Static Equilibrium

When all the forces that act upon an object are balanced, then the object is said to be in a state of equilibrium. Even though these forces are balanced, it does not mean that they are equal. Objects in equilibrium have a net force and acceleration of zero. This comes from Newton's First Law of Motion. 

Even though the object has an acceleration of zero, it does not mean that it is at rest. It can also mean that the object is in motion and continuing in motion with the same speed and direction. 

However, when an object is at rest and is in equilibrium, it is said to be in "static equilibrium". Static means stationary or at rest. 

A bridge is an example of a system in static equilibrium. The bridge undergoes no motion. Static equilibrium is an important concept when building a bridge whether it is small or huge. It is important because these structures need to maintain static equilibrium under all expected loading conditions. A bridge reaches static equilibrium when the weight on top is equal to the resistance on the bottom of the bridge. If the bridge was not in static equilibrium, then it would not be able to hold all the weight it was supposed to, and it would collapse.