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Catapult Physics: Principles & Equations

Instructor: Amanda Robb
In this lesson, you'll learn how to apply motion equations and design a catapult. By the end of the project, you'll understand how to apply projectile motion equations to determine the maximum range for a catapult.

Introduction

Goal: To study how the launch angle of a projectile influences its displacement
Age: High school and up (To fully understand the project, students must have some knowledge of basic geometry.)
Safety concerns: Never launch a projectile at any living thing. Even something small like a cotton ball can hurt it.
Time to complete: Two hours

The Middle Ages were a grandiose time, with castles towering over villages and kings and queens covered in beautiful jewels reigning from the towers. The Middle Ages were also known for their siege weapons. Tall castle towers made it difficult for enemy troops to storm the gate, so people came up with ways to launch weapons over the walls. A catapult is one weapon that uses stored energy in a spring to launch projectiles over a distance.

Properties of the projectiles can be calculated using kinematic equations for projectile motion. If you need to review how to apply projectile motion equations, check out this lesson on Projectile Motion: Definition and Examples and try some projectile motion practice problems.

Today, we're going to look at how changing the angle from which a projectile is flung influences how far it can travel.

Materials

  • 8 popsicle sticks
  • 5 thick rubber bands
  • Plastic bottle cap
  • Craft glue
  • 1 or more cotton balls
  • Protractor
  • Ruler
  • Pencil and paper
  • Book

Steps

1. Place six popsicle sticks on top of each other and secure them with rubber bands on either side.

2. Glue the plastic bottle cap to the end of an additional popsicle stick. This will be your launching stick. Let it dry.

3. Attach the launching stick perpendicular to the stack of popsicle sticks. Secure it in place with a rubber band.

4. Secure the base of the catapult by attaching another popsicle stick to the bottom of the launching stick. This will force the launch stick at an angle over the stack of sticks made earlier.

Construction of the catapult
catapult construction

5. Measure the launch angle. The launch angle is the angle from the horizontal to where the projectile is launched. Line your protractor up against the top of the launch stick parallel to the surface it is resting on to measure the launch angle from the bottle cap.

Measure the launch angle from the bottle cap to the horizontal.
launch angle

6. Now it's time to launch your projectile. Place a cotton ball in the bottle cap, pull the launch stick down and let it fly! Measure how far your projectile flew and record it on paper. Safety Tip: Always launch your projectile out in the open away from other people and objects.

7. Change the launch angle by setting it on the edge of a book at an angle to produce a tilt. Repeat steps 5-6.

Troubleshooting

Make sure that your rubber bands are secure. If they're not secure, there won't be enough stored energy to put your catapult into motion.

Discussion Questions

How did the launch angle affect the distance the projectile traveled?

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