Unit Plan 13 (Grade 8 Science): Balanced & Unbalanced Forces

Focus: Investigate how net force and mass determine changes in motion, using force diagrams, motion data, and student-designed investigations.

Grade Level: 8

Subject Area: Science (Physical Science • Forces & Motion • Scientific Inquiry)

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

I. Introduction

Students revisit and deepen their understanding of balanced and unbalanced forces by focusing explicitly on net force and mass as the key factors that determine changes in motion. They connect intuitive ideas (“harder push,” “heavier object”) to more structured models: force diagrams, vector addition of forces, and motion evidence (speed, direction changes). Through guided experiences and a culminating investigation they plan themselves, students gather evidence that the change in an object’s motion (acceleration) depends on the sum of the forces acting on it and its mass, in alignment with MS-PS2-2.

Essential Questions

• What does it mean for forces on an object to be balanced or unbalanced, and how does this relate to net force?
• How can we use force diagrams to determine the net force on an object?
• How do net force and mass together determine how an object’s motion changes (speeds up, slows down, changes direction)?
• What makes an investigation about forces and motion good enough to provide convincing evidence?
• How can evidence from our own experiments support the idea that the change in motion depends on the sum of the forces and the object’s mass?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Define net force as the vector sum of all individual forces acting on an object and distinguish between balanced and unbalanced force situations.
2. Draw and interpret force diagrams in one dimension (and simple two-dimensional cases) to determine whether the net force is zero or non-zero.
3. Predict how changes in net force and/or mass will affect the change in motion (acceleration) of an object in common scenarios.
4. Plan an investigation that systematically changes either the net force or the mass (or both) while measuring changes in motion (speed/direction), aligning with MS-PS2-2.
5. Collect and analyze data (qualitative and/or quantitative) to identify patterns between net force, mass, and changes in motion.
6. Use data, force diagrams, and scientific reasoning to explain and support the claim that the change in an object’s motion depends on the sum of the forces and its mass.

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 design and conduct investigations with carts, masses, and applied forces, analyze motion data, and use force diagrams to interpret net force and mass effects on motion.

Success Criteria — Student Language

• I can look at all the forces on an object and figure out the net force (and whether forces are balanced or unbalanced).
• I can draw a force diagram that shows all the main forces on an object and use it to decide how its motion will change.
• I can plan and carry out an investigation where I change the net force or mass, measure motion, and look for patterns.
• I can use data, graphs, and force diagrams to explain how net force and mass together affect how an object speeds up, slows down, or changes direction.
• I can state and support the idea that the change in motion depends on the sum of the forces and the object’s mass, not on one force alone.