4.2MechanicsCore

Resultant Moments

When several forces act on a rigid body, their turning effects combine. The resultant moment about a point is found by adding anticlockwise moments and subtracting clockwise ones.

22 min Video by Zeeshan Zamurred Moments

Edexcel A Level Maths: 4.2 Resultant Moments (Forces)Watch the full walkthrough before the notes below.

Open on YouTube

What you'll be able to do

Combine several moments about a point
Take a consistent positive sense
Find the resultant moment
State its direction

Combining moments

Choose a positive sense (usually anticlockwise). The resultant moment is the sum of the anticlockwise moments minus the sum of the clockwise moments.

resultant = \sum M_{anti} - \sum M_{clock}

Net turning effect.

8 - 3 = 5

N m.

2Positive, so anticlockwise.

Answer

5

N m anticlockwise.

Tip — Pick one positive direction and stick with it throughout the problem.

Formula recap

resultant = \sum M_{anti} - \sum M_{clock}

Resultant moment.

Common mistakes to avoid

Adding all moments regardless of direction.

Subtract opposing (clockwise) moments.

Switching the positive sense mid-problem.

Keep one consistent positive direction.

Key takeaways

Choose a positive sense (usually anticlockwise).
Resultant = Σ anticlockwise − Σ clockwise.
State the direction of the resultant.

Test yourself

Ready to lock in Resultant Moments? Pick a mode and earn XP & Dobloons.