Unit Plan 11 (Grade 8 Science): Newton’s Third Law in Action

Focus: Apply Newton’s Third Law to design a solution that uses equal and opposite force pairs to move or control the motion of two interacting objects.

Grade Level: 8

Subject Area: Science (Physical Science • Forces & Motion • Engineering Design)

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

I. Introduction

Students extend their understanding of forces and motion by focusing on Newton’s Third Law: for every action, there is an equal and opposite reaction. They explore real-world interactions—pushing off a wall, two carts colliding, rockets, and balloon-powered motion—to identify force pairs and see how they act on different objects. Building on these ideas, students take on a design challenge: create a simple device or system (e.g., balloon cart, push-off launcher, “rocket” on wheels) that uses action–reaction force pairs to solve a motion-related problem, all within clearly defined criteria and constraints.

Essential Questions

• What does Newton’s Third Law mean in terms of forces on interacting objects?
• How can we identify action–reaction force pairs in everyday situations like walking, jumping, or rocket motion?
• Why do objects with different masses respond differently even though the forces in an action–reaction pair are equal in size and opposite in direction?
• How can we design a solution that takes advantage of equal and opposite forces to produce motion or control it?
• Why are criteria and constraints important when designing a Newton’s Third Law demonstration or device?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. State Newton’s Third Law in their own words and identify action–reaction force pairs in simple interactions (e.g., foot & floor, hand & wall, cart & cart).
2. Draw force diagrams for each object in an interaction, showing equal and opposite forces acting on different objects.
3. Explain why equal and opposite force pairs can result in different changes in motion when the objects have different masses.
4. Define a design problem involving the motion of two interacting objects (e.g., a cart and expelled air, two colliding carts) and specify clear criteria and constraints (MS-ETS1-1).
5. Design and build a prototype device that uses Newton’s Third Law to produce motion or accomplish a simple task (e.g., move a cart a certain distance, knock down a target).
6. Test the prototype, collect qualitative or quantitative data about its motion, and revise the design based on how well it meets the criteria.
7. Use diagrams, data, and written/oral explanations to show how their solution demonstrates equal and opposite force pairs and how this relates to the motion of the interacting objects (MS-PS2-1).

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

• MS-PS2-1 — Apply Newton’s Third Law to design a solution to a problem involving the motion of two interacting objects.
• MS-ETS1-1 — Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution.

Success Criteria — Student Language

• I can find and label action–reaction force pairs in a situation and show that they are equal in size and opposite in direction.
• I can draw force diagrams for each object in an interaction, not just one.
• I can describe a design problem that involves two interacting objects and list clear criteria and constraints.
• I can build and test a device that uses Newton’s Third Law to move or affect the motion of an object.
• I can explain, using force pairs, diagrams, and data, how my design works because of equal and opposite forces.