I have designed this project to be a practical way to demonstrate launching projectiles at fixed angles. The catapult has a stop pin that is adjustable in 5° increments from a 25° stop angle to a 90° stop angle. The throwing arm has a max tension indicator (stop). The indicator is there to assist in creating equal tension on every launch, so that students can compare what happens to the distance when we change the launch angle.
I started with knowledge of the strength of plastic and determined around a 3x3-4x4mm bar would be my stop. I then went into AutoCAD and sketched out an arc of 4x4mm holes with around 3mm of material between them for strength. After I had this sketch it set the basis for the size of my catapult structure.
Next I exported a DXF and imported into Fusion 360. Then, I worked on creating the side frame for the catapult. After I had a side I was happy with I modeled the front attachment support, the bottom support, the stop bar, then finally the throwing arm.
I printed each piece, and did a test.
The original throwing arm had some flaws which required a couple revisions.
Then finally I came to a design I was happy with.
Started with a sketch of the stops in AutoCAD
Fusion 360 - Design of the side.
Projectile physics and trajectory. The goal of this project is to help students grasp an understanding of projectile physics and trajectory through a practical application and physical testing.
Step 1 - Background Knowledge
- Ask students to write down what they think the results are going to be based on the catapult
- Discuss vectors
- Discuss gravity
- Discuss friction and air resistance
- Discuss projectile physics
- Discuss trajectory
- Print and use catapult as a discussion aide
Step 2 - Design / Build
- Print catapult as is from Thing files
Option 2 (bonus):
- Have students design a projectile to launch
Option 3 (alternate / advanced):
- Have students design a catapult to meet your criteria.
- Have students print their catapult.
Step 3 - Practical Testing
- Find a suitable firing range.
- Setup a firing line and lay out a grid for determining range.
- Assemble catapult as shown in the video.
- Fire 2 - 3 shots (or more) at each angle 25° - 90° with 5° (1 stop hole)
- Measure each shot and record the data
**NOTE: Try to ensure each shot is the same. The tension stop was put on the throwing arm for this very reason.
Option 3 from above:
- Hold a contest to determine who met your launching criteria with their design.
Step 4 - Calculations
- Find the average shot range for each angle (assuming you performed multiple shots to average)
- Plot the data.
- Determine what the optimal angle is for the firing conditions.
Step 5 - Review
- Review the data.
- Discuss what the results were and why. Did the answer to the best angle vary from 45°? why?
- What could we do different?
- What are the real world applications for projectile physics? (beyond pumpkin chunkin or military)
- How did the results compare to what students wrote down at the beginning?
The following materials and items can be helpful with this project.
- Rubber bands for powering the catapult
- projectile (students could design one as an optional addon)
- measuring device - for calculating the distance
- pencil and paper - for taking notes on results
- safety gear - projectiles involved!
- projectile physics
How does gravity affect the projectile?
How does friction affect the projectile?
What are vectors?
What is the optimal launch angle with no wind?