Is Perpetual Motion Possible?

Category: Learning cycle/Investigation

Learning Objectives:

1. Students will build a set-up which demonstrates the conversion of potential energy to kinetic energy to thermal energy.
2. Students will be able to state and explain the law of conservation of energy.
3. Students will design a plan for a perpetual motion machine and determine that such a machine, while good in theory, will not work in the real world.

Materials, equipment and/or facilities:

• marble
• foam cup
• grooved ruler
• balloon, tape
• wooden tongue depressor
• various other small, simple devices of teacher's choosing
• paper and pencil

Sequence and duration of each part of the lesson:

Exploration: 15 to 20 Minutes
Divide students into four or five groups and give each group a set of materials listed above. Make sure that all groups have the same materials with which to work. Assign the groups the task of using their materials to transfer energy in such a way that their marble moves the greatest distance when they release it from a designated starting point. Inform them that throwing the marble or pushing it along the course is not permitted. They are simply to release the marble from the starting point, and, using as many energy transfers as possible, cause it to travel as far as possible before stopping on its own.

When they feel that they have developed the best set-up, instruct them to list the energy transfers involved. After all groups have completed this task, a "play-off" may be held between groups to determine the best overall class set-up.

Concept Invention: 5 to 10 Minutes
Invite the winning group to explain for the class what energy transfers were involved in their set-up. The teacher may need to ask guiding questions such as:

"What type of energy did the marble have at the starting line?" (energy of position or potential energy).
"What type of energy did the marble have when it rolled through the set-up?" (energy of motion or kinetic energy).
"Why did the marble eventually stop?" (the kinetic energy was transferred to heat or thermal energy because of friction).

Once students understand the different forms of energy involved and that one form could be converted to another, ask them, "Was any energy gained or lost in the marble experiment?" Students should respond that no energy was added to the marble as it traveled (they were not allowed to push it along the way) and that no energy was lost (it simply changed forms from potential to kinetic to thermal).

The teacher should then introduce the law of conservation of energy, which states that energy can't be created out of nothing, nor can it be destroyed. Energy may be changed to other forms or transferred to other objects, but the total amount of energy remains the same. To further develop student understanding of this concept, the teacher may use the analogy of money exchanges. Money is constantly being exchanged between people as they purchase items, lend and borrow, make change, or even convert to different world currencies. The total amount of money in circulation, however, is kept constant and closely regulated by government officials. Similarly, energy is constantly being converted from one form to another, but the total amount of energy remains constant. Students may argue that money can be "lost" as it falls behind cushions, etc. However, such money still exists and often eventually finds its way back into circulation.

Application: 25 to 30 Minutes
Introduce students to the idea of perpetual motion machines, or machines that, once set in motion, continue to work forever with no additional energy required. One example of such a machine is a battery which powers a motor which runs and generator which recharges the battery. Assign students to return to their groups and brainstorm how they might redesign their marble set-up. Instruct them to draw a perpetual motion set-up through which the marble will travel again and again. Tell students that they need not actually build their new design, and they can use any materials they can think of. They must complete a drawing and simple explanation of the marble's path.

When all groups have a finished plan, invite each to present their design to the class. Ask other class members whether or not they believe each design will work. Having just studied the law of conservation of energy, some may respond in the affirmative, stating that no energy will be lost. However, students should be lead to understand that some energy used to power the machine will be transferred to objects outside the machine. Therefore, more energy must be added to keep the machine working. Often the energy given off by the machine is in the form of heat due to friction and this friction will eventually wear away the machine's parts causing it to break down. Therefore, there is no such thing as a perpetual motion machine.

As final discussion questions, the teacher may ask,"Is there perpetual motion in Earth Systems?" (no--for reasons already mentioned). "What are some ways energy is transferred in Earth systems?" (earthquakes, volcanoes, winds, tides, etc.)

Evaluation:
The teacher may determine whether or not students have met the objectives by observing the workings of each set-up in the opening "play-off," and listening to the group presentations of perpetual motion designs. As a final check, (perhaps a wrap-up pop quiz), students may be asked to write answers to the following:

1. State the law of conservation of energy.

2. Is a perpetual motion machine possible? Why or why not?

3. Describe three energy transfers within Earth systems.