In

/ref> but they have also been developed for the study of the mechanics of deformable bodies.

_{''i''} (''t'') (''i'' = 1,...,''n''), and express r(''t'') and in terms of the generalized coordinates. That is,
$$\backslash mathbf(t)\; =\; \backslash mathbf(q\_1,q\_2,\backslash dots,q\_n;t),$$
$$\backslash mathbf(t)\; =\; \backslash mathbf(q\_1,q\_2,\backslash dots,q\_n;t).$$
Then, the derivative of the variation is given by
$$\backslash frac\backslash delta\; \backslash mathbf\; =\; \backslash frac\backslash epsilon\backslash mathbf\; =\; \backslash sum\_^n\; \backslash frac\; \backslash epsilon\backslash dot\_i,$$
then we have
$$\backslash delta\; W\; =\; \backslash int\_^\; \backslash left(\backslash sum\_^n\; \backslash mathbf\; \backslash cdot\; \backslash frac\backslash epsilon\backslash dot\_i\backslash right)dt\; =\; \backslash sum\_^n\; \backslash left(\backslash int\_^\backslash mathbf\backslash cdot\backslash frac\backslash epsilon\backslash dot\_i\; ~dt\backslash right).$$
The requirement that the virtual work be zero for an arbitrary variation is equivalent to the set of requirements
$$Q\_i\; =\; \backslash mathbf\; \backslash cdot\; \backslash frac\; =\; 0,\; \backslash quad\; i=1,\; \backslash ldots,\; n.$$
The terms ''Q_{i}'' are called the ''generalized forces'' associated with the virtual displacement δr.

_{i}'' , ''i'' = 1, 2, ..., ''m'' and M''_{j}'' , ''j'' = 1, 2, ..., ''n'' are the applied forces and applied moments, respectively, and ''δ''r''_{i}'' , ''i'' = 1, 2, ..., ''m'' and ''δφ_{j}'', ''j'' = 1, 2, ..., ''n'' are the virtual displacements and virtual rotations, respectively.
Suppose the system consists of ''N'' particles, and it has ''f'' (''f'' ≤ 6''N'') _{k}'' , ''k'' = 1, 2, ..., ''f'' are defined such that the virtual movements can be expressed in terms of these generalized coordinates
In analytical mechanics, generalized coordinates are a set of parameters used to represent the state of a system in a configuration space. These parameters must uniquely define the configuration of the system relative to a reference state.,p. 39 ...

:
$$\backslash delta\; W\; =\; \backslash sum\_^f\; \backslash left;\; href="/html/ALL/l/\backslash left(\_\backslash sum\_^m\_\backslash mathbf\_i\_\backslash cdot\_\backslash frac\_+\_\backslash sum\_^n\_\backslash mathbf\_j\_\backslash cdot\_\backslash frac\_\backslash right)\_\backslash delta\_q\_k\_\backslash right.html"\; ;"title="\backslash left(\; \backslash sum\_^m\; \backslash mathbf\_i\; \backslash cdot\; \backslash frac\; +\; \backslash sum\_^n\; \backslash mathbf\_j\; \backslash cdot\; \backslash frac\; \backslash right)\; \backslash delta\; q\_k\; \backslash right">\backslash left(\; \backslash sum\_^m\; \backslash mathbf\_i\; \backslash cdot\; \backslash frac\; +\; \backslash sum\_^n\; \backslash mathbf\_j\; \backslash cdot\; \backslash frac\; \backslash right)\; \backslash delta\; q\_k\; \backslash right$$
where the _{k}'' are defined as
$$Q\_k\; =\; \backslash sum\_^m\; \backslash mathbf\_i\; \backslash cdot\; \backslash frac\; +\; \backslash sum\_^n\; \backslash mathbf\_j\; \backslash cdot\; \backslash frac\; ,\; \backslash quad\; k\; =\; 1,\; 2,\; \backslash dots,\; f\; .$$
Kane shows that these _{''i''} and moments M_{''j''} vanishes if it is in equilibrium. Therefore, the generalized forces ''Q''_{''k''} are zero, that is
$$\backslash delta\; W=0\; \backslash quad\; \backslash Rightarrow\; \backslash quad\; Q\_k\; =\; 0\; \backslash quad\; k\; =1,\; 2,\; \backslash dots,\; f\; .$$

