Static Equilibrium

Static Equilibrium

When all forces on an object are balanced, then an object is said to be in a state of equilibrium. Objects that are in a state of equilibrium must have an acceleration of 0 M/S/S. This does not necessarily mean that the object is at rest, though. An object could be moving at a constant 75 MPH but still be in a state of equilibrium as it is not accelerating. This derives from Newton's first law, or: 

•  An object is in rest and stays at rest, or
• An object is in motion and stays in motion with constant speed and direction

If an object is at rest and in a state of equilibrium, then the object would be in a state of "static equilibrium" with static meaning stationary or at rest. Bridges use static equilibrium to balance their forces. Structures on the ends of bridges, or abutments, help equalize the forces on the bridge so that the bridge does not collapse. In an arched bridge, the abutments at the end of the arch take the brute of the force and help dissipate it throughout the bridge. Some bridges such as suspension bridges try to dissipate it through the ground with the help of anchorages, towers, and cables.  

Forces at work in an arched bridge.

Another example of static equilibrium are balancing toy birds. The birds have weights in their wings and beaks that help the forces spread out and balance. This helps them reach static equilibrium and helps them balance on almost all surfaces.