What is defined as a condition where the sum of forces and moments acting on a body is zero?

The condition described is static equilibrium, which occurs when a system is at rest, and there is no unbalanced force or moment acting on it. In static equilibrium, all the forces acting on a body are perfectly balanced, resulting in a net force of zero. Additionally, the moments (or torques) about any point in the system are also balanced, leading to a net moment of zero.

This principle is essential in various fields, including engineering and physics, as it ensures that structures or objects do not move or rotate when the forces and moments are in equilibrium. An example of this can be seen in a book resting on a table; the weight of the book is countered by the normal force from the table, leading to a state of balance.

Dynamic equilibrium refers to a state where an object moves at a constant velocity without accelerating, which involves forces that still sum to zero but is in motion. Kinetic equilibrium is less commonly used but implies an object in motion with constant speed and direction. Rotational equilibrium focuses solely on the balance of rotational forces or moments but does not specifically account for translational forces. Static equilibrium is the most appropriate term here as it encompasses both the forces and the moments acting on a body at rest.