What condition must be met for an object to be in static equilibrium?

• A. The object must be in motion
• B. Only the sum of forces acting on it is zero
• C. Both the sum of forces and the sum of torques must be zero
• D. It must be at a constant speed

Answer: (C)

An object is in static equilibrium when it is not moving and remains at rest. For this to occur, two essential conditions must be satisfied: the sum of all forces acting on the object must be zero, and the sum of all torques (moments) acting on it must also be zero.

When the forces acting on an object balance each other out, there is no net force to cause acceleration in any direction. Similarly, for the object to maintain its position without rotation, the sum of the torques about any point (usually taken to be the center of mass or a pivot point) must also equal zero. This ensures that there is no tendency for the object to start rotating.

Meeting both conditions ensures that the object does not translate or rotate, thus satisfying the requirements for static equilibrium. This concept is fundamental in mechanics and crucial in analyzing structures, ensuring their stability under various loads.