Equilibrium of any stationary body occurs when external forces, either concurrent or parallel forces are to act upon a rigid body but no movement or rotation is resulted as its reaction. In that position the body is said to be in equilibrium. In this chapter you are going to find:
 Conditions of equilibrium for the external forces both concurrent and parallel separately.
 Moment of force on a point and in an axis.
 Varignon’s theorem.
 Free body diagram.
 Vectorial representation of moments and couples.
 Many types of support reactions.
 And what is the determination of reaction will be explained.
In the next portion, equations of equilibrium will be focused, such as:

 Equations of equilibrium: When studying equilibrium of rigid body, then there will be mentions on coplanar forces or as it is explained as concurrent and parallel forces. In the equilibrium state of the stationary body all mathematical outcome of external forces will become zero with the moments about the point and axis. The equation will be written as F = 0 and M = 0. The first equation is known as force law of equilibrium and the second one is known as the moment law of equilibrium. The forces are generally resolved into horizontal and vertical components. Hence equation (first) is written as F_{x} = 0 F_{y}= 0 whereF_{x}= Algebraic sum of all horizontal components andF_{y}= Algebraic sum of all vertical components.
 Equation for nonconcurrent forces: This equilibrium occurs when all other forces and moments reach a mathematical result of zero. The equation is F_{x} = 0, F_{y }= 0 and M = 0.
 Equation for Concurrent forces: In this equilibrium all lines of actions are found in one point and so ultimately making moments of those forces also zero. Here M = 0 becomes unnecessary and the important equations remain as in F_{x}= 0 and F_{y}= 0.
 Action and reaction: In the third law of Newton, all actions have an equal amount of opposite reaction. This is an equal power of opposite features. In the figure below there are two balls placed horizontally. They can move freely horizontally but not vertically down. The ball will place an action which wants to move downward. The support will place an equal amount of opposite force to stop that ball to go downward than is calld reaction.
Links of Previous Main Topic:
 Introduction to statics
 Introduction to vector algebra
 Two dimensional force systems
 Introduction concept of equilibrium of rigid body
 Friction introduction
 Introduction about distributed forces
 Area moments of inertia in rectangular and polar coordinates
 Mass moment of inertia introduction
 Work done by force
 Kinematics of particles
 Position vector velocity and acceleration
 Plane kinematics of rigid bodies introduction
 Combined motion of translation and rotation
 Rectilinear motion in kinetics of particles
 Work and energy
 Linear momentum
 Force mass acceleration
 Simple stress introduction
 Normal strain
 Statically indeterminate system
 Introduction to thermodynamics
 Statement of zeroth law of thermodynamics with explanation
 Heat and work introduction
 First law of thermodynamics for a control mass closed system undergoing a cycle
 Open system and control volume
 Conversion of work into heat
 Introduction to carnot cycle
 Clausius inequality entropy and irreversibility introduction
 Ideal gas or perfect gas
 Introduction about air standard cycles
 Properties of pure substances introduction
 Vapour compression refrigeration cycle introduction
 Basic fluid mechanics and properties of fluids introduction
 Fluid statics introduction
 Manometers measurement pressure
 Fluid kinematics
 Bernoullis equation
 Basics and statics of particles introduction
Link of Next Mechanical Engineering Topics: