Lesson Conservation of Energy:
Pushing It Off a Cliff

Learning Objectives

After this lesson, students should be able to:

• Describe the types of mechanical energy.
• List the various forms of potential energy.
• Apply the conservation of energy to problems strictly between gravitational potential energy and kinetic energy.
• Explain why mechanical energy is not always conserved.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Pre-Req Knowledge

A working understanding of Newton's laws and flow charting. A firm understanding of how to use Microsoft Excel® or another spreadsheet program before attempting the dry lab (however, this can also be incorporated into the explanation of the lab).

Introduction/Motivation

Now that we have come up with a few ideas about the hybrid cars, we know that we need to know more about energy. So today we will learn about the two most basic types of energy and how they can be equated to each other. So, let's talk about energy (continue with lecture information provided in the Teacher Background section).

Lesson Background and Concepts for Teachers

Content Information

Pedagogically, make the first logical step in the conservation of energy between just two forms of mechanical energy. Typically, do this between kinetic energy, which is the energy of motion (KE = ½ mv²), and gravitational potential energy, which is the energy of position (GPE = mgh). However, consider using other forms of energy (that is, thermal, chemical, nuclear, electric, electromagnetic, elastic potential energy and energy carried by longitudinal waves [typically in the form of sound waves or seismic waves]).

The conservation of energy states that the total energy at any two points in time for a close system. For basic purposes, the assumption that we always deal with a closed system can be taken.

E₁ = E₂ (1)

Here, we assume that the only forms of energy that can be dealt with are kinetic and gravitational potential energies.

KE₁ + GPE₁ = KE₂ + GPE₂ (2)

Expanding,

½ mv₁ ² + mgh₁ = ½ mv ₂ ² + mgh₂

Reducing,

½ v₁ ² + gh₁ = ½ v₂ ² + gh₂ (3)

Therefore, we see that (3) is true regardless of the mass of the object. To start out, problems should be solely KE at one point and GPE at the other. This also requires careful placement of your ground level. So (3) can reduce to:

gh₁ = ½ v₂ ² (4)

Only after a few problems, should the case in which we have to include other terms to have (3) be true should be attempted. Just before the second associated activity Energy on a Roller Coaster should the concept of adding other forms of energy be discussed and the law of Conservation of Energy always be expressed as in (1).

Lecture Information

Write the word "energy" on the classroom board. List some of the reasons (from the last lesson) that relate to conservation of energy, and ask: What are some of the forms of energy? Expect students to give answers such as "solar' or "wind," but steer them into the actual names of the forms of energy. Flow chart these responses on the board (which also serves as a lead-in for a later lesson). Circle kinetic and gravitational potential energy, and tell them these two are our focus first.

Choose two student volunteers (pick two who are friends) and pick two lightweight objects from the classroom such as a sponge and a plastic water bottle. Ask the class: Which object will transfer more energy (hurt more) if thrown at a person at the same speed? Listen to the class answers and have the volunteers throw/receive the objects to confirm (this is why friends are chosen). Write on the board that KE a mass. Then, ask the class: Which transfers more kinetic energy (hurts more), an object thrown at low speed or the same object at a higher speed? The class answers and the volunteers test and confirm (again, it helps if they are friends in order to avoid any serious throwing). Then place velocity into the equation to have "KE a mv." Next, explain that KE is proportional to the square of velocity and that a constant is associated with KE. Then place the ½ and the square into the equation and replace "a" with "=" to have "KE = ½ mv²."

Next, determine the equation for gravitational potential energy. Pick two objects from around the room, but this time make sure they have a larger difference in mass (for example, a sponge and the CRC Handbook of Chemistry and Physics). Remind students about the storage of mechanical energy. Ask the class: Which object has more energy stored in it if held at the same height. Expect many to answer correctly, and some to not be sure. Explain that we can tell from dropping them and seeing which transfers the most energy to the floor. One way to determine which transfers the most energy is by the loudness of the impact. Expect them to immediately determine the answer, but demonstrate anyway. Then, write on the board, "GPE a m." Ask: Which one has more stored energy, a book at 1 m from the floor or 2 m from the floor? Demonstrate this. Then include height in the equation for "GPE a mh." Remind students that the attraction of the object to the ground is also determined by something else. Wait until students determine that it must be gravity. Then include g and substitute "=" for "a" for "GPE = mgh.

For further background on the physcis presented in this lesson, select the lecture 11 video at: https://archive.org/details/MITclassical_mech. Optionally, you could show students clips from this video, or if you feel appropriate, show students the entire lecture.

At this point, have students conduct the Energy Skate Park associated activity. Assign finishing the virtual lab as homework.

Then work on the classroom board several problems from the text on the conservation of energy between these two forms of energy. Inform students about the real-life presence of friction, as well as what friction does. Next, have students conduct the Energy on a Roller Coaster associated activity.

Associated Activities

• Energy Skate Park - Using an online simulation set at a skate park, students make graph predictions before using the virtual laboratory to create graphs of energy vs. time under different conditions. Students' comprehension of potential and kinetic energy and conservation of energy is strengthened as they predict behavior based on their understanding of these energy concepts and experimentation with the simulation.
• Energy on a Roller Coaster - Students use a roller coaster track to collect position data. They calculate velocity and energy data, and then relate the conversion of potential and kinetic energy to the conversion of energy used in a hybrid car, learning about energy lost to friction.

Vocabulary/Definitions

conservation of energy: The total amount of energy within a closed system is equal at any two points in time.

friction: Transfer of energy from mechanical energy into other forms.

gravitational potential energy: Energy stored due to position in the vertical dimension.

kinetic energy: Energy of motion.

potential energy: Energy that is stored.

Assessment

Informal Assessment: While doing problems with the class, make sure to only do the first problem for them. Then solve another with help from the class. Have students individually solve all additional problems, with individual help from the teacher and other students who have had their work checked by the teacher.

Lab Reports: Evaluate student lab reports from the virtual lab and dry lab. Have the reports be in a format that the teacher is comfortable with, but that also allows for the checking of the understanding of the concepts by students.

Quiz: At lesson end and after both associated activities have been conducted, administer the Conservation of Energy Quiz. Review student answers to gauge their depth of comprehension.

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were developed under National Science Foundation RET grant nos. 0338092 and 0742871. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.