The Marshmallow Catapult: Harnessing Levers and Elastic Energy (#78)
A marshmallow catapult converts stored elastic potential energy in a stretched rubber band into kinetic energy through a lever arm, launching lightweight projectiles across your kitchen table. In about 20 minutes, you can build a simple machine from craft sticks that demonstrates mechanical advantage, energy transformation, and projectile motion, all while flinging mini marshmallows at your siblings.
This experiment sits at the sweet spot where fun meets physics. You're not just building a toy; you're creating a teaching tool that shows how ancient siege weapons worked, why rubber bands snap back, and how levers multiply force. Let's build one.
Experiment at a Glance
Age Range: 5–12 years
Cost: Under $5
Difficulty: Easy
Time Required: 20 minutes
Mess Level: Low (unless marshmallows get sticky)
Supervision: Minimal (younger kids may need help with rubber band placement)
What You'll Need
Gather these materials before you start:
- 7–10 craft sticks (the wide popsicle-stick kind)
- 4–6 rubber bands (varying sizes work well)
- 1 plastic spoon (or bottle cap)
- Mini marshmallows (the ammo)
- Masking tape (optional, for reinforcement)
- Ruler (optional, for measuring launch distance)
You probably have most of this in your junk drawer already. If not, a dollar store run will cover everything.
Building Your Catapult: Step-by-Step
Step 1: Create the Base Stack
Take five craft sticks and stack them neatly on top of each other. Make sure the ends line up, sloppy stacking means a wobbly catapult.
Wrap a rubber band tightly around one end of the stack, about half an inch from the edge. Then wrap another rubber band around the opposite end. These bands hold your base together and give it stability.
This stack becomes the fulcrum, the pivot point your lever will rotate around.
Step 2: Build the Lever Arm
Take one craft stick (this becomes your throwing arm). Lay it flat on the table.
Now grab your plastic spoon. Place the bowl of the spoon at one end of the craft stick, with the spoon's handle extending past the stick. Wrap a rubber band around both the stick and the spoon handle several times to secure it. This creates your "bucket" for holding marshmallows.
If you don't have a spoon, tape a bottle cap to the end instead. You need something with a slight cup shape to cradle the marshmallow before launch.
Step 3: Attach the Lever to the Fulcrum
Position your lever arm (the craft stick with the spoon) perpendicular to your base stack, forming a cross or plus sign shape.
Slide the lever arm between the top stick and the second stick of your base stack, right in the middle of the stack, where your rubber bands aren't covering it.
Take another rubber band and wrap it in an "X" pattern around the intersection where the two pieces meet. Wrap it several times until it's snug but still allows the lever arm to pivot up and down.
Test the motion. When you press down on the spoon end, the opposite end should lift up. When you release it, the spoon should snap upward. That's your catapult action.
Step 4: Add the Tension Arm
Take one more craft stick and position it so it creates a diagonal brace from the top of the base stack to the underside of the lever arm (near the non-spoon end).
Wrap rubber bands around both connection points, one where it touches the base stack and one where it meets the lever arm. This diagonal stick adds elastic tension to your system.
The more you pull down on the spoon end now, the more this diagonal stick stretches the rubber bands, storing potential energy. This is where the magic happens.
Step 5: Reinforce (Optional)
If your catapult feels loose or wobbly, add masking tape to any joint that needs extra support. Wrap it around the rubber bands or directly onto the sticks.
You can also double up rubber bands for more tension. Thicker rubber bands store more energy but require more force to pull back.
Launching Your Marshmallow
Place a mini marshmallow in the spoon. Press down on the spoon end of the lever arm, you're storing elastic potential energy in those stretched rubber bands.
Release.
The marshmallow should fly forward in a beautiful arc. If it dribbles off the spoon or flies backward, adjust your rubber band tension or reposition the lever arm connection point.
Pro tip: Aim slightly upward (about 30–45 degrees from horizontal) for maximum distance. Straight horizontal launches lose altitude too quickly; straight vertical launches waste energy on height instead of distance.
Mark where your marshmallow lands with a piece of tape. Now try changing variables: pull back further, use a heavier projectile (like a grape), or adjust the angle. See what gives you the longest flight.
The Science Behind the Fling
Lever Mechanics
Your catapult is a Class 1 lever, the same type as a seesaw. The fulcrum (your base stack) sits between the effort (where you press down) and the load (the marshmallow in the spoon).
When you push down on one end, the lever rotates around the fulcrum, and the opposite end moves in the opposite direction. This is mechanical advantage in action. A small downward movement on your end creates a large, rapid upward movement on the spoon end.
Ancient armies used this same principle to launch stones and flaming projectiles at castle walls. Your marshmallow launcher is a medieval siege weapon in miniature.
Storing Elastic Potential Energy
The rubber bands are your catapult's energy storage system. When you pull the lever arm down, you stretch those rubber bands. Stretching them takes work, you're transferring energy from your muscles into the rubber band molecules.
The amount of potential energy stored follows this relationship:
PE = ½ kd²
Where k is the spring constant (how stiff the rubber band is) and d is the distance you stretch it. Notice that d is squared, that means doubling your stretch distance quadruples the stored energy. Pulling back twice as far doesn't just give you twice the power; it gives you four times the power.
Different rubber bands have different spring constants. A thick, stiff rubber band stores more energy at the same stretch distance than a thin, flimsy one. Experiment with different bands to see how this changes your launch distance.
