The principle of moments states that when a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot.The book is saying that if an object is in equilibrium, then the resultant moment about a pivot (in fact, about any point) is zero. However, the converse is not true. In other words, an object having zero resultant moment about the pivot might not be in equilibrium. Why is that so?
There are two types of equilibrium: translational equilibrium and rotational equilibrium. An object is in translational equilibrium when the resultant force is zero. An object is in rotational equilibrium when the resultant moment about the pivot is zero. An object having zero resultant moment might not have zero resultant force (see diagram below).
Acknowledgement: The diagram is obtained from University Physics with Modern Physics (13th edition) by H. Young and R. Freedman.
In general, students are able to understand the principle of moments. However, they might not know when they should apply the principle. One way is to emphasize that we often need to apply the principle of moments when we are dealing with extended bodies (for examination, objects with dimensions provided).
Since I am on the topic of principle of moment, let me end off by posing a challenge to you: Is the stationary uniform metre rule shown in the diagram below in equilibrium? =P
How about the following question, which is obtained from Cambridge International A-level?
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