_{''A''} at a point ''A'' located by the coordinate vector r_{''A''} on the bar. The lever then exerts an output force F_{''B''} at the point ''B'' located by r_{''B''}. The rotation of the lever about the fulcrum ''P'' is defined by the rotation angle ''θ''.
Let the coordinate vector of the point ''P'' that defines the fulcrum be r_{''P''}, and introduce the lengths
$$a\; =\; ,\; \backslash mathbf\_A\; -\; \backslash mathbf\_P,\; ,\; \backslash quad\; b\; =\; ,\; \backslash mathbf\_B\; -\; \backslash mathbf\_P,\; ,$$
which are the distances from the fulcrum to the input point ''A'' and to the output point ''B'', respectively.
Now introduce the unit vectors e_{''A''} and e_{''B''} from the fulcrum to the point ''A'' and ''B'', so
$$\backslash mathbf\_A\; -\; \backslash mathbf\_P\; =\; a\backslash mathbf\_A,\; \backslash quad\; \backslash mathbf\_B\; -\; \backslash mathbf\_P\; =\; b\backslash mathbf\_B.$$
This notation allows us to define the velocity of the points ''A'' and ''B'' as
$$\backslash mathbf\_A\; =\; \backslash dot\; a\; \backslash mathbf\_A^\backslash perp,\; \backslash quad\; \backslash mathbf\_B\; =\; \backslash dot\; b\; \backslash mathbf\_B^\backslash perp,$$
where e_{''A''}^{⊥} and e_{''B''}^{⊥} are unit vectors perpendicular to e_{''A''} and e_{''B''}, respectively.
The angle ''θ'' is the generalized coordinate that defines the configuration of the lever, therefore using the formula above for forces applied to a one degree-of-freedom mechanism, the generalized force is given by
$$Q\; =\; \backslash mathbf\_A\; \backslash cdot\; \backslash frac\; -\; \backslash mathbf\_B\; \backslash cdot\; \backslash frac\; =\; a(\backslash mathbf\_A\; \backslash cdot\; \backslash mathbf\_A^\backslash perp)\; -\; b(\backslash mathbf\_B\; \backslash cdot\; \backslash mathbf\_B^\backslash perp).$$
Now, denote as ''F''_{''A''} and ''F''_{''B''} the components of the forces that are perpendicular to the radial segments ''PA'' and ''PB''. These forces are given by
$$F\_A\; =\; \backslash mathbf\_A\; \backslash cdot\; \backslash mathbf\_A^\backslash perp,\; \backslash quad\; F\_B\; =\; \backslash mathbf\_B\; \backslash cdot\; \backslash mathbf\_B^\backslash perp.$$
This notation and the principle of virtual work yield the formula for the generalized force as
$$Q\; =\; a\; F\_A\; -\; b\; F\_B\; =\; 0.$$
The ratio of the output force ''F''_{''B''} to the input force ''F''_{''A''} is the

_{A}'' of the input gear to the angular velocity ''ω_{B}'' of the output gear, known as the speed ratio, or _{''A''} acting on the input gear ''G''_{''A''} is transformed by the gear train into the output torque ''T''_{''B''} exerted by the output gear ''G''_{''B''}. If we assume, that the gears are rigid and that there are no losses in the engagement of the gear teeth, then the principle of virtual work can be used to analyze the static equilibrium of the gear train.
Let the angle ''θ'' of the input gear be the generalized coordinate of the gear train, then the speed ratio ''R'' of the gear train defines the angular velocity of the output gear in terms of the input gear, that is
$$\backslash omega\_A\; =\; \backslash omega,\; \backslash quad\; \backslash omega\_B\; =\; \backslash omega/R.$$
The formula above for the principle of virtual work with applied torques yields the generalized force
$$Q\; =\; T\_A\; \backslash frac\; -\; T\_B\; \backslash frac\; =\; T\_A\; -\; T\_B/R\; =\; 0.$$
The _{''B''} to the input torque ''T''_{''A''}, and the above equation yields
$$MA\; =\; \backslash frac\; =\; R.$$
Thus, the speed ratio of a gear train also defines its mechanical advantage. This shows that if the input gear rotates faster than the output gear, then the gear train amplifies the input torque. And, if the input gear rotates slower than the output gear, then the gear train reduces the input torque.

_{i}, i=1,...,n, and let the resultant of the applied forces on each body be the force-torque pairs, F_{i} and T_{i}, ''i'' = 1,...,''n''. Notice that these applied forces do not include the reaction forces where the bodies are connected. Finally, assume that the velocity V_{i} and angular velocities ω_{i}, ''i''=1,...,''n'', for each rigid body, are defined by a single generalized coordinate q. Such a system of rigid bodies is said to have one

Dynamics, Theory and Applications

McGraw-Hill, NY, 2005. $$Q^*\_j\; =\; -\backslash left(\backslash frac\; \backslash frac\; -\backslash frac\backslash right),\; \backslash quad\; j=1,\; \backslash ldots,\; m.$$

_{j}''. This condition yields ''m'' equations,
$$Q\_j\; +\; Q^*\_j\; =\; 0,\; \backslash quad\; j=1,\; \backslash ldots,\; m,$$
which can also be written as
$$\backslash frac\; \backslash frac\; -\backslash frac\; =\; Q\_j,\; \backslash quad\; j=1,\backslash ldots,m.$$
The result is a set of m equations of motion that define the dynamics of the rigid body system, known as _{j} are derivable from a potential energy ''V''(''q''_{1},...,''q''_{''m''}), then these equations of motion take the form
$$\backslash frac\; \backslash frac\; -\backslash frac\; =\; -\backslash frac,\; \backslash quad\; j=1,\backslash ldots,m.$$
In this case, introduce the

Examples applications of the virtual work principle

mechanics
Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objec ...

