O Level PhysicsO Level Physics - Forces & Moments

⚙️ Mastering the 'Moment' of Truth: Essential Calculations for Turning Forces

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10/19/2025

Do you panic when you see a see-saw problem on your O Level Physics exam? You’re not alone! The topic of **Moments**, or the **Turning Effect of a Force**, is crucial and frequently tested, often distinguishing between an 'A' and a 'B' grade.

The Golden Rule: Moment = Force ×\times Perpendicular Distance

A moment is the measure of a force’s ability to cause an object to rotate about a pivot (fulcrum). The formula is simple:

Moment=Force×Perpendicular Distance\text{Moment} = \text{Force} \times \text{Perpendicular Distance}

The most critical mistake students make is misidentifying the **perpendicular distance**—the shortest distance from the pivot to the line of action of the force. Get this wrong, and your whole calculation is off!

The Principle of Moments

For an object to be in **equilibrium** (balanced), the sum of the **clockwise moments** about the pivot must be equal to the sum of the **anticlockwise moments**. This is known as the **Principle of Moments**.

∑Clockwise Moments=∑Anticlockwise Moments\sum \text{Clockwise Moments} = \sum \text{Anticlockwise Moments}

Your Exam Edge: Solved Example

**Problem:** A 20 N20 \text{ N} weight is placed 50 cm50 \text{ cm} from the pivot. What force must be applied 20 cm20 \text{ cm} on the opposite side to balance the system?

  • **Clockwise Moment:** 20 N×0.5 m=10 Nm20 \text{ N} \times 0.5 \text{ m} = 10 \text{ Nm}
  • **Anticlockwise Moment:** Force (FF) ×0.2 m\times 0.2 \text{ m}
  • **Equilibrium:** F×0.2 m=10 NmF \times 0.2 \text{ m} = 10 \text{ Nm}
  • **Solve for F:** F=10/0.2=∗∗50 N∗∗F = 10 / 0.2 = **50 \text{ N}**

Mastering the unit conversions and identifying that perpendicular distance will make these exam questions simple. Ready to dig deeper? Enroll in our dedicated Physics session next week!