A simple catapult is a fun school project that shows how stored energy can make an object move. In this 30 minute build, popsicle sticks, rubber bands, a plastic spoon, and a bottle cap become a mini launcher for soft pom-poms or marshmallows. The project feels like a castle workshop, but it teaches real physics used in levers and machines.
It is safest when only soft items are launched and everyone stands away from the launch path.
The catapult works like a lever, with the stacked sticks acting as the base and fulcrum and the spoon acting as the lever arm. When you pull the spoon down, the rubber bands stretch and store elastic potential energy. When you let go, that energy changes into motion and sends the pom-pom in a curved path called a trajectory.
Changing the lever arm length, rubber band tension, or launch angle can change how far the pom-pom travels.
Understanding Build a Simple Catapult Project
A useful catapult investigation changes one variable at a time. If the aim is to test lever length, keep the same projectile, rubber bands, starting pullback point, launch surface, and release method for every trial. A longer spoon arm can move its cup through a larger arc, but it may not always produce the greatest distance.
The position of the fulcrum matters because it changes the balance between force and movement. A fulcrum closer to the cup makes the cup move farther for each small turn, though more effort may be needed to pull it down. This is the same tradeoff seen in scissors, fishing rods, and playground seesaws.
The projectile does not receive every bit of energy stored in the rubber bands. Some energy moves the spoon and sticks. Some becomes sound, heat from rubbing parts, and vibration in the frame.
A loose catapult bends or slides, which wastes energy and makes results less predictable. This explains why a stronger pull does not always give a neatly proportional increase in distance. Students should watch the frame during launch.
If the base lifts, twists, or skids, the design needs adjustment before comparing data. A heavier base, firmer bands, or a non-slip surface can improve consistency.
After leaving the spoon, the projectile follows a curved path because gravity pulls it downward all the time. Its forward motion continues while its downward speed increases. The launch angle affects how these two motions combine.
A very low angle sends the projectile forward quickly but gives little time in the air. A very high angle gives more air time but less forward travel. For a catapult launched from the same height onto level ground, a middle angle often travels farthest.
Real results can differ because soft pom-poms have air resistance, spin, and an uneven shape. A marshmallow can squash in the spoon, changing the release.
Good measurements make the project more scientific. Mark the launch line with tape. Measure from that fixed line to the first landing point, not to where the projectile rolls afterward.
Record several trials for each lever length and find the average distance by adding the distances and dividing by the number of trials. Results that vary widely may show inconsistent pullback, changing aim, or a loose connection. A simple table with lever length, trial distance, average distance, and observations helps reveal patterns.
Keep launches below face level, use only soft projectiles, and make sure the landing area is clear. Never aim toward people, animals, windows, or breakable objects.
Key Facts
- A catapult is a simple machine that uses a lever to launch an object.
- A lever has a fulcrum, an effort force, and a load.
- Elastic potential energy is stored when a rubber band is stretched.
- More stretch usually means more stored energy, but too much stretch can break the catapult.
- Distance = final position - starting position.
- Average speed = distance / time.
Vocabulary
- Catapult
- A catapult is a machine that launches an object by using stored energy.
- Lever
- A lever is a stiff bar that turns around a fixed point to move a load.
- Fulcrum
- A fulcrum is the fixed point where a lever pivots or turns.
- Elastic potential energy
- Elastic potential energy is energy stored in a stretched or squeezed object, such as a rubber band.
- Trajectory
- A trajectory is the curved path an object follows after it is launched.
Common Mistakes to Avoid
- Launching hard objects is unsafe because they can hurt people or damage things. Use only soft pom-poms, cotton balls, or marshmallows.
- Pulling the spoon back too far is a mistake because it can snap rubber bands or break the popsicle-stick frame. Test small pulls first and increase slowly.
- Forgetting to measure from the same starting line gives unfair results. Always place the catapult in the same spot before each launch.
- Changing many parts at once makes the test confusing because you cannot tell what caused the new launch distance. Change only one variable, such as rubber band tightness or spoon angle, at a time.
Practice Questions
- 1 A pom-pom lands 45 cm from the catapult on the first launch and 60 cm on the second launch. How much farther did the second pom-pom travel?
- 2 A catapult launches a marshmallow 80 cm in 2 seconds. What is its average speed?
- 3 If two catapults are the same except one rubber band is tighter, explain which one will probably launch the pom-pom farther and why.