Equilibrium in Action

Year Level and Curriculum Alignment

Year Level: Year 12
Subject: Science – Physics
Curriculum: Australian Curriculum – Senior Secondary Physics (Units 3 & 4)
Strand: Physical Sciences
Topic: Forces – Static and Dynamic Equilibrium
Relevant Content Descriptions:

• ACSPH097: Analyse the motion of objects and examine the relationships between force, mass and acceleration.
• ACSPH099: Understand the concept of equilibrium and identify examples of static and dynamic equilibrium in systems.

---

Lesson Duration

Total Time: 60 minutes
Class Size: 20 students

---

Learning Intentions

By the end of the lesson, students will:

• Define and distinguish between static and dynamic equilibrium.
• Identify forces acting on objects in equilibrium.
• Apply Newton’s Laws to investigate real-world systems in equilibrium.
• Engage in collaborative problem-solving and physical demonstrations to reinforce practical understanding of forces in balance.

---

Success Criteria

Students will:

• Accurately describe the conditions for static and dynamic equilibrium.
• Successfully complete a practical task demonstrating knowledge of forces in balance.
• Correctly analyse a real-life scenario involving equilibrium and present findings to peers.

---

Resources Needed

• Whiteboard and markers
• String, pulleys, hooks, and hanging weights (for force table activity)
• Trolley, rope, mass pieces, and incline plane (for dynamic demonstration)
• Student worksheets with guided diagrams
• Laptops or tablets (optional, for collaborative modelling)
• A3 paper, coloured markers
• Access to Newton’s Laws reference sheet

---

Lesson Sequence

0–5 mins: Welcome & Set the Scene

• Introduction and brief recall of previous topics (forces, Newton’s Laws).
• Pose a probing question: “Can a skydiver be moving but still be in equilibrium?”

5–15 mins: Direct Instruction – Equilibrium Defined

Static Equilibrium

• Object at rest, net force = 0, net torque = 0.
• Examples: ladder against a wall, book on table.

Dynamic Equilibrium

• Object in constant velocity motion, net force = 0, balanced forces despite movement.
• Examples: skydiver at terminal velocity, car on highway using cruise control.

Teacher Modelling

• Use diagrams to showcase free-body forces.
• Emphasise balance of vertical & horizontal forces.

Engagement Technique: Include a bizarre example (e.g., a tightrope-walking robot) to spark curiosity and discussion.

---

15–30 mins: Practical Demonstration – Force Table & Tensions

Activity: “Tug of Three” – Equilibrium on a force table
Split class into 4 x groups of 5.

Each group:

1. Hangs weights at different angles with pulleys using string and clips.
2. Adjusts weights and angles until central ring remains static (no movement).
3. Records all force vectors (magnitude and angle).

Task:

• Use vector resolution to prove that the net force = 0.
• Draw vector triangles on worksheet.

Teacher Circulates:

• Ask leading questions like “What would happen if we increased just one mass?”
• Reinforce the condition x- and y-components must both balance to confirm equilibrium.

---

30–45 mins: Interactive Mini-Demo – Dynamic Equilibrium

Scenario-Based Challenge “Why does the trolley move at constant speed down the incline?”

Students (in pairs) experiment with:

• A trolley on a ramp with adjustable angle
• Adding weight behind the trolley with string and pulley

Instructions:

• Adjust the counterweight until the trolley glides down the ramp at steady speed.
• Measure angle, weight values, and time.

Students fill in:

• Forces acting on the trolley (gravity component, tension, friction)
• Discussion: What makes the velocity constant?

Outcome:
Students determine that net force = 0 when dynamic equilibrium is achieved. This supports understanding that equilibrium does not imply stillness.

---

45–55 mins: Class Collaboration – Poster Creation

Now working in pairs, students create an A3 poster titled:

“Equilibrium in Everyday Life”

Requirements:

• One static and one dynamic example from real life (not discussed in class).
• Diagram of forces.
• Explanation of how Newton’s Laws apply.
• Identify what would happen if equilibrium is disrupted.

Examples to prompt students:

• Suspended traffic lights (static)
• Person walking at constant pace (dynamic)

Teacher Checks-In:

• Challenge higher-order thinking: Ask students to find the moment equilibrium breaks in their scenarios.

---

55–60 mins: Share & Reflect

• Two pairs present their posters.
• Class contributes “1 big idea, 1 question” for each.

Final Reflection Prompt (written in science journals):

“How has your understanding of equilibrium changed? When have you seen equilibrium in real life without realising it?”

---

Differentiation Strategies

• For advanced learners: Ask students to calculate resultant vectors and forces mathematically.
• For students needing support: Use scaffolded worksheets with diagrams partially completed.
• Visual learners: Leverage colour-coded force arrows and real-world imagery.
• Kinesthetic learners: Fully engage with physical manipulatives in practical activities.

---

Assessment Opportunities

Formative:

• Observation during practical activities.
• Group poster work and explanation to peers.
• Responses in reflection journal.

Summative (optional):

• End-of-lesson quiz (extra task) with scenarios:
  “Is this object in static/dynamic equilibrium? Why?”

---

Teacher Reflection Guide

After the lesson, consider journaling:

• Which equilibrium examples resonated most with students?
• Were students able to distinguish clearly between static and dynamic?
• Did group tasks encourage deeper understanding?
• What could be adjusted for a future cohort?

---

Extension Opportunities

• Set a flipped classroom challenge:
  Students film themselves at home demonstrating a static or dynamic equilibrium example (e.g. balancing objects, constant-speed bike ride).

• Integrate with engineering: Design a system (e.g. mobile sculpture or balanced bridge) that relies on static equilibrium.

---

Closing Thought

Equilibrium surrounds us—in the stillness of a balanced scale or the smooth glide of a skateboarder down a hill. By unpacking these forces, students begin to see physics not just as theory, but as the silent architecture of the everyday world.