Understanding the turning effects of forces is crucial for secondary school students preparing for their GCE O Level Physics exam. This article aims to delve into the concept of forces through carefully crafted examples and explanations developed by our teachers.

By doing so, we explore the principles behind how forces cause objects to rotate or remain at equilibrium. You will have a solid foundation in comprehending how forces move objects both linearly and cause rotational effects.


Moment of a Force

The moment of a force equals the product of the force (F) and the perpendicular distance (d) from the pivot to the force’s line of action. Its SI unit is Newton-metre (NM).

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The moment of a force (torque), force (F), and the perpendicular distance (d) from the pivot point is expressed as:

Moment of a Force (or Torque) = F x d

This relationship quantifies how effectively forces can cause rotation around a pivot point, with the unit of moment expressed as Nm.

Take a moment to consider the following question to better grasp the concept.



Conditions of Equilibrium caused by Forces

The conditions of equilibrium caused by forces involve specific requirements for an object to remain balanced and stationary under external forces. These conditions are fundamental in ensuring that an object does not accelerate or rotate.

Moreover, when an object is at equilibrium, the sum of clockwise moments caused by forces around a pivot is equal to the sum of anticlockwise moments about the same pivot.

The conditions for an object to remain in equilibrium are:

  1. The resultant forces acting on it must be zero so as to ensure no net force, and
  2. The resultant moment around any pivot point must also be zero, so as to achieve balance in rotational forces.

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Principles of Moments & Forces

Applying the principle of moments (also known as the principle of torque) involves using the concept that the sum of clockwise moments caused by forces about a pivot point must equal the sum of anticlockwise moments to solve problems related to equilibrium and rotational motion.

Take a look at the following question to see how this concept may appear as an exam question on forces.



How Forces Impact the Centre of Gravity

Understanding the centre of gravity is crucial in physics as it influences an object’s stability and equilibrium under the influence of external forces. Thus, it is essential to calculate or locate the centre of gravity so as to ensure that the structures can remain balanced and resistant to the forces acting upon them.

The center of gravity is the point where the entire weight of an object appears to act.

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For a regular body with uniform distribution of mass, its centre of gravity coincides with its centroid. However, for irregular bodies whereby its mass distribution varies, the centre of gravity can be determined by using a plumb line so as to determine its centre of gravity. 


Finding the Centre of Gravity (c.g.) of a Lamina

Understanding and locating the centre of gravity is essential in engineering and physics, as it determines how an object responds to gravitational forces and is crucial in designing balanced structures and systems that can withstand external forces effectively.

The centre of gravity of a lamina is determined by following a step-by-step method involving the balance of gravitational forces as below:

  1. Make three distinct points on the lamina.
  2. Suspend the lamina from a pin, allowing it to hang freely.
  3. Suspend a plumb line with a pendulum bob from the same pin, ensuring it hangs vertically.
  4. Draw a vertical line down the lamina directly beneath the pin.
  5. Repeat steps 2-3 for the other two points marked on the lamina.
  6. The point where all three drawn vertical lines intersect is the centre of gravity of the lamina.


Watch this video to understand how to set up your equipment in the lab for plumbline experiment.


During this process, it is essential that you are aware of the following precautions, so as to ensure precise and reliable results in analysing how gravitational forces affect the balance and stability of structures.

Firstly, ensure the lamina hangs freely and can oscillate around the point of suspension. This freedom of movement ensures that gravitational forces act naturally on the lamina, facilitating the precise intersection of vertical lines used to pinpoint the centre of gravity.

Moreover, when marking points on the lamina, it is essential to minimise the removal of the lamina. This maintains the lamina’s structural integrity and ensures uniform weight distribution, preventing unintended shifts in its center of gravity.

Before tracing the plumb line to identify vertical intersections, ensure the pendulum and lamina are stable and securely positioned. Any instability or movement could lead to inaccurate readings, compromising the reliability of the centre of gravity calculation and subsequent analysis of forces acting on the lamina.


Stability and Types of Equilibrium

Stability measures a body’s ability to return to its original position after being slightly tilted. This concept is also important in understanding how external forces affect objects.


Increasing Stability of an Object

Two main strategies can increase the stability of an object.

  1. By lowering the object’s centre of gravity, we can reduce the tendency of the object to tip over as the gravitational forces acting on it are closer to the ground.
  2. Increasing the object’s surface area enhances stability by providing a broader base, thus distributing forces more evenly so as to reduce the likelihood of tipping.


Types of Equilibrium

Understanding these types of equilibrium is crucial in designing structures that can withstand external forces effectively while maintaining stability. Engineers and designers utilise this understanding to ensure that structures remain balanced and resistant to tipping or displacement when subjected to varying forces such as wind or operational loads.


(1) Unstable Equilibrium

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  • When an unstable object is displaced, the line of action of its weight lies outside its base. Consequently, the gravitational force creates a clockwise moment, causing the object to move further away from its original position.
  • Thus, the object will fall away from its original position. 


(2) Neutral Equilibrium


  • When a neutral object has been displaced, the object comes to rest in its new position.


(3) Stable Equilibrium

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  • When a stable object is displaced, the line of action of its weight still passes through its base. 
  • The moment of the weight produces an anticlockwise moment, causing the object to return to its original position.


Test your Understanding on Forces

Understanding the concept of forces is foundational in both theoretical knowledge and practical applications across various fields, particularly in physics and engineering. Forces govern how objects move, interact, and maintain equilibrium in response to external influences. 

Have you grasped the concept of forces? Test your understanding of forces by attempting this question down below.

Live Q1

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Now, compare your answers with those of Teacher Manpreet to see if you got it right!


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