, virtual work arises in the application of the ''principle of least action
The stationary-action principle – also known as the principle of least action – is a variational principle that, when applied to the '' action'' of a mechanical system, yields the equations of motion for that system. The principle states tha ...

'' to the study of forces and movement of a mechanical system
A machine is a physical system using power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecul ...

. The work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal t ...

of a force acting on a particle as it moves along a displacement is different for different displacements. Among all the possible displacements that a particle may follow, called virtual displacements, one will minimize the action. This displacement is therefore the displacement followed by the particle according to the principle of least action. The work of a force on a particle along a virtual displacement is known as the virtual work.Historically, virtual work and the associated

calculus of variations
The calculus of variations (or Variational Calculus) is a field of mathematical analysis that uses variations, which are small changes in functions
and functionals, to find maxima and minima of functionals: mappings from a set of functions t ...

were formulated to analyze systems of rigid bodies,C. Lánczos, The Variational Principles of Mechanics, 4th Ed., General Publishing Co., Canada, 1970/ref> but they have also been developed for the study of the mechanics of deformable bodies.

History

The principle of virtual work had always been used in some form since antiquity in the study of statics. It was used by the Greeks, medieval Arabs and Latins, and Renaissance Italians as "the law of lever". The idea of virtual work was invoked by many notable physicists of the 17th century, such as Galileo, Descartes, Torricelli, Wallis, and Huygens, in varying degrees of generality, when solving problems in statics. Working with Leibnizian concepts,Johann Bernoulli
Johann Bernoulli (also known as Jean or John; – 1 January 1748) was a Swiss mathematician and was one of the many prominent mathematicians in the Bernoulli family. He is known for his contributions to infinitesimal calculus and educating Le ...

systematized the virtual work principle and made explicit the concept of infinitesimal displacement. He was able to solve problems for both rigid bodies as well as fluids. Bernoulli's version of virtual work law appeared in his letter to Pierre Varignon
Pierre Varignon (1654 – 23 December 1722) was a French mathematician. He was educated at the Jesuit College and the University of Caen, where he received his M.A. in 1682. He took Holy Orders the following year.
Varignon gained his first ...

in 1715, which was later published in Varignon's second volume of ''Nouvelle mécanique ou Statique'' in 1725. This formulation of the principle is today known as the principle of virtual velocities and is commonly considered as the prototype of the contemporary virtual work principles. In 1743 D'Alembert published his ''Traité de Dynamique'' where he applied the principle of virtual work, based on Bernoulli's work, to solve various problems in dynamics. His idea was to convert a dynamical problem into static problem by introducing ''inertial force''.René Dugas, A History of Mechanics, Courier Corporation, 2012 In 1768, Lagrange
Joseph-Louis Lagrange (born Giuseppe Luigi LagrangiaD'Alembert's principle
D'Alembert's principle, also known as the Lagrange–d'Alembert principle, is a statement of the fundamental classical laws of motion. It is named after its discoverer, the French physicist and mathematician Jean le Rond d'Alembert. D'Alembert ...

, was given in his ''Mécanique Analytique'' of 1788. Although Lagrange had presented his version of least action principle prior to this work, he recognized the virtual work principle to be more fundamental mainly because it could be assumed alone as the foundation for all mechanics, unlike the modern understanding that least action does not account for non-conservative forces.
Overview

If a force acts on a particle as it moves from point $A$ to point $B$, then, for each possible trajectory that the particle may take, it is possible to compute the total work done by the force along the path. The ''principle of virtual work'', which is the form of the principle of least action applied to these systems, states that the path actually followed by the particle is the one for which the difference between the work along this path and other nearby paths is zero (to the first order). The formal procedure for computing the difference of functions evaluated on nearby paths is a generalization of the derivative known from differential calculus, and is termed ''the calculus of variations''. Consider a point particle that moves along a path which is described by a function $\backslash mathbf(t)$ from point $A$, where $\backslash mathbf(t=t\_0)$, to point $B$, where $\backslash mathbf(t=t\_1)$. It is possible that the particle moves from $A$ to $B$ along a nearby path described by $\backslash mathbf(t)\; +\; \backslash delta\; \backslash mathbf(t)$, where $\backslash delta\; \backslash mathbf(t)$ is called the variation of $\backslash mathbf(t)$. The variation $\backslash delta\; \backslash mathbf(t)$ satisfies the requirement $\backslash delta\; \backslash mathbf(t\_0)\; =\; \backslash delta\; \backslash mathbf(t\_1)\; =\; 0$. The scalar components of the variation $\backslash delta\; r\_1(t)$, $\backslash delta\; r\_2(t)$ and $\backslash delta\; r\_3(t)$ are called virtual displacements. This can be generalized to an arbitrary mechanical system defined by thegeneralized coordinates
In analytical mechanics, generalized coordinates are a set of parameters used to represent the state of a system in a configuration space. These parameters must uniquely define the configuration of the system relative to a reference state.,p. 39 ...

