Lesson 2: Balanced vs Unbalanced Forces & Net Force Calculation
Exploring forces, net force calculations, motion graphs, energy, and real-world NZ physics applications.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Net force (F_net = ∑F), balanced forces (F_net = 0 N) causing constant velocity or rest, and unbalanced forces (F_net ≠ 0 N) causing acceleration or deceleration.
Calculate net force values and directions from multi-force vector diagrams and predict changes in motion.
🎥 Media Anchor & Pedagogical Scaffold
Net Force & Balanced vs Unbalanced Motion
Video Clip: Balanced and unbalanced forces — when objects move and when they stay still (Runtime: 2m 34s).
🧠 1. Before Viewing (Activate & Predict)
If a car is moving at a steady 100 km/h on a flat highway, are the forces acting on it balanced or unbalanced?
👁️ 2. During Viewing (Watch With a Job)
- Define: in your own words, what is the difference between balanced forces and unbalanced forces?
- Calculate: if a force of 15 N pushes right and 10 N pushes left, what is the net force and which direction does the object move?
- Predict: if an object is moving at constant speed, are the forces on it balanced or unbalanced? Explain why.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Demonstrate a tug-of-war on a low-friction skateboard to visualise F_net direction and acceleration.
Immediate Task: Complete Worksheet 2: Net Force Calculation Matrix & Motion Prediction Table.
⚡ Whakaoho | Do Now: Two Teams Pull the Rope. Nothing Moves.
Is there no force here, or are there forces? Two minutes: draw it, arrows and all.
Then answer the harder half: what would one team have to change to get the rope moving, and roughly by how much? "Nothing is moving" and "nothing is pushing" are very different statements.
📖 Activity 1: Add the Arrows, Then Predict the Motion
Calculate the net force (13 min). Work through the six force diagrams. For each, give Fnet with a size and a direction. Watch your signs — if right is positive, a leftward force is negative, and about half of all errors in this topic are sign errors rather than arithmetic.
Say what happens next (12 min). For each diagram state the motion: stays still, moves at constant speed, speeds up, slows down. Two of the six are traps — balanced forces acting on something that is already moving. It keeps going at the same speed. Find both.
📝 Activity 2: Year 10 Physics Worksheet & Explanation Scaffolding (20 mins)
Logbook Section 2. Record: (1) six net-force results with size and direction; (2) a motion prediction for each with a one-line reason; (3) the two balanced-but-moving cases explained in your own words. That idea is the one most often lost in the exam.
🏫 Kaiako Planning & Pedagogy Notes
Year 10 Curriculum Alignment: NZ Curriculum Phase 4 Science — Physical World strand. Builds foundational physics problem-solving skills for NCEA Level 1 Mechanics and Level 2 Physics.