TheInfoList

OR:

In
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 objects re ...
, virtual work arises in the application of the '' principle of least action'' 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 ...
of a force acting on a particle as it moves along a
displacement Displacement may refer to: Physical sciences Mathematics and Physics * Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...
is different for different displacements. Among all the possible displacements that a particle may follow, called
virtual displacement In analytical mechanics, a branch of applied mathematics and physics, a virtual displacement (or infinitesimal variation) \delta \gamma shows how the mechanical system's trajectory can ''hypothetically'' (hence the term ''virtual'') deviate very s ...
s, 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 L ...
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 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 $\mathbf\left(t\right)$ from point $A$, where $\mathbf\left(t=t_0\right)$, to point $B$, where $\mathbf\left(t=t_1\right)$. It is possible that the particle moves from $A$ to $B$ along a nearby path described by $\mathbf\left(t\right) + \delta \mathbf\left(t\right)$, where $\delta \mathbf\left(t\right)$ is called the variation of $\mathbf\left(t\right)$. The variation $\delta \mathbf\left(t\right)$ satisfies the requirement $\delta \mathbf\left(t_0\right) = \delta \mathbf\left(t_1\right) = 0$. The scalar components of the variation $\delta r_1\left(t\right)$, $\delta r_2\left(t\right)$ and $\delta r_3\left(t\right)$ are called virtual displacements. This can be generalized to an arbitrary mechanical system defined by the
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. 3 ...
$q_i$, $i = 1,2,...,n$. In which case, the variation of the trajectory $q_i\left(t\right)$ is defined by the virtual displacements $\delta q_i$, $i = 1,2,...,n$. 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 $W = \int_^ \mathbf \cdot d\mathbf = \int_^ \mathbf \cdot \frac~dt = \int_^\mathbf \cdot \mathbf ~ dt,$ 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 $\bar = \int_^ \mathbf \cdot d(\mathbf+\epsilon \mathbf)=\int_^ \mathbf \cdot \frac~ dt = \int_^\mathbf \cdot (\mathbf +\epsilon \dot) ~ dt .$ The variation of the work ''δW'' associated with this nearby path, known as the ''virtual work'', can be computed to be $\delta W = \bar-W = \int_^ (\mathbf \cdot \epsilon \dot) ~dt.$ 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''''i'' (''t'') (''i'' = 1,...,''n''), and express r(''t'') and in terms of the generalized coordinates. That is, $\mathbf(t) = \mathbf(q_1,q_2,\dots,q_n;t),$ $\mathbf(t) = \mathbf(q_1,q_2,\dots,q_n;t).$ Then, the derivative of the variation is given by $\frac\delta \mathbf = \frac\epsilon\mathbf = \sum_^n \frac \epsilon\dot_i,$ then we have $\delta W = \int_^ \left(\sum_^n \mathbf \cdot \frac\epsilon\dot_i\right)dt = \sum_^n \left(\int_^\mathbf\cdot\frac\epsilon\dot_i ~dt\right).$ The requirement that the virtual work be zero for an arbitrary variation is equivalent to the set of requirements $Q_i = \mathbf \cdot \frac = 0, \quad i=1, \ldots, n.$ The terms ''Qi'' are called the ''generalized forces'' associated with the virtual displacement δr.

# Static equilibrium

Static equilibrium 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 and ...
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 sy ...
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''. 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 $\delta W = \sum_^m \mathbf_i \cdot \delta\mathbf_i + \sum_^n \mathbf_j \cdot \delta\mathbf_j = 0 ,$ where F''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 ''δφj'', ''j'' = 1, 2, ..., ''n'' are the
virtual displacement In analytical mechanics, a branch of applied mathematics and physics, a virtual displacement (or infinitesimal variation) \delta \gamma shows how the mechanical system's trajectory can ''hypothetically'' (hence the term ''virtual'') deviate very s ...
s and virtual rotations, respectively. Suppose the system consists of ''N'' particles, and it has ''f'' (''f'' ≤ 6''N'')
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. 3 ...
''qk'' , ''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. 3 ...
. That is, $\delta \mathbf_i (q_1, q_2, \dots, q_f; t), \quad i = 1, 2, \dots, m ;$ $\delta \phi_j (q_1, q_2, \dots, q_f; t), \quad j = 1, 2, \dots, n .$ The virtual work can then be reparametrized by the
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. 3 ...
: where 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 A system is a group of interacting or interrelate ...
''Qk'' are defined as $Q_k = \sum_^m \mathbf_i \cdot \frac + \sum_^n \mathbf_j \cdot \frac , \quad k = 1, 2, \dots, f .$ Kane shows that these
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 A system is a group of interacting or interrelate ...
can also be formulated in terms of the ratio of time derivatives. That is, $Q_k = \sum_^m \mathbf_i \cdot \frac + \sum_^n \mathbf_j \cdot \frac , \quad k = 1, 2, \dots, f .$ The principle of virtual work requires that the virtual work done on a system by the forces F''i'' and moments M''j'' vanishes if it is in equilibrium. Therefore, the generalized forces ''Q''''k'' are zero, that is $\delta W=0 \quad \Rightarrow \quad Q_k = 0 \quad k =1, 2, \dots, f .$

