statics Statics is the branch of classical mechanics that is concerned with the analysis of force and torque (also called moment) acting on physical systems that do not experience an acceleration (''a''=0), but rather, are in static equilibrium with t ...

and

structural mechanics Structural mechanics or Mechanics of structures is the computation of deformations, deflections, and internal forces or stresses (''stress equivalents'') within structures, either for design or for performance evaluation of existing structures. ...

, a structure is statically indeterminate when the

static equilibrium In classical mechanics, a particle is in mechanical equilibrium if the net force on that particle is zero. By extension, a physical system made up of many parts is in mechanical equilibrium if the net force on each of its individual parts is zero ...

equations force and moment equilibrium conditions are insufficient for determining the

internal forces Internal may refer to: *Internality as a concept in behavioural economics *Neijia, internal styles of Chinese martial arts *Neigong or "internal skills", a type of exercise in meditation associated with Daoism *''Internal (album)'' by Safia, 2016 ...

and reactions on that structure.

Mathematics

Based on

Newton's laws of motion Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows: # A body remains at rest, or in moti ...

, the equilibrium equations available for a two-dimensional body are: :

\sum \mathbf F = 0 :

the vectorial sum of the

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

s acting on the body equals zero. This translates to: ::

\sum \mathbf H = 0 :

the sum of the horizontal components of the forces equals zero; ::

\sum \mathbf V = 0 :

the sum of the vertical components of forces equals zero; :

\sum \mathbf M = 0 :

the sum of the

moment Moment or Moments may refer to: * Present time Music * The Moments, American R&B vocal group Albums * ''Moment'' (Dark Tranq