Beacon Lesson Plan Library

A Mysterious Roll-Back Can

Louise Kent


Amaze students with a can that rolls away a few feet, mysteriously stops, hesitates, and then rolls back to where it started. Then introduce Newton and his Second Law.


The student knows that energy cannot be created or destroyed, but only changed from one form to another.


-Empty Cheeto can or a one-pound coffee can with a plastic top
-Heavy duty rubber band or ¼-elastic band (that would be used in sewing)
-Fishing weight 2 ounces
-10 penny nail
-Crayons and construction paper (optional)
-Glue or tape


1. Gather materials.
2. Construct a can as an example.


Note to the teacher:
1.This is a lesson to introduce Newton's Second Law and Conservation of Momentum.

2. Make the can (instructions are below) and show it working to the students. Roll it out and allow it to roll back to you. Do not tell the students why or what is happening. Ask then to spectulate what is happening and why. Let the students guess at every thing. Nothing is incorrect yet.
Begin explaining Newton's Second Law and the Law of Conservation of Momentum.

3. The secret is in the weight and rubber band inside the can. As you roll the can away from you, the rubber band winds up against the weight. The weight (because it is heavy) will remain suspended vertically as the can rolls and the rubber band twists up. This twisting stores the potential energy that is then released as kinetic energy as the can rolls back. This storing of energy can also be considered conservation of energy.

Building the Come Back Can

1.Take a coffee can, Cheeto can or any tin can four or five inches in diameter with a replaceable plastic top. (It works better if both ends of the can have been cut out and two replaceable plastic tops have been substituted, one on each end. The fact that there are two equal tops makes the can roll evenly in a straight line. If only one plastic end is used, the diameter of one end is larger than the other end and the path of the can becomes curved.) For best results roll the can on a plastic tile or concrete floor. The can will not move well on carpets or rough, bumpy surfaces.
2. Two holes about one inch apart are punched into the top and bottom of the can using the nail.
3. Cut a rubber band apart or cut some sewing elastic and tie it in place (as shown in the drawing in the Associated Files) with the weight suspended as shown. Do not use a really fat rubber band as it will not store up much energy and will not work properly. If you can’t find the correct size rubber band, try doubling or tying two or more smaller ones together.
4. When the student first uses the can, roll the can in one direction such as clockwise for a few turns. This will allow the can to store energy in the beginning. Roll the can on the floor. The can will pause and then return to its original position.
Students may decorate the can with construction paper and crayons.

After students have completed making their cans allow them to play with them and ask the students to describe what is happening. The answers will be much different from the beginning.

EXPLANATION: The secret is in the rubber band inside the can. The rubber band passes through the weight and the weight hangs below the axis, so that the center of mass is below the axis. The figure is therefore always erect because of the turning moment of its center of mass. The rubber bands winds up as the can is rolled. This twist stores elastic energy in the rubber band. The can is brought to rest when the original energy is spent in friction and in the act of twisting the rubber band. The elastically stored energy in the rubber band provides a torque, which causes the can to roll back. Please understand that the rubber band along the axle of the pair wheels makes a connection at the hubs in such a manner as to permit a torque to arise from the twist. Torque is a force that produces rotation. The rate of change of momentum with respect to the table will be proportional to the impressed force.

Diagram is located in Associated files.


The teacher will make the initial can and cover it so the students can not see what the mysterious can is made of. The teacher will roll the can in his/her hands as he/ she is talking (like a magician distracts people while he is setting up the act). Then the teacher will gently roll out the can on a flat surface slowly. The can will roll along, come to a stop, and then roll back to the teacher. The teacher should act surprised when the can returns on its own power. Look up at the students and ask with BIG eyes, "What happened?"
Then the teacher can start the discussion about Newton's Second Law and the Law of Conservation of Momentum.
Have the students take notes while the teacher is explaining the toy to the students.

The students will answer the following questions to indicate understanding. The questions can be answered individually or in a whole group setting. Students who do not grasp the concept about energy changing and storing will need further instruction. This is not intended to be a summative assessment.

Elementary students: To illustrate in a single toy:
1. The concepts of elastic energy
2. The center of mass
3. Potential energy
4. Kinetic energy
5. Newton's Second Law

Questions (for possibly third grade and above)

(Note: I also have purchased a "Sesame Street" Big Bird Come Back Toy to use as a demonstrator. There are a lot of toys that are first designed to play with and then can be studied for the Physics that goes into making them work.)


1. What is Newton’s second law of motion?
2. Where is the center of the toy?
3. How far did your can roll?
4. How many turns did you wind up your toy?
5. Did your toy roll back all the way?
6. Can you make a bigger toy and what will it do?
7. Can you compare your can to other classmates’ cans and how far did they roll?
8. Were all the cans the same size?
9. Did all the cans roll the same distance?
10. What made this can roll?

Intermediate or secondary students: To illustrate in a single toy
1. The concept of elastic energy
2. Torque
3. Different kinds of friction
4. Newton’s Second law applied to rotational motion.
5. Conservation of momentum
6. Potential energy
7. Kinetic Energy
8. Center of mass


1. What is Newton’s Second Law?
2. What is friction?
3. Where is the center of mass?
4. Is there acceleration when all the forces are in equilibrium?
5. What does the twist in the elastic band store?
6. What does this connection of the axle and wheel permit?
7. What law is central to the problem of the toy?
8. Will a perfectly elastic body keep its shape and when will it lose its shape?
9. What is momentum?
10. What is stress?


1. What conditions must be met for rotation to ensue and not just translation?
2. If energy is stored excessively in the twisted rubber band, the wheels will spin after the can comes to a rest at the end of the run, and after spinning briefly, they may take hold. Why does this take place? It might happen on a car. It might also happen that the can spins violently before the motion of the wheels engage.


ADVANCED PHYSICS STUDENTS: These students may substitute a 55-gallon metal drum, a 150-pound anvil and an inner tube. (Suggest safety glasses be worn)
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