Lesson 2: Newton's Laws & Free-Body Force Diagrams
Demonstrating understanding of mechanical principles, motion, forces, and energy for NCEA Level 2 Physics.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Newton's First Law (inertia/balanced forces), Second Law (F_net = ma), and Third Law (action-reaction pairs); normal force, friction, tension, and gravitational weight (F_g = mg).
Construct accurate free-body force diagrams, calculate net force (F_net) vectors on inclined planes and connected bodies, and derive acceleration.
🎥 Media Anchor & Pedagogical Scaffold
Newton's Laws & Free-Body Force Diagrams
Video Clip: Types of forces and free body diagrams — Khan Academy Physics (Runtime: 8m 03s).
🧠 1. Before Viewing (Activate & Predict)
How do balanced vs unbalanced forces govern whether an object accelerates or remains in uniform motion?
👁️ 2. During Viewing (Watch With a Job)
- List: name FOUR types of forces the video identifies. For each, state whether it is a contact or non-contact force.
- Sketch: draw a free body diagram for a book sitting on a table — label all forces with arrows showing direction and approximate size.
- Apply Newton's 1st Law: a car travels at 100 km/h on a motorway. Are the forces on it balanced or unbalanced? Explain what this tells you about its acceleration.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Emphasise that Newton's Third Law action-reaction pairs NEVER act on the same object (common NCEA exam trap).
Immediate Task: Complete Section 2 of your Mechanics Portfolio: Free-Body Force Vector Diagrams & Inclined Plane Acceleration.
⚡ Whakaoho | Do Now: Physics Mechanics Recall (10 mins)
Nothing throws you forward in a crash. The car stops. You keep going. People describe a force flinging them at the windscreen; there isn't one.
Two minutes: state which law explains what actually happens to your body, and then name the force that genuinely does act on you — the one from the seatbelt, and which direction it points. Getting this the wrong way round is the most common Level 2 error.
📖 Activity 1: Mechanical Investigation & Vector Problem Solving (25 mins)
Draw the forces that exist (15 min). Build free-body diagrams for four situations, including a box on a slope and a lift accelerating upward. Rule: every arrow must be a real interaction with a named other object. If you cannot name what is pushing, the arrow does not go on — this alone removes most invented 'centrifugal' and 'forward' forces.
Find the pair (10 min). For three scenarios, identify the Third Law action-reaction pair. The two forces must act on different objects. Then explain why an action-reaction pair never cancels out, which is the trap in almost every exam question on this.
📝 Activity 2: Level 2 Physics Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Portfolio — Section 2. Submit: (1) four free-body diagrams with every force attributed to a named object; (2) three action-reaction pairs identified with the object each acts on; (3) a paragraph explaining the crash using inertia and the seatbelt force.
🏫 Kaiako Planning & Pedagogy Notes
NCEA Level 2 Alignment: Direct preparation for Level 2 Physics (Demonstrate understanding of mechanics). Emphasise linking mathematical working directly to physical concept explanations for Merit/Excellence grades.