Museum of Science and Industry

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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 an, 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.

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