Instructables
Picture of K'nex catapult (the orange arch)
This is a simple 10 minute catapult that I built using the 521 piece K'NEX set. Using a strong rubber band I have been able to fire a marble about 20 feet.

 
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Step 1: Parts list

Picture of Parts list
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Here's a parts list.

4 red rods
3 yellow rods
3 bendable orange rods
2 white rods
11 green rods

6 grey 45 degree connectors
6 red 90 degree connectors
4 green 135 degree connector
4 orange connectors
2 grey sliding connectors
2 blue sliding connectors
2 purple clips
2 tan connector clips
2 back rod clips
1 small gear

Step 2: The main body

Picture of The main body
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first connect 2 red connectors with a green rod (X3)

then attach the two grey sliding connectors, with the two blue connectors.

next connect both of the blue grey connectors with a green rod (as shown below)

Step 3: Connect main body parts

Picture of Connect main body parts
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connect the two red assemblies with the blue green assembly as shown in the picture.

Step 4: Connect body parts 2

Picture of connect body parts 2
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then create the vertical boom piece of the catapult with two yellow rods, and a red assembly. 

then attach two purple pieces with green connectors too the center piece, as shown below.

Step 6: Attaching the arm to the body

Picture of attaching the arm to the body
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slide a yellow rod through the two purple pieces and attach the arm.

Then add two tan, two black, and a small gear as show in the picture.

Step 7: Add strength to the body

Picture of add strength to the body
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now to make the pieces to help strengthen the body.

attach two green connectors with a green rod, and two more light grey pieces to that with 2 more green connectors.

now (X2) connect a green connector with a green rod to a orange straight connector, and an orange bendy rod on the other side of the green connector.
Nice!
singhjr96 (author)  sandroknexmaster1 year ago
Hope it's good enough to win a prize in the contest.
Don't misunderstand me, but I think this will not win a price. It's not very innovative,
and almost everyone could build this.
But no offense I like it anyway! :)