A model trebuchet is a fun school project that shows how stored energy can turn into motion. Instead of using springs or rubber bands, a trebuchet uses a falling counterweight to swing a long arm. Building one from craft sticks, cardboard, string, tape, and a small cup helps students see engineering ideas in action.
Use only soft projectiles such as pom-poms or crumpled paper, and test in a clear area with adult supervision.
When the counterweight drops, gravity pulls it downward and makes the beam rotate around a pivot. The short side of the arm holds the counterweight, while the long side carries the sling or launch cup, so the projectile moves faster than the counterweight. Changing the counterweight mass, arm length, sling length, or release angle can change how far the projectile travels.
Careful measuring and repeated trials turn the build into a real science investigation.
Key Facts
- Gravitational potential energy is PE = mgh, where m is mass, g is 9.8 m/s^2, and h is height.
- A larger counterweight can store more potential energy before the launch.
- Torque is τ = rF, where r is the distance from the pivot and F is the force applied.
- The pivot is the fixed point where the throwing arm rotates.
- Projectile range depends on launch speed, launch angle, and air resistance.
- A fair test changes only one variable at a time, such as counterweight mass.
Vocabulary
- Trebuchet
- A trebuchet is a simple machine that launches a projectile by using a falling counterweight to swing a throwing arm.
- Counterweight
- A counterweight is a mass that drops under gravity and provides energy for the launch.
- Pivot
- A pivot is the point or axle around which the throwing arm turns.
- Torque
- Torque is the turning effect of a force applied at a distance from a pivot.
- Projectile
- A projectile is an object that moves through the air after being launched.
Common Mistakes to Avoid
- Using hard or sharp projectiles, which is unsafe because a trebuchet can launch objects farther than expected. Use soft items like pom-poms, marshmallows, or crumpled paper.
- Changing several parts at once, which makes the test unfair because you cannot tell what caused the range to change. Change only one variable per round.
- Making the base too light or narrow, which is wrong because the frame can tip and waste energy during launch. Add a wider cardboard base or tape the frame down securely.
- Placing the pivot off center by accident, which can stop the arm from swinging smoothly. Measure both sides and make sure the axle is straight before testing.
Practice Questions
- 1 A counterweight has a mass of 0.20 kg and starts 0.30 m above its lowest point. Using PE = mgh with g = 9.8 m/s^2, how much gravitational potential energy does it have?
- 2 A class tests three counterweights: 50 g, 100 g, and 150 g. The launch distances are 0.8 m, 1.4 m, and 1.9 m. What is the increase in distance from 50 g to 150 g?
- 3 If two trebuchets use the same counterweight but one has a longer throwing arm, explain why the longer arm might launch the projectile farther.