$q\_i$, $i\; =\; 1,2,...,n$. In which case, the variation of the trajectory $q\_i(t)$ is defined by the virtual displacements $\backslash delta\; q\_i$, $i\; =\; 1,2,...,n$.
Virtual work is the total work done by the applied forces and the inertial forces of a mechanical system as it moves through a set of virtual displacements. When considering forces applied to a body in static equilibrium, the principle of least action requires the virtual work of these forces to be zero.
Mathematical treatment

Consider a particle ''P'' that moves from a point ''A'' to a point ''B'' along a trajectory r(''t''), while a force F(r(''t'')) is applied to it. The work done by the force F is given by the integral $$W\; =\; \backslash int\_^\; \backslash mathbf\; \backslash cdot\; d\backslash mathbf\; =\; \backslash int\_^\; \backslash mathbf\; \backslash cdot\; \backslash frac~dt\; =\; \backslash int\_^\backslash mathbf\; \backslash cdot\; \backslash mathbf\; ~\; dt,$$ where ''d''r is the differential element along the curve that is the trajectory of ''P'', and v is its velocity. It is important to notice that the value of the work ''W'' depends on the trajectory r(''t''). Now consider particle ''P'' that moves from point ''A'' to point ''B'' again, but this time it moves along the nearby trajectory that differs from r(''t'') by the variation , where ''ε'' is a scaling constant that can be made as small as desired and h(''t'') is an arbitrary function that satisfies . Suppose the force is the same as . The work done by the force is given by the integral $$\backslash bar\; =\; \backslash int\_^\; \backslash mathbf\; \backslash cdot\; d(\backslash mathbf+\backslash epsilon\; \backslash mathbf)=\backslash int\_^\; \backslash mathbf\; \backslash cdot\; \backslash frac~\; dt\; =\; \backslash int\_^\backslash mathbf\; \backslash cdot\; (\backslash mathbf\; +\backslash epsilon\; \backslash dot)\; ~\; dt\; .$$ The variation of the work ''δW'' associated with this nearby path, known as the ''virtual work'', can be computed to be $$\backslash delta\; W\; =\; \backslash bar-W\; =\; \backslash int\_^\; (\backslash mathbf\; \backslash cdot\; \backslash epsilon\; \backslash dot)\; ~dt.$$ If there are no constraints on the motion of ''P'', then 3 parameters are needed to completely describe ''Ps position at any time ''t''. If there are ''k'' (''k'' ≤ 3) constraint forces, then ''n'' = (3 − ''k'') parameters are needed. Hence, we can define ''n'' generalized coordinates ''q''Static equilibrium

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 z ...

is a state in which the net force and net torque acted upon the system is zero. In other words, both linear momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass a ...

and angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...

of the system are conserved. The principle of virtual work states that ''the virtual work of the applied forces is zero for all virtual movements of the system from 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 z ...

''. This principle can be generalized such that three dimensional rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

s are included: the virtual work of the applied forces and applied moments is zero for all virtual movements of the system from static equilibrium. That is
$$\backslash delta\; W\; =\; \backslash sum\_^m\; \backslash mathbf\_i\; \backslash cdot\; \backslash delta\backslash mathbf\_i\; +\; \backslash sum\_^n\; \backslash mathbf\_j\; \backslash cdot\; \backslash delta\backslash mathbf\_j\; =\; 0\; ,$$
where F''degrees of freedom
Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

. It is sufficient to use only ''f'' coordinates to give a complete description of the motion of the system, so ''f'' generalized coordinates
In analytical mechanics, generalized coordinates are a set of parameters used to represent the state of a system in a configuration space. These parameters must uniquely define the configuration of the system relative to a reference state.,p. 39 ...

''qgeneralized coordinates
In analytical mechanics, generalized coordinates are a set of parameters used to represent the state of a system in a configuration space. These parameters must uniquely define the configuration of the system relative to a reference state.,p. 39 ...

. That is,
$$\backslash delta\; \backslash mathbf\_i\; (q\_1,\; q\_2,\; \backslash dots,\; q\_f;\; t),\; \backslash quad\; i\; =\; 1,\; 2,\; \backslash dots,\; m\; ;$$
$$\backslash delta\; \backslash phi\_j\; (q\_1,\; q\_2,\; \backslash dots,\; q\_f;\; t),\; \backslash quad\; j\; =\; 1,\; 2,\; \backslash dots,\; n\; .$$
The virtual work can then be reparametrized by the generalized forces Generalized forces find use in Lagrangian mechanics, where they play a role conjugate to generalized coordinates. They are obtained from the applied forces, Fi, i=1,..., n, acting on a system that has its configuration defined in terms of generaliz ...

