In mechanics, what does it mean to be in equilibrium?

Being in equilibrium in mechanics refers to a state where all the forces acting on a system are balanced, resulting in no net force acting on it. This means that the sum of all forces equals zero, and, as a result, the object remains at rest or continues to move at a constant velocity. This principle is fundamental to understanding applications such as static structures or objects at rest, as well as dynamic systems in uniform motion.

The balance of forces ensures that there is neither acceleration nor change in motion, which is a key characteristic of equilibrium. For example, if two people are pushing on opposite sides of a cart with equal force, the cart remains in place because those forces are balanced, illustrating the concept of equilibrium.

Other options do not accurately describe the condition of equilibrium. A lever being at a 90-degree angle does not inherently indicate that it is in equilibrium—it could be in motion. Similarly, merely applying effort without resistance does not imply a balanced force scenario. Lastly, the alignment of the fulcrum does not pertain to the forces acting on the lever system and does not determine whether it is in equilibrium. Thus, the concept of balanced forces is central to the definition and illustration of equilibrium in mechanics.