Physics Application: Projectile Motion

The Variables in Projectile Motion on a trebuchet:

Release angle: As shown in the picture above; too high of a release angle and the projectile will go high rather than far. But too low of a release angle and the  projectile goes far, but not as far as a projectile that has height. Therefore if 90 degrees is too high and 0 degrees is too low, the perfect angle is 45 degrees.

Gravity: Gravity will always affect the projectile no matter where its released at. The projectile's inertia wants it to go at what ever angle released, but gravity impedes on the inertia causing the projectile to come back to the ground. That's why its key to have height in your projectile because if too low, gravity will act down really quickly since the projectile is close to the ground, causing the distance to be short.

The speed of the projectile: The higher speed the projectile has, the faster it will go. The faster it goes, the more time it has to travel before gravity forces the projectile back onto the ground. Therefor if the projectile is slow, it wont travel as far. 

Modifications:

Perfect Release Angle: If the projectile is launched at the perfect release angle of 45 degrees, it has the perfect balance of distance and height giving it the best chance of increasing range.

Speed: The faster the projectile is going, the more distance you can cover. Therefor putting as much weight as possible will increase energy and force causing an increase in speed of the projectile allowing it to cover as much distance as possible before gravity effectively puts it back on the ground.

Physics Application: Energy

A: The counterweight has all Gravitational Potential energy at this point. It is not moving, but has a lot of mass and is being suspended above the ground. Giving it a chance to move downward. As it does the Gpe turns into kinetic energy as the counterweight speeds up, and the Gpe diminishes because there is less and less space to travel as the weight is moving down.

B: The projectile at this point has no Gpe, no kinetic energy, it is all regular potential energy as the sling is taught to the arm. But as the arm flies up it gains kinetic energy and starts losing its potential energy as the arm moves, as it rises in elevation it also gains Gravitational potential energy. Once it reaches the top and the projectile is released, the kinetic energy is still climbing, the original potential energy is gone, but the gravitational potential energy is now at its peak. As the projectile flies in the air and comes back down its kinetic energy is still gaining energy due to the dwindling gravitational potential energy as the projectile lowers elevation. Finally once it hits the ground all potential energy is lost and the rest of the kinetic energy falls due to friction.

Modifications to increase range would be to increase the gravity potential energy in the counterweight as much as possible without exceeding structural capacity. This way the most energy can be transferred to the arm which gives the arm the best chance to release the projectile with as much energy as possible letting it go the farthest distance possible.

Physics Application: Force

Modifications that could be made to increase range:

Release Angle of the sling: The release point of the sling is the most crucial part of the trebuchet. Release too low and your balloon goes backwards or straight up. Release too high and your balloon goes straight down with not only the force of the counterweight, but gravity added at the last second. Therefor releasing the catapult at the perfect angle gives a balance of distance and height causing it to go as far as possible.

Weight of counterweight: This is pretty simple. The more weight you have, the farther the balloon will go. But, too much weight and your catapult will either break under the pressure, or the whole thing will be unbalanced and toppled over. At the same time, if you don't have enough weight, you wont have enough force to not only move the weight of the arm, but the sling as well. So, you must have a decent amount of weight to move the arm and the sling, but also move it fast enough to get velocity out of the balloon.

Height of the swinging arm: Due to restrictions you cant make the height more than 5 feet. But if restrictions were abolished essentially the longer the arm, the more centripetal force would be acted on the arm and especially the swing, causing a larger velocity. Rather than a short arm only being able to move a short distance, a longer arm would create a longer distance to travel causing a greater buildup of velocity at release. But then again too much length of an arm and the arm could break under the pressure of the weight unless the arm was really sturdy and thick, preferably the trunk of a tree.

Testing and Modification

Trial 1

Trial 2

Trial 3

Process of Design and Construction

2X4 Wood acquired for building

Lacrosse mesh (soon to be transformed into a sling)

Metal Rod

Base being built

Base built (with wood underneath to hold weight)

Base arms being added

First swinging arm being built

Metal rod being attached to support swinging arm

Weights added, Batman I ready to go

R.I.P. Batman I

ditch the first lacrosse head, take the second lacrosse head (pictured at top) and use its mesh for sling mechanism.

Sling

re-use first swinging arm, attach sling to end

testing weight hold and modifying sling mechanism

reinforcers added

One last sling modifier

(after spray paint dried) trigger installed at base of catapult

Log of Meetings

Log of Meetings:

May 17th, 2014: Ben and Andrew both present, raw materials gathered, base built, first arm built, first trebuchet built

May 18th, 2014: Ben and Andrew both present, scrapped first arm, built second arm, built sling system

May 19th, 2014: Ben and Andrew both present, final arm modified, trigger built, base reinforced, more weight added.

Ben Hogan and Andrew Yoches' 4th 9 Weeks Project

This is our electronic portfolio for the trebuchet catapult.