1 Westwood Catapults 2014 Elliot Rose March 19,

2 What this group will do Meet on Wednesdays at 2:10pm for about 1 hour. –Phase 1: Everyone will learn about catapults, build a catapult, and test their catapult. –Phase 2: Science Field Day team will practice with the 2 best catapults to get ready for the SFD competition, which is on Saturday May 10 th. Weds, Mar. 19: Learn about catapults, the Science Field Day rules, process for choosing the competitors, form teams of 2. Weds, Mar. 26: Share design ideas, answer question, Bring in what you’ve built so far. Meet at the Basketball courts. Weds, April 2nd: Test the Catapults. Submit SFD team suggestions to the organizers. Meet at the Basketball courts.

3 Science Field Day Rules Students must compete in 2 or 3 events. Catapult Rules: Device must be free-standing, not hand-held and capable of throwing a tennis ball at a target that will be between 2 and 7 meters away. Bring your catapult to the site immediately upon arrival. Print the school’s name prominently on the catapult. Testing or practicing is not allowed at the event. A regular tennis ball is used and will be provided by the person running the competition. It is a good idea to make a chart to show how to set your catapult for a given distance. Bring any tools and spare parts you may need. The ball may be shot, slung, or lobbed at the target. The “launch force” must be provided by gravity or elastic solids (springs, rubber bands, bungee) The last point on the device touching the tennis ball may not be more than 50 cm (19.7 inches) above the ground before, during or after shooting the tennis ball. –Exception: for trebuchet-type designs, the flexible sling may swing higher than 50 cm during launch as long as the rigid arm attached to the sling does not exceed 50 cm. Each catapult should be designed and built by the students with minimal adult assistance. They should be made to operate safely at all times. The catapult will sit on the ground and be fired at the target, also at ground level. The target will be a clearly marked point in the center of a sand filled area approximately one meter in diameter. The distance from the shooting line to the center of the target will be between 2m -7m. The actual distance will be announced the day of the event. Participants may place their catapult at any point behind the shooting line, up to 2 meters. (Continued)

4 Science Field Day Rules (cont.) Scoring: The contestants will have 6 minutes to fire their catapults five times. Each team will have two practice shots and three scoring shots. The distance from where the tennis ball initially lands to the target will be measured in centimeters after each shot. Teams will be allowed to make adjustments following each shot. Hitting any part of the “bullseye” will be scored as a zero, while missing the sand filled area entirely will be automatically given a score of 150 cm. The cumulative distance after 3 scoring shots will be the team's score. The smallest score wins.

5 Keys to Success Safety – There is some risk in building and using a catapult, so take your time and be careful. You will need an adult to assist you in building the catapult. Simplicity – Simple designs tend to work better, be more reliable, easier to adjust, and easier to build. Keep it Simple!!! Also, a catapult capable of hitting something 50 meters away is probably not very accurate at 5 meters. Range – Must be able to shoot as long as 7 meters and as short as 4 meters (can move the catapult as much as 2 meters behind the line). Consistency – If your catapult hits about the same spot every time, then you can adjust it to hit the target. If it changes every time, you cannot aim well. Stability – The catapult may tend to “hop” when you fire it, so you need a way to hold it steady (large base). Robust – There is a lot of energy when you fire the catapult, so your catapult must be strong enough to not break when used. Practice as a Team – The 2 practice shots and 3 real shots must be finished within 6 minutes, so the teammates must work quickly and well together to succeed. Have Fun! – It’s more important to learn and have fun, than to win.

6 Catapult Design Options Basic Types: Bottom-Hinged, Center-Hinged, Ballista, & Trebuchet Material: Wood, PVC Pipe, Metal Springs: Bungee, Rubber Bands, Metal Springs (Twist or Stretch) Ball Holder: Plastic Cup, Tupperware, Half a ball, –Do not want ball to move around in the cup; not consistent. Ball should be stable. Aiming Device: Sights to line up the target Distance Adjustment: –Change Spring, –Change Start Point, –Change Release Point, –Change the Arc –Okay to adjust in big steps, as you can fine tune by moving the catapult back from the line. –Want to have a chart worked out ahead of time for adjustment

7 Examples of Bottom Hinged Catapults Good: Repeatability, Long travel makes adjustment easy Bad: Heavy Arm = Need for Strong Spring & Strong Structure

8 Examples of Center-Hinged Catapults Good: Arm is more balanced, so not as many “Heavy Arm” issues Bad: Shorter stroke means a stiff spring is needed and adjustment is harder

9 Examples of Ballista Catapults Good: Simpler than Arm-type, low trajectory works well with raised-cup target, Small moving weight Bad: Friction a bigger factor = Variability

10 Projectile Equation Distance a projectile will travel: –d = distance –v = velocity (speed) when released –g = gravity (constant) – term means distance is longest when the angle is 45 degrees. (Ignores air friction.)