Catapult Project. For our catapult project, we based our design off of two different types of historical

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Johnson 1 Cameron Johnson Nathan Ericksen Physics 221 November 18, 2012 Catapult Project For our catapult project, we based our design off of two different types of historical designs and then added

1 Johnson 1 Cameron Johnson Nathan Ericksen Physics 221 November 18, 2012 Catapult Project For our catapult project, we based our design off of two different types of historical designs and then added our own modifications and designs in order to maximize distance and accuracy. The historical designs we modeled our catapult after were the Ballista and the Mangonel. These two designs are very different in their approach to launching projectiles, but they both have very distinctive elements that we thought would be useful in our design. The Ballista has a design that can be thought of as somewhat similar to a giant crossbow. It consists of a bow, or some type of flexible plank which is attached to a rope that spans across the length and is connected at both ends. The bow or plank is also mounted in the center to a stationary base of some kind. When the center of the rope is pulled, the bow or planks flex, creating tension and potential energy in the direction opposite of which the rope has been pulled. A sling is attached at the center of the rope and this point can also be secured in place by some kind of locking mechanism. A projectile is then placed in the sling and when the locking Figure 1: Example of a Ballista. 2 talent caliber.jpg Wikipedia Commons Web. 18 Nov < talent_caliber.jpg>.

Johnson 2 mechanism is triggered to release, the bow or plank springs back into its natural straight position, pulling the rope and sling back with it.

2 Johnson 2 mechanism is triggered to release, the bow or plank springs back into its natural straight position, pulling the rope and sling back with it. This can create an enormous amount of force which quickly accelerates the projectile, launching it at unsuspecting foes or castles. The Mangonel is another approach which is probably closer to what most people would think of as a traditional catapult. It consists of an arm mounted to a base by twisted ropes connected horizontally to the arms fulcrum. On the end of the arm opposite the fulcrum, a scoop of some kind is mounted where projectiles to be launched are placed. The scoop side of the catapult is pulled down so that the rope at the fulcrum twists, creating tension from torsion. The scoop is locked in place and is loaded with a projectile. When Figure 2: Example of a Mangonel. Watson, Chris. Goblin Mangonel. N.d. Elfwood Web. 18 Nov < Mangonel html>. the locking mechanism is triggered to release, the rope at the fulcrum spins back into its original orientation which flips the arm upward, and the projectile is lobbed through the air. Generally, for a full scale Ballista or Mangonel, the force to pull the launching element into place is far too great for a person to achieve without a mechanical advantage. To resolve this issue, a crank type of mechanism is mounted to the base of the catapult on the side that the scoop or sling is located when it is ready to fire. A rope is attached to the crank mechanism at one end and the other to the scoop or sling. When the crank is turned, the rope wraps around a rod and pulls the scoop or sling into position with much less force than would be

3 Johnson 3 needed without such a mechanism. In our project, however, we decided that because we are merely launching tennis balls and not thousand pound boulders, this mechanism would not be needed as we would be able to cock the catapult with just the strength of our arm. Before starting the design phase of our catapult, we discussed our main objectives, and the rules and limitations of the contest. Our main goal was to achieve maximum distance and accuracy. The limiting factors that we discussed were the 50cm maximum arm length from fulcrum to center of projectile and the materials that we could use: mainly wood, rope, and no metal whatsoever. We had to come up with a design that could create the force we needed to achieve maximum distance without breaking the wood used to make the catapult, or tweaking the wood to make it inaccurate. We reviewed different types of catapult designs and discussed the advantages and disadvantages of each. We discussed whether we should use a design that would use a counterweight, torsion, or a bow/plank type of mechanism to create the force needed and what type of launching mechanism we would use. We then began drawing rough diagrams and researched various methods and approaches for launching projectiles. The sources we used are listed at the end of this report. Figure 3: Some examples of our initial rough diagrams. Johnson, Cameron. Nov 2012.

4 Johnson 4 In the end, we decided that we would use the bow/plank approach from the Ballista because we could produce a lot of tension without adding a lot of weight to the catapult. This method also made it very easy to adjust the distance of the projectile because we could adjust the tension by just pulling the launcher back to different points. We also decided that we would use an arm like that from the Mangonel because it would allow us to use mechanical advantage to maximize the power from the bow/plank approach by making the part where the rope attaches to the arm much closer to the fulcrum than the part where the projectile would be placed. We then began drawing more detailed diagrams using actual measurements. Figure 4: Some examples of our more detailed sketches with measurements. Johnson, Cameron. Nov We decided that we would want the projectile to be released at a 45 angle because we learned in the Ballistic Pendulum lab in our Engineering Physics class that this was the optimum angle for maximum distance for a projectile. After calculating our measurements, we picked up our materials which consisted of 2x4 s and 2x10 s for the main structure; oak rods which we used for dowels, bars and beams; 8 oak planks which we stacked for the bow; 3/8 inch rope; and a perfectly shaped wood serving spoon that we found at Ross to place the tennis ball in. We then put the catapult together to our specifications and when we did our first test launch, we were surprised to find that it worked quite well. It was very accurate and

11cm. We found that if we moved it farther than that, the arm started to crack so we knew we had reached its strength limitation.

5 Johnson 5 consistent, and could launch the ball a reasonable distance of about 35 or so feet. We decided that we wanted to try to make the ball launch farther so we added more tension to the planks by moving them farther away from the arm by approximately 11cm. We found that if we moved it farther than that, the arm started to crack so we knew we had reached its strength limitation. This adjustment ended up almost doubling our maximum distance which averaged around 60 to 65 ft. Figure 5: Detailed diagram of our final catapult, Front View. Ericksen, Nathan. Nov Figure 6: Detailed diagram of our final catapult, Side View. Ericksen, Nathan. Nov 2012.

Johnson 6 Overall we were very happy with the results of our catapult and are looking forward to when the snow starts falling because we feel it will work quite well in snowball fights.

6 Johnson 6 Overall we were very happy with the results of our catapult and are looking forward to when the snow starts falling because we feel it will work quite well in snowball fights. Figure 7: Photo of our catapult in action at the 2012 WVC Catapult Contest. Bonnickson, Mike. Physics in motion WVC students learn to appreciate medieval catapult makers The Wenatchee World, Wenatchee. Web. 16 Nov < Bibliography: Gurstelle, William. Backyard Ballistics. Chicago: Chicago Review Press, Incorporated, Print. Gurstelle, William. "Early Adopter: Build This Onager Catapult." Popular Mechanics. Hearst Communication, Inc, Web. 18 Nov "How to Build a Catapult." Storm the Castle. N.p.. Web. 18 Nov < to builda catapult.htm>. Moubarak, Bilal. "Fun with Catapults." IN Framez. N.p., Web. 18 Nov <

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