Unit Plan 24 (Grade 8 Science): Energy in Mechanical Systems

Focus: Apply energy models (graphs, bar models, system diagrams) to mechanical devices such as springs, ramps, and pendulums, showing how kinetic and potential energy change and relate to mass, speed, and position.

Grade Level: 8

Subject Area: Science (Physical Science • Energy • Forces & Motion)

Total Unit Duration: 5 sessions (one week), 50–60 minutes per session

I. Introduction

Students bring together earlier ideas about kinetic energy, potential energy, and energy transfer by analyzing familiar mechanical systems: carts on ramps, masses on springs, and pendulums swinging. They observe and measure how speed and position change over time and use graphs and energy bar models to tell “energy stories” for these systems. By examining how kinetic energy depends on mass and speed and how potential energy depends on mass, height, and separation, students develop and interpret models that show energy moving between kinetic and potential forms while the total energy of the system remains consistent (in idealized cases). By the end of the week, they can explain how springs, ramps, and pendulums store and release energy using quantitative patterns and qualitative models.

Essential Questions

• How does kinetic energy change as an object’s mass or speed changes in mechanical systems like ramps and swings?
• How does stored potential energy depend on mass, height, and distance/stretch in systems like ramps, springs, and pendulums?
• How can we use graphs and energy bar models to describe energy changes and transfers in a mechanical system?
• In what ways can mechanical devices convert potential energy to kinetic energy and back again?
• How do energy models help engineers design safer and more efficient mechanical systems (rides, tools, devices)?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Describe qualitatively how kinetic energy depends on an object’s mass and speed, and predict which of two objects in a system has more kinetic energy.
2. Construct and interpret graphs that show the relationship between kinetic energy and mass (at constant speed) and between kinetic energy and speed (at constant mass) for simple mechanical systems (e.g., carts on ramps).
3. Develop models for gravitational potential energy in ramps and pendulums that show how stored energy increases with mass and height above a reference level.
4. Develop models for elastic potential energy in springs that show how stored energy depends on stretch or compression (distance between objects).
5. Use energy bar models and system diagrams to describe how energy in mechanical systems can shift between kinetic and potential forms during motion (e.g., rolling down/up a ramp, swinging pendulum, oscillating spring).
6. Use graphs and models together to explain and compare energy changes in at least two different mechanical systems (e.g., ramp vs. pendulum).

Standards Alignment — 8th Grade (NGSS-based custom)

• MS-PS3-1 — Construct and interpret graphical displays of data to describe the relationships between kinetic energy and mass or speed.
  • In this unit, students collect or use data from ramp and cart setups and create graphs showing how kinetic energy changes when mass or speed changes.
• MS-PS3-2 — Develop a model to describe how stored potential energy depends on mass, height, and distance between objects.
  • In this unit, students model gravitational potential energy in ramps/pendulums and elastic potential energy in springs, highlighting dependence on mass, height, and stretch.

Success Criteria — Student Language

• I can look at a motion situation and say which object has more kinetic energy by thinking about mass and speed.
• I can make and read graphs that show how kinetic energy changes with mass or speed in a mechanical system.
• I can use models to show that gravitational potential energy increases when mass or height increases.
• I can use models to show that elastic potential energy increases as a spring is stretched or compressed more.
• I can draw energy bar models for a ramp, spring, or pendulum that show energy shifting between kinetic and potential forms as the system moves.