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Mousetrap Catapult


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#1 The Thing

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Posted 28 February 2006 - 02:25 AM

I'm trying to build a mousetrap catapult. It would be launching acorns. I was wondering how I can get the acorn to fly as far as possible? Is a shorter arm or longer arm going to throw the acorn further, and what other features, such as rubber bands, can I add onto the catapult to make it more powerful?
Thanks.
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#2 Phi for All

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Posted 28 February 2006 - 03:41 AM

Longer arm = better trajectory, to a point. Weight will be a factor. I suggest you cut the U part of the trap at one end (leaving the upright that's attached to the spring), straighten it and then solder or braze a small wire hoop to hold your acorn. Play with the hoop design for best release. Releasing too soon will get you height but not necessarily distance. Releasing too late will just roll it along the floor.

I can't think of any way to improve it with rubber bands. Try lengthening that throwing arm for different results. There will have to be a point where the arm is too long and heavy to throw it farther.

Google the word "atlatl" and see if that gives you any ideas.
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#3 Norman Albers

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Posted 28 February 2006 - 04:57 AM

If the arm is too short you won't get much velocity. IF it is too long you will have a hard time swinging it around. It's some kind of kinematic optimal. David s'posedly did wipe out Goliath with a good sling, so I say go for it! There was an article in SciAm a handful of years ago about folks making catapults. Gloriously, the first shot went straight up, and predictably downward. Dear, dear. This was like a small European car body. Actually I will seize this chance to plug the cartoon Prince Valiant. You will, as a steady follower, learn many such techno hints and cultural goodies.
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#4 JohnB

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Posted 28 February 2006 - 07:47 AM

You will need to add a "Stop Arm" for the thing to work. Otherwise the acorn will just slam into the floor as the trap completes it's full swing.

For best results, make it adjustable so you can vary the trajectory.

For extra fun, try building a Trebuchet.:D
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#5 Norman Albers

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Posted 28 February 2006 - 03:21 PM

Thanks! I spent time wondering why not open the outer side of the bucket, thinking of the centrifugal force. What matters is the instantaneous velocity, which is tangential...................What's big out here in Oregun are potato guns. I saw one fueled with hairspray shoot 100 yds.
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#6 ecoli

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Posted 28 February 2006 - 03:42 PM

Here's a practical hint... use a rat trap instead. I launched a ball with a rat trap about 10 ft... think of what an acorn could do!
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#7 Phi for All

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Posted 28 February 2006 - 03:52 PM

You will need to add a "Stop Arm" for the thing to work. Otherwise the acorn will just slam into the floor as the trap completes it's full swing.

Really good thinking! Probably save some fingers too, especially if he takes Ecoli up on the rat trap idea.

Do you recommend leaving the trap bar "as is" so you can use the trigger mechanism? With a big rat trap you don't have to worry so much about trimming weight.
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#8 Norman Albers

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Posted 28 February 2006 - 03:53 PM

Kinematics can be learned by looking at momentum and energy balance when one 'ball' hits another. Do the simple in-line case first, and vary the relative size. Clearly different exchanges happen when a small ball knocks a large one, or the opposite. I had a good discussion with a young friend just getting a rifle, and we talked about momentum transfer viv-a-vis bullet mass. Similar questions as here........A few years ago I injured a disc or two in my spine. Now nearly what I was, to my pleasure I found that a six-pound splitting maul is so much easier to lift and develop a swing, compared with the eight-pounder that is now too much to swing, that I get more momentum to split wood with the six. As a youth I disdained it. We change.
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#9 Norman Albers

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Posted 28 February 2006 - 04:56 PM

This seems non-intuitive and bothers me. I would set up such projectile problems with the statement that we supply a force acting through a distance on some mass. This is not the same as identifying a specific impulse which is force over time. Here, the heavier shot comes out slower, but the energy input is fixed, say E. Then, E = (1/2)mv^2, so v = SQRT(2E/m), and we can write mv = SQRT(2mE). How can we keep getting more momentum with a heavier weight? If we put my eight pound wedge on your rattrap, it's hard to see it going fast enough to hurt me. Should I then not identify momentum with 'hurt'? Seems like it's really E that counts. Talkin' crash-test dummies.
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#10 gcol

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Posted 28 February 2006 - 05:19 PM

1. Study Roman and medieval siege catapults. They had it to a fine art.

2. Assuming the power of the trap spring is a constant, then the length of the throwing arm should be tuned to the weight of the acorn, to get maximum terminal velocity. This would be my priority, the stop and release mechanism would be second consideration.

