Proof of Efficacy on Trebuchet
Design
For our trebuchet we went for a simple design. We used two arms with two evenly drilled holes in them. We put o rounded stick through it as an axle. We then got a long skinny board and drilled a hole near the bottom. We then put a screw on the bottom and a hole it the very end of the arm. We the attached 6 rubber bands from the screw to the hole. We put a small nail at the top of the arm as the release hook. We used a clay ball attached to a string as our projectile. That was our final design of our trebuchet.
8 improvements
1. Weights to rubber bands
Based of my original thoughts I assumed that weights would be superior to rubber bands when used as a counterweight because that's what they used in the annual Punkin Chunkin competition. After we tested it I learned that weights are actually way worse than rubber bands. We did multiple tests with weights then when we switched to rubber bands our launches averaged about 20 m farther
2. String length
Originally our string was way too long but then once we edited to be only about 10 inches it launched way better. When it was too long it would bottom out and not whip around and when it was short it wouldn't gain enough speed. We found the perfect length to be about 10 inches.
3. Ball weight
Our ball was originally only 5 grams and it lacked momentum in the air and sometimes wouldn't even release. Based on another groups information we changed our balls mass to 10 grams. With this it wasn’t to heavy that it was hard to throw but it still had an efficient flight.
4. arm stabilizers
To reduce wobble in our arm we added stabilizers. They were pvc pipes around our axle that stopped the arm from swaying too much to one side. It allowed the launch to get the maximum distance by launching completely straight and not at a slant.
5. Rubber Band to a string
At the beginning design phase I assumed that attaching our projectile to a rubber band instead of a string would let it launch farther. When we used the rubber band to launch it it instead launched straight down. After swapping to a string it went 35m because it didn't absorb any of the arms initial movements.
6. Launch nail hook
At first we used a hook as our release nail. We used it under the assumption that it would hold the rope then release it at the top of the arms swing. Instead the hook never released the launch string. We then switched it too a small nail with a 50 degree angle upwards. This nail released the string perfectly at the point of the swing we wanted and maximized the launch angle.
7. Launch arm proportions
Based off our prior knowledge of simple machines our group built our arm so that the side being pulled was smaller then the side being flung and launching the weight. For once our assumption was correct. Our launch arm gave our trebuchet a mechanical advantage of 3.34.
8. Distance arm is pulled down during launch
During our launches we found that if we pulled the arm down to about a 45 degree angle to the ground it would launch the best. This let the ball not get pulled to sharply so it would still gain speed but not fall off early.
Personal experiment results
When launching a small scale trebuchet it's more effective to use rubber bands as a counter weight then to use a weight. We first started by using different weights in our trebuchet to see what weight would prove to be the most effective. We began using them but only ever got up to 5 meters even with very heavy weights. We decided that the weights were just a bad idea in general. Instead we swapped over to rubber bands. We first used 10 bands that sent the projectile about 20m. After we used 15 rubber bands that sent the projectile about 17m. We realized that the weights could only move at a max of 9.8 m/s due to acceleration due to gravity. Also we learned that with too many rubber bands you can have too much force that takes away the full swing of the catapult. In conclusion about ten rubber bands has the best amount of power for our trebuchet.
Calculations/Content
Mass of projectile- 13.5 grams / amount of matter in the ball
Horizontal distance- 35m / distance the ball travelled on the ground
Time in air- 4.89 seconds / time the ball was in the air between launch and landing
Vertical distance- 28.2 m / distance the ball went in the air
Horizontal Velocity-7.29 m/s / speed of the ball parallel to the ground
Vertical Velocity- 28.2 m/s / speed of the ball straight up
Total velocity-29.1 / speed of the ball in the real launch pattern
Angle of release- 55 degrees / angle the ball took off from the trebuchet
Spring constant-122.5 n/m / Amount the rubber bands will stretch due to a weight
Initial spring potential energy- 8.575 / stored energy in the rubber bands while stretched
Kinetic energy of the ball- 5.71 J / energy being used during the balls flight
Percent energy converted- 66.5% / amount of energy converted from the rubber bands to the ball
Selling Points
Easy to launch Reusable
Easy to build efficient trebuchet
Cheap fun for the whole family
My Part
For our project we were told to create a trebuchet that could launch an object the farthest. I helped design our trebuchet and did a lot of the building. I mainly did the research and calculations and I made kept our group on task. I found all the measurements for our trebuchet and I did testing in the string length to try and improve our distance. I created the ideal ball. but eventually our trebuchet became too powerful that it would rip apart the clay balls during launch and our distance decreased.
Reflection
During this project I did a good job of getting work done. Even when I didn't know what to do I tried to do as much as I could to stay on task. I gave my teammates work to do and we all did the assigned work. I kept the team together and working and I put actions to the groups words. When they said we needed to test the trebuchet I always went over and got us too test it as best as possible. I also took a lead on doing the calculations.
I need to let my teammates do more and have a leadership role. I micromanage the group too much and it annoys me when they don't do the work I told them too. I need too let my teammates do what they want as long as its productive. I also need to be nicer to teammates because i would get mean when i was irritated by them.
I need to let my teammates do more and have a leadership role. I micromanage the group too much and it annoys me when they don't do the work I told them too. I need too let my teammates do what they want as long as its productive. I also need to be nicer to teammates because i would get mean when i was irritated by them.