Unit Plan 3 (Grade 3 Science): Balanced & Unbalanced Forces

Focus: Explain how balanced and unbalanced forces combine to change an object’s motion (start, stop, speed up, slow down, change direction).

Grade Level: 3

Subject Area: Science (Physical Science)

Total Unit Duration: 5 sessions (one week), 45–60 minutes per session

I. Introduction

In this unit, students deepen their understanding of forces by investigating how pushes and pulls can add together or cancel out. They explore real-world examples (tug-of-war, sliding books, rolling balls) to see when forces are balanced (no change in motion) and when they are unbalanced (motion changes). Through hands-on investigations, diagrams, and simple data tables, students learn to describe and explain motion changes using the idea of combined forces.

Essential Questions

• What is the difference between balanced and unbalanced forces?
• How do multiple forces acting on an object combine to affect how it moves?
• How can we use observations and data to decide if forces on an object are balanced or unbalanced?
• Why is understanding forces and motion important in everyday life (sports, safety, transportation)?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Describe balanced forces as forces that are equal in strength and opposite in direction, causing no change in an object’s motion.
2. Describe unbalanced forces as forces that are not equal, causing an object to start moving, stop, speed up, slow down, or change direction.
3. Plan and conduct simple investigations (e.g., tug-of-war, rolling objects, sliding objects) to observe how motion changes when forces are combined.
4. Use force arrows/diagrams to represent forces acting on an object and explain whether they are balanced or unbalanced.
5. Use evidence from observations or data tables to explain how changes in force affect motion, in student-friendly language.

Standards Alignment — 3rd Grade (NGSS-Aligned)

• 3-PS2-1 — Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
  • Example: Compare how a cart moves when pushed from one side vs. from both sides with equal or unequal forces.

Success Criteria — Student Language

• I can tell the difference between balanced and unbalanced forces using examples.
• I can describe how an object’s motion changes when forces are unbalanced (starts, stops, speeds up, slows down, or turns).
• I can plan and carry out a simple test to see what happens when we change the forces on an object.
• I can draw force arrows to show the direction and relative strength of pushes and pulls on an object.
• I can use observations or data to explain why an object’s motion changed or stayed the same.