Turning Forces & Moments
Year 8 🚀 Forces & Motion Apply the principle of moments: moment = force × distance.
🔩 Moments: Turning Effects of Forces
A moment (or torque) is the turning effect of a force about a pivot. The further from the pivot the force acts, the greater the turning effect.
🔩 Moment Formula
$$M = F \times d$$
$M$ = moment (Nm) · $F$ = force (N) · $d$ = perpendicular distance from pivot (m)
🔧 Example: A 20 N force applied 0.5 m from a pivot:
Moment = 20 × 0.5 = 10 Nm
Same force, 1 m away: Moment = 20 × 1.0 = 20 Nm — double the turning effect!
Moment = 20 × 0.5 = 10 Nm
Same force, 1 m away: Moment = 20 × 1.0 = 20 Nm — double the turning effect!
⚖️ Principle of Moments
For an object in equilibrium (balanced), the total clockwise moment equals the total anticlockwise moment about any pivot.
⚖️ Balanced Lever
$$\sum \text{Clockwise moments} = \sum \text{Anticlockwise moments}$$🎢 See-saw example: Child A (300 N) sits 2 m from pivot → clockwise moment = 600 Nm. Child B (200 N) must sit 3 m away → anticlockwise = 200 × 3 = 600 Nm. ✅ Balanced!
| 🔧 Type of lever | 📍 Pivot position | 🔍 Example |
|---|---|---|
| 1st class | Between effort and load | See-saw, scissors |
| 2nd class | Load between pivot and effort | Wheelbarrow, nutcracker |
| 3rd class | Effort between pivot and load | Tweezers, fishing rod |
🏋️ Centre of Mass
The centre of mass is the point where all of an object's mass appears to be concentrated. An object topples if its centre of mass moves outside its base.
Stability tips: Lower centre of mass = more stable. Wider base = more stable. That's why racing cars are wide and low!
🧘 Balancing: A ruler balanced on a finger tip is at its centre of mass (usually the midpoint). Non-uniform shapes have their centre of mass shifted towards the heavier end.
Ready to test yourself? Click the Quiz tab above to answer questions on this topic!
🔩 Moments & Balance Calculator