======Rollback Can======


<WRAP right box>
**Materials: **{{$demo.materials_description}}\\
**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
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**Categories:** {{$demo.categories}}
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**Alternative titles:** Magic Rollback Can, Comeback Can
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====Summary====                   
{{$demo.summary}}



====Procedure====  
  - Remove or place the plastic lid on the can’s bottom to work on a flat surface.
  - Carefully punch a small central hole in the metal can bottom and in the lid.
  - Tape a weight across the midpoint of a rubber band so the band makes a loop with the weight centered.
  - 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.
  - 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.
  - Set the can on its side on a smooth floor and gently roll it forward several turns so the internal band twists.
  - Watch as the can slows, stops, and then rolls backward as the band unwinds.




====Links==== 
Comeback Can - Mad Science of London: 
{{youtube>m8P4d1gebqg?}}\\

Rolling Back Can Demonstration - phyisfun: 
{{youtube>Z4M08LEjcaY?}}\\

📄🎞️ 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/]]\\



====Variations====
  * 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.



====Safety Precautions====
  * 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.




====Questions to Consider====
  * 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?