3. Norman: In yor own rather unique and minimalist literary style, is your log-splitting illustration in any way similar to this:

My son, who is light and "whippy", could throw a small stone twice as far as his friend, who was stocky and muscular. He, in turn, could throw a heavier stone considerably further.
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#11 s pepperchin

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Posted 28 February 2006 - 07:46 PM

This seems non-intuitive and bothers me. I would set up such projectile problems with the statement that we supply a force acting through a distance on some mass. This is not the same as identifying a specific impulse which is force over time. Here, the heavier shot comes out slower, but the energy input is fixed, say E. Then, E = (1/2)mv^2, so v = SQRT(2E/m), and we can write mv = SQRT(2mE). How can we keep getting more momentum with a heavier weight? If we put my eight pound wedge on your rattrap, it's hard to see it going fast enough to hurt me. Should I then not identify momentum with 'hurt'? Seems like it's really E that counts. Talkin' crash-test dummies.


Remember that since momentum is force times velocity we can increase the momentum by increasing mass or velocity in this situation we are increasing the mass and decreasing the velocity, however we are doing it in such a way that the momentum still goes up.

Another thing you want to remember when you build your catapult is that when the arm reaches the stop arm it could flip the whole thing over. So make sure you have a base which will keep it upright.
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#12 Phi for All

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Posted 28 February 2006 - 09:01 PM

Another thing you want to remember when you build your catapult is that when the arm reaches the stop arm it could flip the whole thing over. So make sure you have a base which will keep it upright.

If you extended/braced the front of the trap (in the direction of the throw) would the energy saved (which might have made it flip) be transferred into a more forceful throw or would it be transferred into the brace? Either way it's a good idea.
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#13 Connor

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Posted 28 February 2006 - 09:17 PM

heck, maybe you could even get your hands on a beaver trap ; )
those things have some massive power- got my hand caught in one once.
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#14 gcol

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Posted 28 February 2006 - 09:22 PM

Watch out for beaver, it always smells of fish.
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#15 Norman Albers

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Posted 28 February 2006 - 11:19 PM

Very nice, now I have some meditations while I string a set of bass strings, especially your observations on the boys, gcol.
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#16 Bluenoise

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Posted 1 March 2006 - 12:41 AM

Ha ha from the title I was so sure you mean a mouse trap that kills the mouse by catapulting it.

LMAO
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#17 Norman Albers

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Posted 1 March 2006 - 12:53 AM

Yes, with cholera, up into the attic nest.
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#18 ma65p

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Posted 10 November 2007 - 09:50 PM

Remember that since momentum is force times velocity we can increase the momentum by increasing mass or velocity in this situation we are increasing the mass and decreasing the velocity, however we are doing it in such a way that the momentum still goes up.

Another thing you want to remember when you build your catapult is that when the arm reaches the stop arm it could flip the whole thing over. So make sure you have a base which will keep it upright.


I really really disagree. First thing, E= Potential energy + kinetic energy= mgh+(1/2)mv^2. Now, we are trying to get the object move by using kinetic energy only since potential energy of the object only pull it down (that also explain why the object move in projectile motion). Increasing mass will increase kinetic AND potential energy. This does not help you much while increase the mass of the object only allowed it to be pulled down faster. I really don't think increasing mass will help, unless you figure out an optimum mass that allow maximum kinetic energy. That would be great.
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#19 Scotsquatch

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Posted 16 December 2007 - 11:52 PM

Check out my website, www.siegethedayinc.com . I make and design catapults for living. While I have a copywright on all of my designs and they are my intellectual property, I don't mind if you reverse-engineer them for your own use...in other words, not for profit.

That said, I have designed mousetrap and rat trap catapults, and there are pictures of them on the website.

Also, feel free to contact me through the website for advice or questions.

In regards to having to add anything to a mousetrap to increase its power for launching acorns: Don't bother! My design will easily launch an acorn 35+ feet with authority...lol!

Scotsquatch
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#20 chemkid

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Posted 17 December 2007 - 12:47 AM

Hollow out your acorn as best you can and get a nice small one.
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