AQA AS Level Physics Paper 2, June 2024: Question 31

✅ Correct Answer: C

Statement C: The lines of action of the forces must pass through the centre of mass of the body.

Why it's the correct choice: For equilibrium, the lines of action of three coplanar forces must simply intersect at a single common point (making the net moment zero about any point), but that common point does not have to be the centre of mass of the body. Therefore, statement C is not required.

💡 Key Knowledge

• First Condition of Equilibrium: The vector sum of the forces in any direction must be zero (Statement A). Graphically, this means three forces must form a closed triangle.
• Second Condition of Equilibrium: The sum of the moments about any point in the plane must be zero (Statement B).
• Triangle of Forces: For three forces to be in equilibrium, the resultant of any two forces must be equal in magnitude, opposite in direction, and collinear with the third force (Statement D).

🧠 Exam Technique

Read questions containing negative qualifiers like "not" very carefully! Underline or highlight the word not in the exam paper so you actively search for the false statement among three true statements.

❌ Common Errors

Students frequently confuse rotational equilibrium with the centre of mass. While forces acting through the centre of mass produce no linear acceleration, an object can be in rotational equilibrium even if forces act off-centre, provided the net moment about any pivot equals zero.