Unit Plan 17 (Grade 8 Science): Force Models—Graphs & Data

Focus: Interpret motion graphs and connect them to Newtonian models of net force, mass, and kinetic energy, using data from investigations to describe and explain motion.

Grade Level: 8

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

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

I. Introduction

Students shift from “watching things move” to modeling motion with graphs and data. They learn to read and create position–time and speed–time graphs, tell the “story” of an object’s motion from a graph, and connect these stories to Newtonian ideas about net force and mass. Building on previous forces units, students see that changes in motion (curved graphs, changing slope) signal unbalanced forces, while straight-line patterns signal more constant motion. Toward the end of the week, they begin to explore kinetic energy and how it depends on mass and speed, using simple graphical displays as preparation for MS-PS3-1.

Essential Questions

• How can graphs (position–time and speed–time) help us describe and explain how an object moves?
• What features of a graph tell us whether an object’s motion is speeding up, slowing down, or staying constant?
• How do net force and mass relate to the changes in motion that we see in graphs?
• What patterns in data show relationships between kinetic energy and mass or speed?
• Why are graphs and data powerful tools for building and testing models of forces and motion?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Interpret and sketch position–time and speed–time graphs for simple motion scenarios (rest, constant speed, speeding up, slowing down).
2. Connect features of motion graphs (slope, curvature, flat sections) to changes in motion and to whether forces are likely balanced or unbalanced.
3. Use data from a simple motion investigation (e.g., cart on ramp with changing mass or net force) to construct and interpret graphical displays.
4. Explain qualitatively how the change in an object’s motion depends on the sum of the forces and its mass, using graph evidence (MS-PS2-2).
5. Construct and interpret graphs that show how kinetic energy changes with mass and with speed, recognizing that energy grows faster with speed (prep for MS-PS3-1).
6. Write a short explanation that links graph features, force models, and kinetic energy concepts to describe a motion situation.

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

• MS-PS2-2 — Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and its mass.
  • In this unit, students use or extend a simple motion investigation and represent the results with graphs, interpreting how changes in net force/mass affect motion.
• MS-PS3-1 (prep focus) — Construct and interpret graphical displays of data to describe the relationships between kinetic energy and mass or speed.
  • This unit introduces kinetic energy graphs qualitatively (energy vs. mass, energy vs. speed) to build readiness for deeper work in later units.

Success Criteria — Student Language

• I can read and draw basic position–time and speed–time graphs and tell the motion story they represent.
• I can look at a graph and say whether motion is constant, speeding up, or slowing down, and connect that to balanced or unbalanced forces.
• I can use data from an investigation to make a graph and explain how net force or mass changed the motion.
• I can interpret graphs showing how kinetic energy changes with mass or speed and describe the pattern in words.
• I can write an explanation that uses graphs, force ideas, and kinetic energy together to describe what is happening in a motion situation.