# Example of Elastic Potential Energy: The Rollback Can

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#### Updated on Dec 05, 2013

Energy is neither created nor destroyed: it is only transformed. There are many different forms of energy, including kinetic energy (the energy of motion), potential energy (energy that is considered stored because of an object’s position), mechanical energy (energy that performs work), and thermal (heat) energy. Kinetic and potential energy are often paired together in simple demonstrations, like dropping a book, jumping on a trampoline, or building cool machines like rollback cars.

### Problem

Build a rollback can and test it!

### Materials:

• Metal can with plastic lid
• Screwdriver
• 9V battery
• Rubber band
• Duct tape
• 2 Paperclips

### Procedure

1. Place the plastic lid on the bottom closed side of the metal can.
2. Have an adult use the screw driver to punch holes through the center of the plastic lid and the bottom of the can. Be careful when handling the bottom of the can! The punched hole can be very sharp.

1. Tape a rubber band to the top of the 9V battery.

1. Making sure not to scratch or cut yourself on the cut aluminum, thread one end of the rubber band through the hole in the bottom of the can.
2. Secure the rubber band on the outside of the can with a paperclip so it does not fall back through.

1. With the battery inside the can, stretch the rubber band and thread the other free end through the hole in the plastic lid.
2. Secure the rubber band on the outside of the lid with a paperclip so it does not fall back in.

1. Roll the can on a flat surface. Record your observations.

### Results

After rolling forward a short distance, the direction-changing can should roll back to you!

### Why?

The reason that the can moves backwards is very similar to the reason the rubber band racer moves forward: the transition of potential energy to kinetic energy. In the case of the rollback can, kinetic energy is transformed into an easily observable example of elastic potential energy, which is then transformed back into kinetic energy. Let’s take a look at how this works.

When we rolled the can forward, we gave the can kinetic energy. Gravity exerts a lot of force on the heavy battery, pulling it down and causing it to resist the twisting motion of the rubber band. As the can rolls forward, the stubborn, heavy battery doesn’t turn with it, so the rubber band gets all wound up. This converts the can’s kinetic energy into potential energy, and once all the kinetic energy is used up, the rubber band can untwist, pushing against the battery and rolling the can back towards you. Potential energy turns back into kinetic energy!

### Going Further

Curious about how rubber bands are able to store potential energy? Check out this project!

Want to make a simple wind-up toy out of common household materials? Check out this project!