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Come-Back Can

Students make a tin can toy with a secret mechanism. When you roll it away, it comes rolling back!

Adapted from GEM this coffee tin contains a simple elastic powered motor. The "secret" to the Come-Back Can is in the weight that hangs from the rubber band. As long as the weight hangs down as the tin rolls in one direction (and doesn't flip) the kinetic energy of the rolling can will be stored as potential energy in the tightly twisted rubber band (some will be transferred to friction (heat) and sound). Once the band has reached its limit, the stored energy is released and the Come-Back Can moves backwards towards its starting point.

Objectives

  • Build a model that stores potential energy and releases kinetic energy.
    Explain that energy is not created or destroyed, but rather converted from one form to another.

Materials

  • Coffee tin (or plastic container)
    A drill
    Two elastic bands
    Paperclip
    Tape
    Cotton thread or string
    Small weight (e.g. washer)

Key Questions

  • How is potential energy stored in the can? What kind of energy is this converted to? What happens when the elastic band is allowed to unwind?

What To Do

  1. Drill or punch a small hole in both ends of the tin, and thread the first elastic band through the hole at the base. Secure a paperclip to the band outside of the can to prevent the elastic from slipping through the hole. Use tape to secure completely.
  2. Thread the weight or washer onto a piece of string, and use that string to tie together the two elastic bands. This should result in the string holding the weight and also joining the free end of the elastic you secured to the tin in step 1 to the second elastic band.
  3. Take the other end of the second elastic band up through the hole in the lid, and use a paperclip to prevent from slipping through the hole. Close the lid and secure the paperclip with tape.
  4. Roll the tin away from you: This should “wind up” the elastic, causing the can to roll back again. This can be quite mystifying to those seeing it for the first time. 


Tip: Make sure that the weight hangs down below the point where it is tied. If it flips over as you roll the can away from you, the elastic won’t wind up, and your can won’t come back!

Vocabulary:

  • Potential Energy: Stored energy in an object due to its position (i.e. in relation to the ground) or condition (i.e. stretched or compressed in a spring or elastic band).
  • Kinetic Energy: The energy of a moving object. Potential energy is converted into kinetic energy when an object is acted upon by a force.

Extensions

  • Try varying the size of the weight or the type of elastic band, and see how that changes the speed or distance the Come-Back Can rolls. You can wind up your Come-Back Can by shaking the can in a circular fashion. See if it works! What happens when you take a wound-up Come-Back Can and stand it upright?

Other Resources

Science World Resources | Full Unit | Elastic Energy