## Constraint 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 In analytical mechanics, a branch of applied mathematics and physics, a virtual displacement (or infinitesimal variation) \delta \gamma shows how the mechanical system's trajectory can ''hypothetically'' (hence the term ''virtual'') deviate very s ...
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 In analytical mechanics, a branch of applied mathematics and physics, a virtual displacement (or infinitesimal variation) \delta \gamma shows how the mechanical system's trajectory can ''hypothetically'' (hence the term ''virtual'') deviate very s ...
, which means that they need not be considered in this analysis. The two important examples are (i) the internal forces in a
rigid 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 f ...
, 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 In analytical mechanics, a branch of applied mathematics and physics, a virtual displacement (or infinitesimal variation) \delta \gamma shows how the mechanical system's trajectory can ''hypothetically'' (hence the term ''virtual'') deviate very s ...
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

A
lever 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 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 $a = , \mathbf_A - \mathbf_P, , \quad b = , \mathbf_B - \mathbf_P, ,$ 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 $\mathbf_A - \mathbf_P = a\mathbf_A, \quad \mathbf_B - \mathbf_P = b\mathbf_B.$ This notation allows us to define the velocity of the points ''A'' and ''B'' as $\mathbf_A = \dot a \mathbf_A^\perp, \quad \mathbf_B = \dot b \mathbf_B^\perp,$ 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 $Q = \mathbf_A \cdot \frac - \mathbf_B \cdot \frac = a(\mathbf_A \cdot \mathbf_A^\perp) - b(\mathbf_B \cdot \mathbf_B^\perp).$ 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 $F_A = \mathbf_A \cdot \mathbf_A^\perp, \quad F_B = \mathbf_B \cdot \mathbf_B^\perp.$ 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
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 lever, and is obtained from the principle of virtual work as $MA = \frac = \frac.$ 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 scientist ...
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 ''ωA'' of the input gear to the angular velocity ''ωB'' of the output gear, known as the speed ratio, or
gear 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 circles of engaging gears roll on each other without slipping, providing a smooth transmission ...
, of the gear train. Let ''R'' be the speed ratio, then $\frac = R.$ The input torque ''T''''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 $\omega_A = \omega, \quad \omega_B = \omega/R.$ The formula above for the principle of virtual work with applied torques yields the generalized force $Q = T_A \frac - T_B \frac = T_A - T_B/R = 0.$ The
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''''B'' to the input torque ''T''''A'', and the above equation yields $MA = \frac = R.$ 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.

# Dynamic equilibrium for rigid bodies

If the principle of virtual work for applied forces is used on individual particles of a
rigid 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 f ...
, 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 mec ...
(1700–1782).

## Generalized inertia forces

Let a mechanical system be constructed from n rigid bodies, Bi, i=1,...,n, and let the resultant of the applied forces on each body be the force-torque pairs, Fi and Ti, ''i'' = 1,...,''n''. Notice that these applied forces do not include the reaction forces where the bodies are connected. Finally, assume that the velocity Vi and angular velocities ωi, ''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 degree of freedom. 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 This inertia force can be computed from the kinetic energy of the rigid body, by using the formula $Q^* = -\left(\frac \frac -\frac\right).$ A system of n rigid bodies with m generalized coordinates has the kinetic energy which can be used to calculate the m generalized inertia forcesT. R. Kane and D. A. Levinson
Dynamics, Theory and Applications
McGraw-Hill, NY, 2005.
$Q^*_j = -\left(\frac \frac -\frac\right), \quad j=1, \ldots, m.$

