Lesson 7: Rotational Equilibrium & Torque Mechanics
Demonstrating understanding of mechanical principles, motion, forces, and energy for NCEA Level 2 Physics.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Torque / moment of force (tau = Fd where d is perpendicular distance to pivot), conditions for static equilibrium (sum F = 0 AND sum tau = 0), centre of mass.
Calculate clockwise and anticlockwise torques around multiple pivot points, solve seesaw and bridge support reaction forces, and determine object stability.
🎥 Media Anchor & Pedagogical Scaffold
Torque & Rotational Static Equilibrium
Video Clip: Torque — rotational force, moment arm, and the torque equation | Khan Academy Physics (Runtime: 9m 40s).
🧠 1. Before Viewing (Activate & Predict)
Why is it much easier to open a heavy door by pushing at the outer handle rather than near the hinge?
👁️ 2. During Viewing (Watch With a Job)
- Define torque: write the equation τ = rF sin θ. What is the moment arm, and why does angle matter?
- Clockwise vs anticlockwise: for an object in rotational equilibrium, what must be true about the clockwise and anticlockwise torques?
- Apply: a 10 N force is applied 0.3 m from a pivot point at 90°. Calculate the torque. How would doubling the moment arm change the result?
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Demonstrate a metre ruler balanced on a pivot with unequal masses to verify the principle of moments.
Immediate Task: Complete Section 7 of your Mechanics Portfolio: Torque Equilibrium Problems & Centre of Mass Stability Analysis.
⚡ Whakaoho | Do Now: Physics Mechanics Recall (10 mins)
Why is the handle at the far edge of the door? Doors would be cheaper to make with the handle in the middle.
Two minutes: push a door open near the hinge, then at the handle, and notice the difference in effort. State what quantity you changed, and write the relationship between force, distance and turning effect before it is given to you.
📖 Activity 1: Mechanical Investigation & Vector Problem Solving (25 mins)
Balance a beam (15 min). For three loaded beams, calculate clockwise and anticlockwise torques about the pivot and solve for the unknown. Both equilibrium conditions must be satisfied — ΣF = 0 and Στ = 0 — and a common error is checking only one.
Perpendicular distance (10 min). Two of the problems apply force at an angle. Show why the perpendicular distance, not the length of the beam, is what goes into τ = Fd, and calculate the difference it makes.
📝 Activity 2: Level 2 Physics Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Portfolio — Section 7. Submit: (1) three beam problems with both equilibrium conditions shown; (2) two angled-force torques with the perpendicular distance derived; (3) a paragraph on door handle placement using torque.
🏫 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.