''Qgeneralized forces Generalized forces find use in Lagrangian mechanics, where they play a role conjugate to generalized coordinates. They are obtained from the applied forces, Fi, i=1,..., n, acting on a system that has its configuration defined in terms of generaliz ...

can also be formulated in terms of the ratio of time derivatives. That is,
$$Q\_k\; =\; \backslash sum\_^m\; \backslash mathbf\_i\; \backslash cdot\; \backslash frac\; +\; \backslash sum\_^n\; \backslash mathbf\_j\; \backslash cdot\; \backslash frac\; ,\; \backslash quad\; k\; =\; 1,\; 2,\; \backslash dots,\; f\; .$$
The principle of virtual work requires that the virtual work done on a system by the forces FConstraint forces

An important benefit of the principle of virtual work is that only forces that do work as the system moves through a virtual displacement are needed to determine the mechanics of the system. There are many forces in a mechanical system that do no work during a virtual displacement, which means that they need not be considered in this analysis. The two important examples are (i) the internal forces in arigid body
In physics, a rigid body (also known as a rigid object) is a solid body in which deformation is zero or so small it can be neglected. The distance between any two given points on a rigid body remains constant in time regardless of external fo ...

, and (ii) the constraint forces at an ideal joint
A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

.
Lanczos presents this as the postulate: "The virtual work of the forces of reaction is always zero for any virtual displacement which is in harmony with the given kinematic constraints." The argument is as follows. The principle of virtual work states that in equilibrium the virtual work of the forces applied to a system is zero. Newton's laws state that at equilibrium the applied forces are equal and opposite to the reaction, or constraint forces. This means the virtual work of the constraint forces must be zero as well.
Law of the lever

Alever
A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is d ...

is modeled as a rigid bar connected to a ground frame by a hinged joint called a fulcrum. The lever is operated by applying an input force Fmechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...

of the lever, and is obtained from the principle of virtual work as
$$MA\; =\; \backslash frac\; =\; \backslash frac.$$
This equation shows that if the distance ''a'' from the fulcrum to the point ''A'' where the input force is applied is greater than the distance ''b'' from fulcrum to the point ''B'' where the output force is applied, then the lever amplifies the input force. If the opposite is true that the distance from the fulcrum to the input point ''A'' is less than from the fulcrum to the output point ''B'', then the lever reduces the magnitude of the input force.
This is the ''law of the lever'', which was proven by Archimedes
Archimedes of Syracuse (;; ) was a Greek mathematician, physicist, engineer, astronomer, and inventor from the ancient city of Syracuse in Sicily. Although few details of his life are known, he is regarded as one of the leading scientis ...

using geometric reasoning.
Gear train

A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next. For this analysis, we consider a gear train that has one degree-of-freedom, which means the angular rotation of all the gears in the gear train are defined by the angle of the input gear. The size of the gears and the sequence in which they engage define the ratio of the angular velocity ''ωgear ratio
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...

, of the gear train. Let ''R'' be the speed ratio, then
$$\backslash frac\; =\; R.$$
The input torque ''T''mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...

of the gear train is the ratio of the output torque ''T''Dynamic equilibrium for rigid bodies

If the principle of virtual work for applied forces is used on individual particles of arigid body
In physics, a rigid body (also known as a rigid object) is a solid body in which deformation is zero or so small it can be neglected. The distance between any two given points on a rigid body remains constant in time regardless of external fo ...

, the principle can be generalized for a rigid body: ''When a rigid body that is in equilibrium is subject to virtual compatible displacements, the total virtual work of all external forces is zero; and conversely, if the total virtual work of all external forces acting on a rigid body is zero then the body is in equilibrium''.
If a system is not in static equilibrium, D'Alembert showed that by introducing the acceleration terms of Newton's laws as inertia forces, this approach is generalized to define dynamic equilibrium. The result is D'Alembert's form of the principle of virtual work, which is used to derive the equations of motion for a mechanical system of rigid bodies.
The expression ''compatible displacements'' means that the particles remain in contact and displace together so that the work done by pairs of action/reaction inter-particle forces cancel out. Various forms of this principle have been credited to Johann (Jean) Bernoulli (1667–1748) and Daniel Bernoulli
Daniel Bernoulli FRS (; – 27 March 1782) was a Swiss mathematician and physicist and was one of the many prominent mathematicians in the Bernoulli family from Basel. He is particularly remembered for his applications of mathematics to mecha ...

(1700–1782).
Generalized inertia forces

Let a mechanical system be constructed from n rigid bodies, Bdegree of freedom
Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

.
Consider a single rigid body which moves under the action of a resultant force F and torque T, with one degree of freedom defined by the generalized coordinate q. Assume the reference point for the resultant force and torque is the center of mass of the body, then the generalized inertia force Q* associated with the generalized coordinate q is given by
$$Q^*\; =\; -(M\backslash mathbf)\; \backslash cdot\; \backslash frac\; -\; (;\; href="/html/ALL/l/\_R.html"\; ;"title="\_R">\_R$$
This inertia force can be computed from the kinetic energy of the rigid body,
$$T\; =\; \backslash frac\; M\; \backslash mathbf\; \backslash cdot\; \backslash mathbf\; +\; \backslash frac\; \backslash boldsymbol\; \backslash cdot;\; href="/html/ALL/l/\_R.html"\; ;"title="\_R">\_R$$
by using the formula
$$Q^*\; =\; -\backslash left(\backslash frac\; \backslash frac\; -\backslash frac\backslash right).$$
A system of n rigid bodies with m generalized coordinates has the kinetic energy
$$T\; =\; \backslash sum\_^n\; \backslash left(\backslash frac\; M\; \backslash mathbf\_i\; \backslash cdot\; \backslash mathbf\_i\; +\; \backslash frac\; \backslash boldsymbol\_i\; \backslash cdot;\; href="/html/ALL/l/\_R.html"\; ;"title="\_R">\_R$$
which can be used to calculate the m generalized inertia forcesT. R. Kane and D. A. LevinsonDynamics, Theory and Applications