## 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 $\delta W = (Q_1 + Q^*_1)\delta q_1 + \dots + (Q_m + Q^*_m)\delta q_m = 0,$ for any set of virtual displacements ''δqj''. This condition yields ''m'' equations, $Q_j + Q^*_j = 0, \quad j=1, \ldots, m,$ which can also be written as $\frac \frac -\frac = Q_j, \quad j=1,\ldots,m.$ The result is a set of m equations of motion that define the dynamics of the rigid body system, known as Lagrange's equations or the generalized equations of motion. If the generalized forces Qj are derivable from a potential energy ''V''(''q''1,...,''q''''m''), then these equations of motion take the form $\frac \frac -\frac = -\frac, \quad j=1,\ldots,m.$ In this case, introduce the Lagrangian, , so these equations of motion become $\frac \frac - \frac = 0 \quad j=1,\ldots,m.$ 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 the
free 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, which is composed of an infinite number of differential cubes. Let's define two unrelated states for the body: * The $\boldsymbol$-State : This shows external surface forces T, body forces f, and internal stresses $\boldsymbol$ in equilibrium. * The $\boldsymbol$-State : This shows continuous displacements $\mathbf ^*$ and consistent strains $\boldsymbol^*$. 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 $\boldsymbol$-State undergo the
displacement Displacement may refer to: Physical sciences Mathematics and Physics * Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...
s and
deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Def ...
s in the $\boldsymbol$-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
displacement Displacement may refer to: Physical sciences Mathematics and Physics * Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...
s, 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): $F_B \left( u^* + \frac dx \right ) - F_A u^* \approx \frac \sigma dV + u^* \frac dV = \epsilon^* \sigma dV - u^* f dV$ where the equilibrium relation $\frac+f=0$ has been used and the second order term has been neglected. Integrating over the whole body gives: $\int_ \boldsymbol^ \boldsymbol \, dV$ – 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: $\int_ \mathbf u \cdot \mathbf T dS = \int_ \mathbf u \cdot \boldsymbol \sigma \cdot \mathbf n dS$ Applying divergence theorem to the right hand side yields: $\int_S \mathbf dS = \int_V \nabla \cdot \left( \mathbf \cdot \boldsymbol \sigma \right) dV$ Now switch to indicial notation for the ease of derivation. $\begin \int_V \nabla \cdot \left( \mathbf \cdot \boldsymbol \sigma \right) dV &= \int_V \frac \left( u_i \sigma_ \right) dV \\ &= \int_V \left( \frac \sigma_ + u_i \frac\right) dV \end$ To continue our derivation, we substitute in the equilibrium equation $\frac + f_i = 0$. Then $\int_V \left(\frac \sigma_ + u_i \frac\right) dV = \int_V \left(\frac \sigma_ - u_i f_i\right) dV$ The first term on the right hand side needs to be broken into a symmetric part and a skew part as follows: where $\boldsymbol\epsilon$ 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 $\int_ \mathbf dS = \int_V \boldsymbol\epsilon : \boldsymbol\sigma dV - \int_V \mathbf u \cdot \mathbf f dV$ Move the 2nd term on the right hand side of the equation to the left: $\int_ \mathbf dS + \int_V \mathbf u \cdot \mathbf f dV = \int_V \boldsymbol\epsilon : \boldsymbol\sigma dV$ 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 $\delta\ \mathbf \equiv \mathbf^*$ and $\delta\ \boldsymbol \equiv \boldsymbol ^*$ * 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 of
continuous 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 ...
, single-valued functions, 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 the unit dummy force method, 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 Energy principles in structural mechanics express the relationships between stresses, strains or deformations, displacements, material properties, and external effects in the form of energy or work done by internal and external forces. Since ener ...
, 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 thei ...
,
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 The finite element method (FEM) is a powerful technique originally developed for numerical solution of complex problems in structural mechanics, and it remains the method of choice for complex systems. In the FEM, the structural system is modeled b ...
.

* Flexibility method * Unit dummy force method *
Finite element method in structural mechanics The finite element method (FEM) is a powerful technique originally developed for numerical solution of complex problems in structural mechanics, and it remains the method of choice for complex systems. In the FEM, the structural system is modeled b ...
*
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-Lo ...
* 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 ...