# 5.2.1 Equilibrium of Forces

### The Principle of Moments

• The principle of moments states: For a system to be balanced (in equilibrium), the sum of clockwise moments about a point must be equal to the sum of anticlockwise moments (about the same point) Diagram showing the moments acting on a balanced beam

• In the above diagram:
• Force F2 is supplying a clockwise moment;
• Forces F1 and F3 are supplying anticlockwise moments
• Hence: F2 × d2 = F1 × d1 + F3 × d3 #### Exam Tip

Make sure that all the distances are in the same units and you’re considering the correct forces as clockwise or anticlockwise, as seen in the diagram below ### Equilibrium

• A system is in equilibrium when all the forces are balanced. This means:
• There is no resultant force
• There is no resultant torque
• An object in equilibrium will therefore remain at rest, or at a constant velocity, and not rotate
• The system is in an equilibrium state when applying the principle of moments. See “The Principle of Moments” for more notes on this

### Coplanar Forces in Equilibrium

• Coplanar forces can be represented by vector triangles
• In equilibrium, these are closed vector triangles. The vectors, when joined together, form a closed path
• The most common forces on objects are
• Weight
• Normal reaction force
• Tension (from cords and strings)
• Friction
• The forces on a body in equilibrium are demonstrated below:

#### Exam Tip

The diagrams in exam questions about this topic tend to be drawn to scale, so make sure you have a ruler handy! ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close Close