McGraw-Hill, NY, 2005. $$Q^*\_j\; =\; -\backslash left(\backslash frac\; \backslash frac\; -\backslash frac\backslash right),\; \backslash quad\; j=1,\; \backslash ldots,\; m.$$

D'Alembert's form of the principle of virtual work

D'Alembert's form of the principle of virtual work states that a system of rigid bodies is in dynamic equilibrium when the virtual work of the sum of the applied forces and the inertial forces is zero for any virtual displacement of the system. Thus, dynamic equilibrium of a system of n rigid bodies with m generalized coordinates requires that $$\backslash delta\; W\; =\; (Q\_1\; +\; Q^*\_1)\backslash delta\; q\_1\; +\; \backslash dots\; +\; (Q\_m\; +\; Q^*\_m)\backslash delta\; q\_m\; =\; 0,$$ for any set of virtual displacements ''δqLagrange's equations
In physics, Lagrangian mechanics is a formulation of classical mechanics founded on the stationary-action principle (also known as the principle of least action). It was introduced by the Italian-French mathematician and astronomer Joseph-Lou ...

or the generalized equations of motion.
If the generalized forces QLagrangian
Lagrangian may refer to:
Mathematics
* Lagrangian function, used to solve constrained minimization problems in optimization theory; see Lagrange multiplier
** Lagrangian relaxation, the method of approximating a difficult constrained problem with ...

, , so these equations of motion become
$$\backslash frac\; \backslash frac\; -\; \backslash frac\; =\; 0\; \backslash quad\; j=1,\backslash ldots,m.$$
These are known as the Euler-Lagrange equations for a system with m degrees of freedom, or Lagrange's equations of the second kind.
Virtual work principle for a deformable body

Consider now thefree body diagram
A free body diagram consists of a diagrammatic representation of a single body or a subsystem of bodies isolated from its surroundings showing all the forces acting on it.
In physics and engineering, a free body diagram (FBD; also called a force ...

of a deformable body
In physics and materials science, plasticity, also known as plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece ...

, which is composed of an infinite number of differential cubes. Let's define two unrelated states for the body:
* The $\backslash boldsymbol$-State : This shows external surface forces T, body forces f, and internal stresses $\backslash boldsymbol$ in equilibrium.
* The $\backslash boldsymbol$-State : This shows continuous displacements $\backslash mathbf\; ^*$ and consistent strains $\backslash boldsymbol^*$.
The superscript * emphasizes that the two states are unrelated. Other than the above stated conditions, there is no need to specify if any of the states are real or virtual.
Imagine now that the forces and stresses in the $\backslash boldsymbol$-State undergo the displacements and deformations in the $\backslash boldsymbol$-State: We can compute the total virtual (imaginary) work done by ''all forces acting on the faces of all cubes'' in two different ways:
* First, by summing the work done by forces such as $F\_A$ which act on individual common faces (Fig.c): Since the material experiences compatible displacements, such work cancels out, leaving only the virtual work done by the surface forces T (which are equal to stresses on the cubes' faces, by equilibrium).
* Second, by computing the net work done by stresses or forces such as $F\_A$, $F\_B$ which act on an individual cube, e.g. for the one-dimensional case in Fig.(c): $$F\_B\; \backslash left(\; u^*\; +\; \backslash frac\; dx\; \backslash right\; )\; -\; F\_A\; u^*\; \backslash approx\; \backslash frac\; \backslash sigma\; dV\; +\; u^*\; \backslash frac\; dV\; =\; \backslash epsilon^*\; \backslash sigma\; dV\; -\; u^*\; f\; dV$$ where the equilibrium relation $\backslash frac+f=0$ has been used and the second order term has been neglected. Integrating over the whole body gives: $$\backslash int\_\; \backslash boldsymbol^\; \backslash boldsymbol\; \backslash ,\; dV$$ – Work done by the body forces f.
Equating the two results leads to the principle of virtual work for a deformable body:
where the total external virtual work is done by T and f. Thus,
The right-hand-side of (,) is often called the internal virtual work. The principle of virtual work then states: ''External virtual work is equal to internal virtual work when equilibrated forces and stresses undergo unrelated but consistent displacements and strains''. It includes the principle of virtual work for rigid bodies as a special case where the internal virtual work is zero.
Proof of equivalence between the principle of virtual work and the equilibrium equation

