Magic Rollback Can - Sick Science!

Potential and kinetic energy at work in this magical demonstration

In our long line of "magical" science, we introduce the Magic Rollback Can. The Magic Rollback Can appears to be a normal can of coffee or oats, but after you roll it along the ground a little ways and watch it come back, you'll be wondering just how it works.

Materials

Coffee or oats can
Nail or other hard pointed object
9-volt battery or object with similar weight
Rubber band
2 paperclips
Tape

Experiment
Video
Related Experiments
Reviews

Using the nail, make a hole in the middle of the bottom of your coffee or oats can. Be extra careful when using sharp objects. Also, if you are using a coffee can, be careful around the sharp metal edges that you may create when making the hole.
Poke the same kind of hole in the lid of the can.
Tape the 9-volt battery to the middle of the rubber band. Make sure both sides of the rubber band are taped to the bottom of the battery.
Push one end of your rubber band loop through the hole in the bottom of the can and secure it there by attaching one of the paperclips. Once you have it secured, tape the paperclip down.
Stretch the rubber band across the length of the can and push the other end of the rubber band loop through the hole in the lid.
Secure the rubber band with a paperclip and tape it down.
Put the lid on the can. Does the battery rub against the side of the can? If not, you're good to go. If it does, try a shorter rubber band.
Getting the set-up just right may take a bit of experimentation, but you'll get it!
Set the can on its side on a hard surface or short carpet floor and give it a roll. Once the can comes to a stop, try to contain your excitement as it begins to roll back to you!

How does it work?

The Magic Rollback Can is a great example of transfer of energy. When you roll the can, it has kinetic energy. As it slows down, the energy is transferred into potential energy within the twisted rubber band inside the can. The twisted rubber band's potential energy is then transferred back to the can in kinetic energy as it untwists.

The secret to all this energy transfer comes from the weight that you've taped to the rubber band inside the can. While the weight is being pulled down by gravity, it is also being subjected to a twisting force from the rubber band. So long as the force being exerted by gravity on the weight is greater than the twisting rubber band's force on the weight (meaning the weight never goes over the rubber band), the rubber band will continue to twist.

Once all of the kinetic energy from the rolling can has been exhausted by converting to heat (friction) or potential energy (twisted rubber band), the can stops rolling and the weighted rubber band is able to unwind. Because of the weight in the middle of the rubber band, only the ends of the loop are able to unwind and, therefore, the can begins to roll backwards.

Additional Info

If you are looking to take the Magic Rollback Can to the next level, try painting it a solid color. If you do this, observers won't be able to see the apparatus on the ends of the can. This makes the Magic Rollback Can a perfect "Black Box" tool for teachers. Show your students what the Magic Rollback Can does, and have them observe and hypothesize how the can might work.

Battery winds the rubber band

John Lesko - August 31, 2011

This user gave 4/5 stars

Nothing magical about the battery. It's just a weight. The battery remains in an uright position due to gravity. When the can is rolles the battery twists and tightens the rubber band and when you let go the rubber band untwists against the stationary battery and the can has no choice but to roll with the flow.