D'Alembert's Principle

By Deepak Yadav|Updated : October 6th, 2022

D'Alembert's Principle is a variant of Newton's second law of motion proposed by the 18th-century French philosopher Jean Le Rond D'Alembert. In effect, the principle reduces a dynamic problem to a statical problem. In D'Alembert's form, the force F plus the negative of the mass m times acceleration and of the body is equal to zero: F - ma = 0. 

According to the second law, the force F acting on a body is equal to the product of the mass m and acceleration of the body, or F = ma. In other words, the body is in the balance due to the action of the real force F and the imaginary force ma. Inertial force and reversed effective force are other names for fictitious force. Here, we will explore D'Alembert's principle in detail and its mathematical representation and applications.

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D'Alembert's Principle

D'Alembert's principle is another way to express Newton's second law of motion. The principle has been described as "the negative of the product of mass times acceleration. There is balance when this force is added to the impressed force, indicating that the virtual work principle is satisfied." It represents a transfer of the virtual work principle from static to dynamic systems.

Definition of D'Alembert's Principle

"When projected onto any virtual displacement, the total difference between the force acting on the system and the time derivatives of the momenta is zero for a system of mass particles."

D'Alembert's Principle PDF

It is also known as the Lagrange-d'Alembert principle, after the French mathematician and physicist Jean le Rond D'Alembert. It is a variant of Newton's second law of motion. The second law of motion states that F = ma, although D'Alembert's principle states that F - ma = 0. Therefore, when a real force exerts itself on the item, it can be considered in equilibrium. Here, F is the actual force, while ma is a constructed force known as the inertial force.

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Mathematical Representation of D'Alembert's Principle

According to the principle, the sum of the differences between the forces acting on a system of heavy particles and the time derivatives of the system's linear momentum projected onto any virtual displacement consistent with the system's restrictions is zero.

D'alembert's Principle is given by the equation as follows:

D’Alembert’s Principle formula

  • i is the integral used to identify the variable related to the specific particle in the system.
  • Fi denotes the total applied force on the ith position.
  • mi is the mass of the ith particle.
  • The acceleration of the ith particle is denoted by ai
  • The time derivative representation is denoted by miai.
  • The virtual displacement of the ith particle is denoted by σri.

D'Alembert's Principle of Inertial Forces

D'Alembert showed how to convert an accelerating rigid body into an equal static system by combining "inertial force" and "inertial torque" or moment. While the inertial force must pass via the center of mass, the inertial torque can act everywhere. As a result, the system can be assessed as a static system that is affected by internal and external forces. The benefit is that with the analogous static system, one can pause and think about any location (not just the center of mass). This often results in more straightforward calculations because each force (in turn) can be removed from the moment equations by determining the ideal position to apply the moment equation (sum of moments = zero). In Fundamentals of Dynamics and Kinematics of Machines, this approach is used to analyze the forces acting on a link of a machine while it is in motion.

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Applications of the D'Alembert Principle

D'Alembert's principle is founded on the principles of virtual work and inertial forces. Applications of D'Alembert's principle include the following:

  • Gravity causes the mass to descend.
  • Axis parallel theorem.
  • Vertical hoop with a bead that is frictionless.

Important GATE Topics

Hydrostatic ForceDifference Between Microcomputer And Minicomputer
Difference Between While And Do While LoopsPartial Dependency In Dbms
Virtual DisplacementCoefficient Of Restitution
File Handling In C ProgrammingMoment Of Couple
Monostable MultivibratorForce Method Of Analysis Of Indeterminate Structure


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FAQs on D'Alembert's Principle

  • According to D'Alembert's principle, "For reversible displacements, the entire virtual work of the impressed forces plus the inertial forces vanishes." Newton's Law of Motion can also be restated as F - ma = 0. The total amount of energy in a closed system stays constant, according to the rule of conservation of energy.

  • Since the system is subject to both internal and external forces, it can be assessed as a static system. The advantage is that one can take moments about any place in the corresponding static system (not just the center of mass).

  • According to the virtual work principle, any virtual work performed by forces on an object when it is in equilibrium will be equal to zero. The same is stated in Newton's laws, which assert that at equilibrium, the applied forces are equal and opposing.

  • The force that opposes an accelerating force and is equal to the product of the accelerating force and the body's mass is known as an inertial force.

  • A particle's effective force is defined as the product of its mass and acceleration. It will be demonstrated that the system of external forces operating on a system of particles is equivalent to the system's effective forces.



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