The concept of **static balance**more precisely static mechanical equilibrium, is used in physics to describe a **stationary state** in which the relative position of the components of a system does not change with time. It doesn't mean they don't move, they can, what doesn't change is the relative position between the components.

In other words, in the state of static equilibrium **the system is at rest or its center of mass moves at a constant speed**.

This concept is implicit in the Law of Inertia, the first of Newton's three laws:

Every body perseveres in its state of rest or uniform and rectilinear motion unless compelled to change its state by forces impressed upon it.

The most common definition of static equilibrium uses the net force: An object is in static equilibrium when the **sum of the forces acting on it** (net or resultant force) **is equal to zero**. Both translational and torsional forces are taken into account and therefore an object is in static equilibrium if it is **in translational equilibrium and in rotational equilibrium**.

Another broader definition defines the state of static equilibrium as that state of an object whose position in space has a gradient of potential energy equal to zero. In this definition the object can move at a constant speed and implies that, although in our observation frame it may not seem so, it is always possible to find a reference frame with respect to which the object is stationary.

## Driving Forces and the State of Balance

As defined, static equilibrium implies that the resultant of the forces acting on the object is zero, the forces continue to act but there is equilibrium. To understand how forces can act on an object but remain in a state of equilibrium, it is necessary to understand what a driving force is and how it acts.

The effect on the movement, according to Newton's Second Law or Fundamental Law of Dynamics, is proportional to the magnitude of the force and occurs in the straight line in which the force is exerted. Namely, **driving forces are vector forces that are defined by a direction and a magnitude**. When multiple forces are applied to an object, the net force is equal to the vector sum of all the forces, that is, the net force is equal to the resultant vector. **If the net force is of zero magnitude, then the object is in static equilibrium.**.

In mathematical expression, the net force, as stated by Newton in his second Law, is:

Where *F* is the force, *m* is the mass, *v* is the speed and *you* It's the time. In words, the net force is equal to the momentum differential (m×v) in a given time interval. Then, **the object will be in a state of equilibrium if the momentum becomes zero** during that time interval.

If the mass of the object remains constant, the above equation can be expressed as the product of the mass times the acceleration experienced by the object:

Where *F* is the net force, *m* is the mass, which remains constant, and *a* is the acceleration. Since F is zero if the object is in static equilibrium, it can be said that **An object of constant mass is in equilibrium when the sum of the forces acting on it produces zero acceleration.**.

If you read the previous paragraphs carefully, static equilibrium does not necessarily imply that the object does not move. If an object in a vacuum is given a small push, it will begin to move and will remain in motion forever and at a constant speed since there is no force slowing it down or accelerating it (for example, a car moving down the street). a road is slowed down by gravity and friction.). Therefore, the object can move and be in static equilibrium at the same time as long as the resultant force acting on it is zero, which fulfills the Law of Inertia.

### Torsion forces

So far we have talked about translational forces acting on the object. But in addition to translational equilibrium, the state of static equilibrium requires rotational equilibrium. Rotational equilibrium is reached when all torsional forces cancel and their resultant is zero.