cifer
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Making a Catapult

Postby lu6cifer » Sat Oct 03, 2009 12:54 pm UTC

So, for physics, we have to build a catapult, but I'm not a very good builder. Does have past experience with building catapults, or know anything about catapult design? What's the simplest design (i.e, can be built and calibrated in a few hours) that can launch a tennis ball roughly 40 yards?
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Re: Making a Catapult

Postby Sockmonkey » Sat Oct 03, 2009 7:37 pm UTC

http://www.io.com/~beckerdo/other/trebuchet.html

There are plenty of other trebuchet designs on the web you can google if that one doesn't suit you. The advantage of the trebuchet is that you can easilly adjust the range with the mass of the counterweight.

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Re: Making a Catapult

Postby whereswalden90 » Mon Oct 05, 2009 2:17 am UTC

Trebuchets are the simplest and easiest types of catapults. Rather than relying on a source of mechanical tension to propel the projectile, they just use a counterweight, which makes them simpler to build, more mechanically sound, and easier to fix when they break (and it will break).

Now some design notes: unless you're making a very large trebuchet, I suggest using a paint can (or the like) filled with concrete or rocks as your counterweight. It's cheap and simple. I would also suggest using pine for the body of the trebuchet because it's easier to work with and relatively strong (as soft woods go, at least.) I can tell you now the part you will probably have the most trouble with is getting the sling set up properly so that it doesn't just throw your projectile into the ground. The best way I've seen it done was to make the prong at the end of the arm out of a nail with the head clipped off and bent into the appropriate shape. That way, when you're testing the trebuchet, you can just tap the nail into whatever shape you need with a hammer to get the trebuchet to fire the way you want.

Last thing: He suggests fastening the trebuchet to the ground, which is more often than not impractical. If that is the case for you, you might want to put some weight on the back of the trebuchet to keep it from hopping when you fire it. A second concrete or rock-filled paint can will do the trick just fine.

Good luck! Please post pictures when you're finished.
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Re: Making a Catapult

Postby Carnildo » Mon Oct 05, 2009 7:02 am UTC

If your trebuchet is hopping significantly, you didn't build the support frame tall enough. One of the main benefits of the trebuchet over the catapult is that it merely wobbles a bit after firing, rather than jumping around like a catapult -- you don't need to aim anew after each shot.

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Re: Making a Catapult

Postby Comic JK » Mon Oct 05, 2009 2:13 pm UTC

I built a trebuchet in high school for a school fair, from no plans at all (just dead reckoning). Basically, build it so it looks right. The way it made a clean launch was, I had a large metal hook (a weak one, like you might hang a towel on) with a loop of string over it and the projectile on the end of the string. By changing the angle of the hook, I was able to get any desired launch angle.
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Re: Making a Catapult

Postby lu6cifer » Mon Oct 05, 2009 7:09 pm UTC

Some other restrictions...

It has to fit in a 1.5-1.5-1.5 meter box; would that be too small for a trebuchet that would have to sling something ~30 yards?
Also, the catapult has to have a firing mechanism, but that shouldn't be a problem for a counterweight trebuchet, right?
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Re: Making a Catapult

Postby Izawwlgood » Mon Oct 05, 2009 7:48 pm UTC

I would definitely go with the treb over the catapult if you can. Cooler, more physics-y, and less danger of snapping wood in your face. I made a crappy treb with a 1m throwing arm and it launched stuff maybe... I dunno, 20 ft? Had no counterweight, just my buddy and I pushing it down.
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Re: Making a Catapult

Postby lu6cifer » Tue Oct 06, 2009 2:30 am UTC

Okay, so based on the comments I've heard, I'm probably gonna go with a counterweight trebuchet. Many people say that a sling would help increase the range of a projectile, but does it really help? My group was just planning to use a lacrosse stick.
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Re: Making a Catapult

Postby Carnildo » Tue Oct 06, 2009 3:21 am UTC

lu6cifer wrote:Okay, so based on the comments I've heard, I'm probably gonna go with a counterweight trebuchet. Many people say that a sling would help increase the range of a projectile, but does it really help? My group was just planning to use a lacrosse stick.

The improvement is incredible -- figure an order-of-magnitude increase in range.

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Re: Making a Catapult

Postby Izawwlgood » Tue Oct 06, 2009 1:19 pm UTC

It requires a bit of fiddling to get the release cords length correctly detaching, but yeah, it's the difference between a 5 foot toss and 40 ft toss.
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Re: Making a Catapult

Postby Heisenberg » Tue Oct 06, 2009 6:53 pm UTC

The sling is tricky, but incredibly awesome. It gives you a type of mechanical advantage, and allows you to fine-tune the release. If you want to do it without the sling, you'll need to rig up a stop so that the arm stops moving forward at the optimal angle (45 degrees for no air resistance, I forget what it is with air resistance). This will be difficult and jolt the heck out of your catapult every time. The advantage of the sling is that you don't need a stop because the sling setup releases the ball at the perfect moment and the arm gets to continue to swing. Also, if the whole contraption is on wheels, it will increase your distance as the trebuchet will roll forward as it releases, but if you only want to do one tricky thing, make it the sling.

