- Use a hands-on approach to explore and comprehend basic concepts in engineering and physics like stored energy, trajectory, reinforcement, density, levers and hinges, and trusses.
- Acquire an understanding of the different parts of a catapult: the base, the support structure, the arm, and the basket.
- Strengthen critical thinking and motor skills as they build their siege engines and troubleshoot any design flaws that may arise.
Prep work: 2/5
Setup time: 2/5
Each mini siege engine is built differently, so I will outline how to build the individual models in the following steps. For now, let's focus on what these little war machines have in common:
Trusses: I explain to the students what a truss is and why it's so strong and useful. Trusses are basically triangles that are used to strengthen a structure. Trusses do not change their shape easily, unlike squares which can flatten into rhombuses (sometimes I will make a triangle and a square out of craft sticks and show the students how easy it is to squash a square, but not a triangle). Trusses also require very little materials which makes them highly efficient. The catapult in the photo above uses trusses to support the downward pull of the catapult arm. If the trusses were replaced with a pair of beams perpendicular to the base, the whole support structure would collapse if the arm were to be pulled down.
Reinforce: This is easy to explain - reinforce simply means adding more material to make something stronger, like gluing sticks on all sides of the base instead of just one.
Trajectory: I explain that trajectory is the path that an object, or projectile follows through the air. What happens when the projectile is aimed upward at 45 degrees? The trajectory it takes will be a nice smooth arc. Aimed straight forward? Immediately begins to fall toward the ground. ( Just like in Angry Birds - kids can really relate to this!) Changing the trajectory will change how the projectile moves. Most kids understand this intuitively.
Step 1: Simple Catapult
The easiest way to show kids how to build this is by taking the the catapult apart into it's two components: the base + support, and the catapult arm. Point out the key features of the design like where the hinge will be, how the cubes with hinges are not connected with sticks, and how the cubes on the base are connected on three sides to make the base sturdy. Next, show the students how to assemble the catapult and attach the rubberbands - students in grades 1-3 typically need help with this.
Pulling on the catapult arm stores energy in the rubberbands. When the arm is released, the stored energy is transferred into pulling the arm rapidly upward and into the crossbar. The catapult arm stops upon contact with the crossbar, but now the projectile still has momentum and will continue to follow its trajectory.
- Load the projectile
- Firmly hold down the front of the catapult
- Pinch the cube under the basket and pull down, then release