Rollback Can Physics Experiment

Have you ever seen a can that when pushed away from you, comes rolling back to you? You are probably saying “There’s no such thing as a Magic Can”, right? Well we have an experiment where you take a simple can and give it boomerang powers! Roll it away and it will come right back to you. This is an easy and fun way to learn about physics with hands on experimentation.

Rollback Can DIY Toy Project

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In this short and easy physics project for kids, you are going to make a really fun DIY toy. It is a great project for school that can easily be completed in a standard classroom period. It is also a fun idea for STEM club, summer camp or homeschoolers.

A Rollback Can is also known as Comeback Can or Magic Can or Boomerang Can, and it is a fun way for kids to learn about stored energy, forces, and motion using simple materials. Did we mention budget friendly too? So quick, easy, fun and budget friendly. This is such a great STEM project idea!

Let’s make the Rollback can first and then look at the science of this magic can. We don’t want to spoil the surprise!

Rollback Can DIY Toy Instructions

First gather up all of the supplies. A lot of these you should have in the recycle bin and your Tinker Space or No-Prep STEM Teacher Toolkit (which we recommend for all educators!)

Tin Can (like a coffee can, not soup can, it should have a plastic lid)
Scissors
Permanent marker
Pencil
Metal punch, awl, or fat nail
Hammer
Crafting wire
1.5v LR14 or C-Cell battery or a 9v battery works too
Two paperclips
Assortment of medium-length, thick rubber bands (the ones that come on celery work well!)
Roll of duct tape
Ruler

Directions

Getting the can ready

Remove the lid of the can and use a ruler to find the middle of the lid. Some lids have a raised area in the middle on the underside of the lid which can make this step easier.

Use the punch or awl and a hammer to make a hole in the middle. The hole should be just large enough to pull a rubber band through. Remember, to do this on a surface where you won’t cause any damage to the table.

Turn the can upside down and use a ruler and permanent marker to find the middle point.

Use the punch or nail and the hammer to make a hole here. Once again, the hole should be just large enough to pull a rubber band through.

The can is ready!

Making the weight for the inside of the can

Get the battery, a rubber band, and the two paper clips.

Cut a piece of duct tape long enough to go all the way around the battery (about 9cm long). Now cut the length of the duct tape in half. If you leave it full width it is too wide for the battery. If you have your kids working in groups or individually, they can share the other half with one of their peers.

Place the rubber band on top of the battery with equal-sized “tails” on either end stick it down with the duct tape around the girth of the battery.

Installing the battery

Here is a visual of how things will look inside the can when we are done. Remember, if any of this is confusing, watch the video tutorial. It really helps demonstrate the process.

Using the piece of wire, roll the tip over a nail or toothpick to make a small hook to hold the rubber band.

Push the hook side into the bottom of the can and hook one end of the rubber band onto it and gently pull it back out of the hole under the can.

Slide a paper clip onto the rubber band and lay it flat over the hole.

Push the hook into the lid from the top of the lid.

Stretch the rubber band inside of the can up to the top with your other hand and hook it onto the wire hook.

Pull the rubber band through the lid and close the lid.

Slide a paper clip onto the rubber band and lay it flat over the hole.

The Magic Rollback Can is now finished and ready to WOW!

How to Use the Rollback Can

Place the can onto a flat surface and push it away from you. Did it come back to you? Of course, it did!

That’s it! So easy and so cool. Test it out by pushing harder or softer when you roll it to see what happens.

Now let’s look at how the magic happened.

The Science Behind a Rollback Can

So how does a rollback can actually work? What is this secret magic that turns a can into a boomerang? It’s all physics.

Energy Transformation

When you push the can forward, you give it kinetic energy. This is known as the energy of movement. Inside the can, the rubber band stores potential energy. This is energy waiting to be used.

Elastic Potential Energy

As the can rolls away from you, the rubber band twists and stretches. This causes the rubber band to store potential energy inside the band.

Force and Motion (which are part of Newton’s Laws of Energy)

When the can stops, the rubber band releases the stored energy, turning it back into motion (kinetic energy) to roll the can backward. The energy pushes the can backward—that’s why it rolls back!

The force in the rollback can depend on three things:

1. How tightly the rubber band is wound. The more tension on the elastic means more force. The type of elastic used will also change the force.
2. The weight inside the can. If you were to use a larger, heavier battery, the can may slow down, because its weight will cause the can to slow down, but it will have more momentum. The battery should not touch the sides of the can as it will reduce the friction needed for speed.
3. If the surface it rolls on is smooth, friction is reduced, and the can will move faster over the surface. The can will slow down because of friction between the can and the surface if the surface is rough. This is known as inertia.

The rolling stops when all stored energy is used up!

Teaching Prompts

In a classroom situation, the teacher can use the following questions to spark discussions and ideas, before starting a new challenge on increasing the speed or force of a rollback can:

How can you increase rubber band tension?
What would happen if you were to use a lighter or heavier can?
How do you choose the right weight inside?
What could you use for a weight instead of a battery?
Will using a smoother or rougher surface be better?
How can you reduce friction inside the can?

Extension Ideas

If your kids love exploring physics with this experiment, here are a few more fun, hands on ways to play with physics.