Shoot the Monkey
Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely
Categories: Force, Motion
Alternative titles: Projectile Motion Independence
Summary
This classic demonstration shows that the horizontal and vertical components of a projectile’s motion are independent. A projectile fired from a cannon hits a monkey that drops from a tree at the same instant, because both fall with the same vertical acceleration due to gravity.
Procedure
- Suspend a stuffed monkey (with a metal plate or shield attached) from an electromagnet.
- Aim a spring-powered cannon directly at the monkey while it hangs.
- As the cannon fires a steel ball, the circuit releases the electromagnet, causing the monkey to drop at the same moment.
- Observe that the ball follows a parabolic path and strikes the monkey in midair.
- Emphasize that the hit occurs regardless of projectile speed or distance, because both objects fall with the same gravitational acceleration.
Links
Shoot the falling target demo - Rhett Allain:
📄 Shoot the Monkey - Harvard Natural Sciences Lecture Demonstrations: https://sciencedemonstrations.fas.harvard.edu/presentations/shoot-monkey
Variations
- Adjust the cannon’s angle or spring compression to change the projectile’s speed and collision height.
- Vary the monkey’s starting height to demonstrate collisions at different points in space.
- Replace the monkey with another target object to keep the concept but reduce theatrical effect.
Safety Precautions
- Never stand in front of the loaded cannon.
- Secure the cannon firmly to prevent movement when firing.
- Use a shield or barrier to protect the audience from stray projectiles or ricochets.
- Practice loading and firing carefully; the spring mechanism requires significant force.
- Ensure the target is padded or reinforced to withstand repeated impacts.
Questions to Consider
- Why does the projectile always hit the monkey, regardless of speed or angle? (Because both the projectile and monkey fall with the same acceleration due to gravity.)
- What would happen if there were no gravity? (The monkey would not fall, and the projectile would miss.)
- How does this demonstration disprove the Aristotelian view of motion? (It shows that motion is the combination of independent horizontal and vertical components, not a mix of “violent” and “natural” motion.)