We start by looking at the total work done by surface traction on the body going through the specified deformation: $$\backslash int\_\; \backslash mathbf\; u\; \backslash cdot\; \backslash mathbf\; T\; dS\; =\; \backslash int\_\; \backslash mathbf\; u\; \backslash cdot\; \backslash boldsymbol\; \backslash sigma\; \backslash cdot\; \backslash mathbf\; n\; dS$$ Applying divergence theorem to the right hand side yields: $$\backslash int\_S\; \backslash mathbf\; dS\; =\; \backslash int\_V\; \backslash nabla\; \backslash cdot\; \backslash left(\; \backslash mathbf\; \backslash cdot\; \backslash boldsymbol\; \backslash sigma\; \backslash right)\; dV$$ Now switch to indicial notation for the ease of derivation. $$\backslash begin\; \backslash int\_V\; \backslash nabla\; \backslash cdot\; \backslash left(\; \backslash mathbf\; \backslash cdot\; \backslash boldsymbol\; \backslash sigma\; \backslash right)\; dV\; \&=\; \backslash int\_V\; \backslash frac\; \backslash left(\; u\_i\; \backslash sigma\_\; \backslash right)\; dV\; \backslash \backslash \; \&=\; \backslash int\_V\; \backslash left(\; \backslash frac\; \backslash sigma\_\; +\; u\_i\; \backslash frac\backslash right)\; dV\; \backslash end$$ To continue our derivation, we substitute in the equilibrium equation $\backslash frac\; +\; f\_i\; =\; 0$. Then $$\backslash int\_V\; \backslash left(\backslash frac\; \backslash sigma\_\; +\; u\_i\; \backslash frac\backslash right)\; dV\; =\; \backslash int\_V\; \backslash left(\backslash frac\; \backslash sigma\_\; -\; u\_i\; f\_i\backslash right)\; dV$$ The first term on the right hand side needs to be broken into a symmetric part and a skew part as follows: $$\backslash begin\; \backslash int\_V\backslash left(\; \backslash frac\; \backslash sigma\_\; -\; u\_i\; f\_i\backslash right)\; dV\; \&=\; \backslash int\_V\backslash left(\; \backslash frac12\; \backslash left;\; href="/html/ALL/l/\backslash left(\_\backslash frac\_+\_\backslash frac\_\backslash right)\_\; \_\_\_\_\_\_\_\_\_\_+\_\backslash left(\_\backslash frac\_-\_\backslash frac\_\backslash right)\_\backslash right.html"\; ;"title="\backslash left(\; \backslash frac\; +\; \backslash frac\; \backslash right)\; +\; \backslash left(\; \backslash frac\; -\; \backslash frac\; \backslash right)\; \backslash right">\backslash left(\; \backslash frac\; +\; \backslash frac\; \backslash right)\; +\; \backslash left(\; \backslash frac\; -\; \backslash frac\; \backslash right)\; \backslash right$$ where $\backslash boldsymbol\backslash epsilon$ is the strain that is consistent with the specified displacement field. The 2nd to last equality comes from the fact that the stress matrix is symmetric and that the product of a skew matrix and a symmetric matrix is zero. Now recap. We have shown through the above derivation that $$\backslash int\_\; \backslash mathbf\; dS\; =\; \backslash int\_V\; \backslash boldsymbol\backslash epsilon\; :\; \backslash boldsymbol\backslash sigma\; dV\; -\; \backslash int\_V\; \backslash mathbf\; u\; \backslash cdot\; \backslash mathbf\; f\; dV$$ Move the 2nd term on the right hand side of the equation to the left: $$\backslash int\_\; \backslash mathbf\; dS\; +\; \backslash int\_V\; \backslash mathbf\; u\; \backslash cdot\; \backslash mathbf\; f\; dV\; =\; \backslash int\_V\; \backslash boldsymbol\backslash epsilon\; :\; \backslash boldsymbol\backslash sigma\; dV$$ The physical interpretation of the above equation is, ''the External virtual work is equal to internal virtual work when equilibrated forces and stresses undergo unrelated but consistent displacements and strains''. For practical applications: * In order to impose equilibrium on real stresses and forces, we use consistent virtual displacements and strains in the virtual work equation. * In order to impose consistent displacements and strains, we use equilibriated virtual stresses and forces in the virtual work equation. These two general scenarios give rise to two often stated variational principles. They are valid irrespective of material behaviour.Principle of virtual displacements

Depending on the purpose, we may specialize the virtual work equation. For example, to derive the principle of virtual displacements in variational notations for supported bodies, we specify: * Virtual displacements and strains as variations of the real displacements and strains using variational notation such as $\backslash delta\backslash \; \backslash mathbf\; \backslash equiv\; \backslash mathbf^*$ and $\backslash delta\backslash \; \backslash boldsymbol\; \backslash equiv\; \backslash boldsymbol\; ^*$ * Virtual displacements be zero on the part of the surface that has prescribed displacements, and thus the work done by the reactions is zero. There remains only external surface forces on the part $S\_t$ that do work. The virtual work equation then becomes the principle of virtual displacements: This relation is equivalent to the set of equilibrium equations written for a differential element in the deformable body as well as of the stress boundary conditions on the part $S\_t$ of the surface. Conversely, () can be reached, albeit in a non-trivial manner, by starting with the differential equilibrium equations and the stress boundary conditions on $S\_t$, and proceeding in the manner similar to () and (). Since virtual displacements are automatically compatible when they are expressed in terms ofcontinuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous g ...

