A trebuchet is a gravity powered throwing machine that turns the falling motion of a counterweight into the fast motion of a projectile. In a school design challenge, it is a practical way to test physics ideas such as torque, energy transfer, rotational motion, and projectile motion. Small changes in the arm ratio, sling length, release angle, and counterweight mass can strongly affect the range.
This makes the trebuchet a useful engineering project because students can build, measure, redesign, and improve a real device.
Key Facts
- Torque is the turning effect of a force: tau = rF sin(theta).
- Gravitational potential energy of the counterweight is PE = mgh.
- A longer projectile-side arm usually gives higher projectile speed, but it also requires enough torque to rotate quickly.
- Arm ratio = long arm length / short arm length, often tested between about 3:1 and 6:1 for classroom trebuchets.
- Ideal projectile range on level ground is R = v^2 sin(2theta) / g, where theta is the launch angle.
- Increasing counterweight mass can increase range up to a point, but friction, frame flexing, and poor release timing can limit gains.
Vocabulary
- Trebuchet
- A siege engine that launches a projectile by using a falling counterweight to rotate a throwing arm.
- Counterweight
- The mass that drops under gravity and provides energy to swing the trebuchet arm.
- Torque
- A measure of how strongly a force causes an object to rotate around a pivot.
- Arm ratio
- The ratio of the long throwing side of the arm to the short counterweight side of the arm.
- Release angle
- The angle at which the sling lets go of the projectile, setting the direction of launch.
Common Mistakes to Avoid
- Making the frame too weak, which is wrong because flexing and wobbling waste energy that should go into the projectile.
- Adding counterweight mass without retesting the release point, which is wrong because the faster arm motion can change when and where the sling releases.
- Assuming a 45 degree launch is always best, which is wrong because real trebuchets have air resistance, sling motion, release height, and friction that shift the best angle.
- Changing many variables at once, which is wrong because you cannot tell whether range changed because of counterweight mass, arm ratio, sling length, or another factor.
Practice Questions
- 1 A trebuchet has a counterweight of 3.0 kg that drops 0.60 m. How much gravitational potential energy is available before losses? Use PE = mgh with g = 9.8 m/s^2.
- 2 The short side of a trebuchet arm is 0.20 m and the long side is 0.90 m. What is the arm ratio? If the counterweight force is 50 N and acts perpendicular to the short arm, what torque does it produce?
- 3 A team increases the counterweight mass and the range first improves, then gets worse. Explain two possible physics or engineering reasons for the decrease in range.