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Hands-On
Science

physics

Comeback Can

Surprise your friends by making a can that rolls back to you when you tell it to!

Materials

  • Empty can with a lid (like a coffee, oatmeal or paint can)
  • Twist tie or short pipe cleaner
  • Hex nut
  • Large rubber band
  • Scissors

Directions

  1. Punch two holes in the lid and the bottom of the can.
  2. Cut the rubber band once so it is one long strip. Thread the rubber band through the holes in the bottom of the can so that both ends are inside the can.
  3. Wrap the pipe cleaner (or twist tie) around the hex nut. You should have two "bunny ears" of equal length sticking up when you're done.
  4. As you hold the lid away from the can (a partner can help), wrap each "bunny ear" around one of the rubber bands that runs through the inside of the can. The hex nut should hang from the middle of the rubber bands.
  5. Place the can on the floor and gently roll it away from you. Watch what happens!

If you time it just right, you can figure out when the can will start to roll back to you. Then you can "tell" the can to come back, so that it looks like the can is doing what you tell it to do!

What's happening?

To understand how the comeback can works, you have to understand energy. Energy comes in many forms. One form of energy is motion, called kinetic energy. Another form is stored, or potential energy. The comeback can uses both forms.

When you push the can, you give it kinetic energy and it moves away from you. The hex nut holds one length of rubber band still while the rolling can causes the other rubber band to twist around it. The can rolls until the rubber band is completely twisted. This is when kinetic energy becomes potential energy - the can is not moving, but it has the ability to do so. As the rubber band unwinds, the potential energy again becomes kinetic energy and the can rolls back to you.