, single-valued function
In mathematics, a multivalued function, also called multifunction, many-valued function, set-valued function, is similar to a function, but may associate several values to each input. More precisely, a multivalued function from a domain to ...

s, we often mention only the need for consistency between strains and displacements. The virtual work principle is also valid for large real displacements; however, Eq.() would then be written using more complex measures of stresses and strains.
Principle of virtual forces

Here, we specify: * Virtual forces and stresses as variations of the real forces and stresses. * Virtual forces be zero on the part $S\_t$ of the surface that has prescribed forces, and thus only surface (reaction) forces on $S\_u$ (where displacements are prescribed) would do work. The virtual work equation becomes the principle of virtual forces: This relation is equivalent to the set of strain-compatibility equations as well as of the displacement boundary conditions on the part $S\_u$. It has another name: the principle of complementary virtual work.Alternative forms

A specialization of the principle of virtual forces is theunit dummy force method
The Unit dummy force method provides a convenient means for computing displacements in structural systems. It is applicable for both linear and non-linear material behaviours as well as for systems subject to environmental effects, and hence more g ...

, which is very useful for computing displacements in structural systems. According to D'Alembert's principle
D'Alembert's principle, also known as the Lagrange–d'Alembert principle, is a statement of the fundamental classical laws of motion. It is named after its discoverer, the French physicist and mathematician Jean le Rond d'Alembert. D'Alembert ...

, inclusion of inertial forces as additional body forces will give the virtual work equation applicable to dynamical systems. More generalized principles can be derived by:
* allowing variations of all quantities.
* using Lagrange multipliers
In mathematical optimization, the method of Lagrange multipliers is a strategy for finding the local maxima and minima of a function subject to equality constraints (i.e., subject to the condition that one or more equations have to be satisfied e ...

to impose boundary conditions and/or to relax the conditions specified in the two states.
These are described in some of the references.
Among the many energy principles in structural mechanics, the virtual work principle deserves a special place due to its generality that leads to powerful applications in structural analysis
Structural analysis is a branch of Solid Mechanics which uses simplified models for solids like bars, beams and shells for engineering decision making. Its main objective is to determine the effect of loads on the physical structures and their ...

, solid mechanics
Solid mechanics, also known as mechanics of solids, is the branch of continuum mechanics that studies the behavior of solid materials, especially their motion and deformation under the action of forces, temperature changes, phase changes, and ...

, and finite element method in structural mechanics.
See also

* Flexibility method *Unit dummy force method
The Unit dummy force method provides a convenient means for computing displacements in structural systems. It is applicable for both linear and non-linear material behaviours as well as for systems subject to environmental effects, and hence more g ...

* Finite element method in structural mechanics
*Calculus of variations
The calculus of variations (or Variational Calculus) is a field of mathematical analysis that uses variations, which are small changes in functions
and functionals, to find maxima and minima of functionals: mappings from a set of functions t ...

*Lagrangian mechanics
In physics, Lagrangian mechanics is a formulation of classical mechanics founded on the stationary-action principle (also known as the principle of least action). It was introduced by the Italian-French mathematician and astronomer Joseph- ...

* Müller-Breslau's principle
*D'Alembert's principle
D'Alembert's principle, also known as the Lagrange–d'Alembert principle, is a statement of the fundamental classical laws of motion. It is named after its discoverer, the French physicist and mathematician Jean le Rond d'Alembert. D'Alembert ...

References

External links

Examples applications of the virtual work principle

Bibliography

* Bathe, K.J. "Finite Element Procedures", Prentice Hall, 1996. *Charlton, T.M. ''Energy Principles in Theory of Structures'', Oxford University Press, 1973. *Dym, C. L. and I. H. Shames, ''Solid Mechanics: A Variational Approach'', McGraw-Hill, 1973. *Greenwood, Donald T. ''Classical Dynamics'', Dover Publications Inc., 1977, *Hu, H. ''Variational Principles of Theory of Elasticity With Applications'', Taylor & Francis, 1984. *Langhaar, H. L. ''Energy Methods in Applied Mechanics'', Krieger, 1989. * Reddy, J.N. ''Energy Principles and Variational Methods in Applied Mechanics'', John Wiley, 2002. *Shames, I. H. and Dym, C. L. ''Energy and Finite Element Methods in Structural Mechanics'', Taylor & Francis, 1995, *Tauchert, T.R. ''Energy Principles in Structural Mechanics'', McGraw-Hill, 1974. *Washizu, K. ''Variational Methods in Elasticity and Plasticity'', Pergamon Pr, 1982. *Wunderlich, W. ''Mechanics of Structures: Variational and Computational Methods'', CRC, 2002. {{ISBN, 0-8493-0700-7 Mechanics Dynamical systems Structural analysis Linkages (mechanical)