Energy Transformation
When you release the lever, all that stored potential energy converts into kinetic energy, the energy of motion. The rubber bands snap back to their resting length, yanking the lever arm upward. The spoon accelerates rapidly, and the marshmallow (which was sitting still) suddenly gets shoved forward at high speed.
At the moment of release, you can see energy transformation in real time:
- Before release: Maximum elastic potential energy, zero kinetic energy
- During release: Potential energy decreases as kinetic energy increases
- At launch: Potential energy is mostly gone; kinetic energy is at its peak
- After launch: The marshmallow has kinetic energy; the catapult returns to rest
The marshmallow flies until air resistance and gravity slow it down, converting its kinetic energy into heat and eventually bringing it to a stop.
Why Mass Matters
Try launching marshmallows of different sizes. A mini marshmallow flies farther than a jumbo marshmallow when launched with the same energy input. Why?
Both marshmallows receive the same amount of kinetic energy from the catapult (because you pulled back the same distance). But kinetic energy depends on both mass and velocity:
KE = ½ mv²
Since the lighter marshmallow has less mass, it needs more velocity to carry that same amount of energy. More velocity means it travels farther before gravity pulls it down.
Heavier projectiles travel shorter distances at the same energy level. If you want to launch something heavy, you need more stored energy, pull back farther or use stronger rubber bands.
Variables to Test
Turn your catapult into a science experiment by changing one variable at a time:
Launch angle: Try horizontal (0°), medium (30–45°), and steep (60–75°) angles. Which gives the longest distance? Which gives the highest peak?
Rubber band tension: Add more rubber bands or use thicker ones. Does more tension always mean longer flight?
Projectile mass: Launch mini marshmallows, jumbo marshmallows, grapes, small erasers, or crumpled paper balls. Graph mass versus distance.
Lever arm length: Extend your throwing arm by taping another craft stick to it. Does a longer arm increase distance? Why or why not?
Fulcrum position: Move the base stack closer to the spoon end or closer to the opposite end. How does this change mechanical advantage?
Keep a notebook. Write down what you change, what you observe, and what you think is happening. That's how real engineers optimize their designs.
Troubleshooting Common Problems
Marshmallow flies backward: Your lever arm is positioned incorrectly. The spoon should be on the end that moves upward when released, not downward.
Catapult falls apart: Add more rubber bands or reinforce joints with tape. Make sure your base stack is tightly bound.
No power: Your rubber bands aren't stretched enough. Add tension by repositioning the diagonal brace stick or using stiffer rubber bands.
Marshmallow falls out before launch: You're pulling back too slowly. The motion needs to be quick, pull and release in one smooth motion.
Uneven launches: Make sure your fulcrum (base stack) is flat and stable. Wobble in the base creates random launch angles.
Frequently Asked Questions
Can I use something other than marshmallows?
Absolutely. Try pompoms, crumpled paper, grapes, small erasers, or even ping pong balls. Just keep projectiles lightweight, this catapult isn't designed for anything dense or heavy.
How far should it launch?
A well-built marshmallow catapult typically launches mini marshmallows 3–10 feet, depending on rubber band tension and launch angle. If you're getting less than 2 feet, add more rubber bands or pull back farther.
Why do some rubber bands work better than others?
Rubber bands vary in thickness, stretchiness, and material composition. Thicker bands store more energy but require more force to stretch. Experiment with different types to find the sweet spot between ease of use and launch power.
Is this the same kind of lever as a seesaw?
Yes: both are Class 1 levers with the fulcrum between the effort and the load. Other examples include scissors, pliers, and crowbars.
Can I make it more powerful?
You can increase power by adding more rubber bands, pulling back farther, or using a longer lever arm. Be careful with too much tension: rubber bands can snap if overstretched.
What's the best launch angle for distance?
In ideal conditions (no air resistance), 45 degrees gives maximum range. In reality, with air resistance and a lightweight projectile, slightly lower angles (30–40 degrees) often work better.
Can younger kids do this alone?
Kids 5–7 might need help wrapping rubber bands tightly and positioning the diagonal brace. Kids 8 and up can usually build it independently. Always supervise rubber band use to prevent eye injuries from snapped bands.
How does this relate to real catapults?
Ancient trebuchets used counterweights instead of rubber bands, but the lever principle is identical. Ballistas used twisted rope bundles as springs, basically giant rubber bands. Your craft stick version demonstrates the same physics that launched thousand-pound boulders at castle walls.
Taking It Further
Once you've mastered the basic design, try these challenges:
Build a target: Set up paper cups at different distances and assign point values. Play catapult basketball.
Design competition: Challenge friends or classmates to build their own catapults and compete for distance, accuracy, or style points.
Calculate energy: Use your launch distance and projectile mass to calculate the kinetic energy at launch. Compare this to the theoretical potential energy stored in your rubber bands.
Double-arm catapult: Build a second lever arm on the opposite side of the fulcrum and launch two marshmallows simultaneously.
Historical research: Learn about medieval siege weapons. How did trebuchets, mangonels, and ballistas differ? What advantages did each design offer?
This little pile of craft sticks and rubber bands isn't just a toy: it's a functional demonstration of energy storage, mechanical advantage, and projectile physics. The same principles that make your marshmallow fly powered siege weapons, launched early aircraft from carrier decks, and still drive pinball flippers today.
Build it. Launch it. Break it. Rebuild it better. That's engineering.