Rollback Can

A weighted rubber band is suspended inside a can. Rolling the can forward twists the band and stores elastic potential energy; when the can stops, the band unwinds and drives the can to roll back.

1. Remove or place the plastic lid on the can’s bottom to work on a flat surface.
2. Carefully punch a small central hole in the metal can bottom and in the lid.
3. Tape a weight across the midpoint of a rubber band so the band makes a loop with the weight centered.
4. Feed one end of the rubber band loop through the hole in the metal bottom and secure it inside with a paperclip; tape the clip so it cannot slip.
5. Stretch the band through the can and feed the other end through the hole in the lid; secure with a second paperclip and tape. The weight should hang free, centered, and not touch the can walls.
6. Set the can on its side on a smooth floor and gently roll it forward several turns so the internal band twists.
7. Watch as the can slows, stops, and then rolls backward as the band unwinds.

Comeback Can - Mad Science of London:

Rolling Back Can Demonstration - phyisfun:

📄🎞️ Magic Rollback Can - Steve Spangler: https://stevespangler.com/experiments/magic-rollback-can-2/

📄 Physics Project Idea: Rollback Can - Babble Dabble Do: https://babbledabbledo.com/physics-project-idea-rollback-can/

• Change the mass (AA battery, washers) to see how weight affects rollback distance and speed.
• Try different rubber bands (length, thickness) to explore torque and energy storage.
• Offset the internal weight slightly to investigate wobble and stability.
• Paint or wrap the can so students infer the mechanism as a black-box challenge.

• Watch for sharp metal edges when making holes.
• Keep fingers clear of stretched rubber bands to avoid snaps.
• Secure the weight well so it cannot come loose and become a projectile.
• Use appropriate supervision and tools when piercing the can.

• Where is energy stored while the can rolls forward, and how is it returned to create the rollback?
• Why must the hanging weight not flip over the rubber band for the effect to work?
• How do friction and rolling resistance influence how far the can returns?
• What changes if you increase band stiffness or the internal mass?
• How is this similar to a wind-up toy in terms of energy transfer?