A Level Physics Practice Exam

According to the principle of moments, when is a system in equilibrium?

1. The sum of the clockwise moments equals total weight

2. The sum of the clockwise moments equals the sum of the anti-clockwise moments

3. The total force equals zero

4. The applied forces are balanced by the support forces

The correct answer is: The sum of the clockwise moments equals the sum of the anti-clockwise moments

A system is in equilibrium according to the principle of moments when the sum of the clockwise moments equals the sum of the anti-clockwise moments. This principle is derived from the observation that for an object to be in rotational equilibrium, there must be no net moment acting on it. In practical terms, this means that if you take any pivot point, the moments caused by forces acting in a clockwise direction must be balanced by those acting in an anti-clockwise direction. When this balance is achieved, the object will neither rotate about the pivot point nor change its state of motion. While the total force being zero is also a condition for translational equilibrium, it does not specifically address the rotational aspects governed by the principle of moments. The statement about total weight relates to the overall force balance rather than rotational moments. Similarly, the notion of applied forces being balanced by support forces does not encapsulate the rotational characteristics required for a system to be in equilibrium as defined by the principle of moments.