Air resistance is your enemy. Use a baseball if you can, or fill the tennis ball with weights or water. A massive ball won't really impact the release velocity, but it will help your distance.

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Re: Making a Catapult

Postby Tass » Wed Oct 07, 2009 5:51 pm UTC

Also the sling greatly reduces the amount of jolting and bouncing of the machine. Without the sling most of the momentum and energy will at all times be in the counterweight, and all that has to be dissipated. That may be true with the sling as well, but not nearly to the same degree.

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Re: Making a Catapult

Postby lu6cifer » Wed Oct 07, 2009 8:25 pm UTC

What I'm worried most about the sling is if the ball stays in the sling past the point of release, or if the ball falls out of the sling before point of release. Any advice on getting the right balance?
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Re: Making a Catapult

Postby Josephine » Fri Oct 09, 2009 11:52 pm UTC

lu6cifer wrote:What I'm worried most about the sling is if the ball stays in the sling past the point of release, or if the ball falls out of the sling before point of release. Any advice on getting the right balance?

Perhaps combine a trebuchet design with an onager design to stop the throwing arm?
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Re: Making a Catapult

Postby Azrael001 » Sat Oct 10, 2009 12:01 am UTC

When I had to make a ball launching device I made a (roman?) ballista. It was very similar to the one shown here. Though ours lacked a firing mechanism. It had the second largest range, and a firing speed which was second to none.
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Re: Making a Catapult

Postby Sockmonkey » Sun Oct 11, 2009 3:57 am UTC

nbonaparte wrote:Perhaps combine a trebuchet design with an onager design to stop the throwing arm?


That would ruin the stability of the design. Onager means "wild ass" meaning it kicks like one.

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Re: Making a Catapult

Postby Josephine » Sun Oct 11, 2009 4:18 am UTC

Sockmonkey wrote:
nbonaparte wrote:Perhaps combine a trebuchet design with an onager design to stop the throwing arm?


That would ruin the stability of the design. Onager means "wild ass" meaning it kicks like one.


Ah, right. good point. And you need to expend all that extra energy all at once, so no padding or springs. Scratch that.
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Re: Making a Catapult

Postby Heretical Mind » Mon Oct 12, 2009 3:57 am UTC

My brother and I built a trebuchet with an eight foot arm and a sling system, and managed to come within inches of hitting our neighbors car with a melon sized rock- total throwing distance probably 100- 150 yards. I'd advise that you build it so that the counterweight can swing: this seemed to improve our range by a good bit.

The sling is also a must- it can be made out of a strip of cloth with a length of cord at both ends. Nail one cord to the throwing arm, put a loop on the other. As whereswalden90 said, a nail with the butt-end clipped off works well as a hook on the throwing arm.

Last thing- experiment, experiment, experiment. If you haven't made at least five test fires that exceeded your range requirements, then it's not functional. Of course, on the scale, and with the materials you're working with, you're probably going to be doing extensive repairs after a while, but hey. Once you know how, it takes about an hour and a half to build another, so feel free to screw it up once or twice.
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Re: Making a Catapult

Postby lu6cifer » Tue Oct 13, 2009 1:01 am UTC

I am sorry to report that the trebuchet failed miserably.

Well, semi-miserably. It launched a tennis ball about 25 yards, but after that, the sling just wouldn't function correctly, launching the tennis ball backwards or at an incorrect angle.

So, we resorted to a lacrosse stick + bungie cords.
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Re: Making a Catapult

Postby Azrael001 » Tue Oct 13, 2009 3:21 am UTC

You should have made a ballista.
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Re: Making a Catapult

Postby lu6cifer » Tue Oct 13, 2009 3:40 am UTC

Azrael001 wrote:You should have made a ballista.


It's actually funny you say that, because I have a ballista frame from a Science Olympiad competition a couple of years ago. Unfortunately, we have to have some sort of lever arm in the design.
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Re: Making a Catapult

Postby Azrael001 » Tue Oct 13, 2009 3:48 am UTC

Use a lever to pull it back. Have the firing mechanism be the part of the lever that hooks onto the pouch thing, then to fire, make the trigger pull it down, out of the way.
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Re: Making a Catapult

Postby RogerMurdock » Tue Oct 13, 2009 6:17 pm UTC

You wouldn't happen to go to a Northern Viriginia High School and take physics from a certain Mr. Bowman would you?

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Re: Making a Catapult

Postby lu6cifer » Tue Oct 13, 2009 9:53 pm UTC

RogerMurdock wrote:You wouldn't happen to go to a Northern Viriginia High School and take physics from a certain Mr. Bowman would you?


Not quite. I go to an Eastern Pennsylvania high school.
lu6cifer wrote:"Derive" in place of "differentiate" is even worse.

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Re: Making a Catapult

Postby RogerMurdock » Tue Oct 13, 2009 11:05 pm UTC

Ah ok, I was just wondering as we have a teacher at my high school that literally just assigned this project with the same specifications for size, and it seemed suspicious.

Carry on :)


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