DAlembert’s Principle

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Created by: Team Physics - Examples.com, Last Updated: May 9, 2024

DAlembert’s Principle

D’Alembert’s Principle is an important concept in physics that helps simplify the analysis of dynamics. It states that for a system in motion, the sum of the applied forces and the inertial forces acting on each particle of the system results in a state of equilibrium.

What is DAlembert’s Principle?

D’Alembert’s Principle is a method used in classical mechanics to analyze the dynamics of a system. Instead of directly working with accelerating forces, this principle simplifies dynamic problems by transforming them into equivalent static problems.

DAlembert’s Principle Formula

The formula associated with D’Alembert’s Principle is:



  • 𝐹ᡒ: External (applied) force on the i-th particle.
  • π‘šα΅’β€‹: Mass of the i-th particle.
  • π‘Žα΅’: Acceleration of the i-th particle.
  • π›Ώπ‘Ÿα΅’β€‹: Virtual displacement of the i-th particle.

This equation represents that the sum of the applied forces and the fictitious inertial forces (due to mass and acceleration) yields zero virtual work. Thus, the system can be treated as if it were in static equilibrium, simplifying the analysis of dynamic systems.

DAlembert’s Principle Derivation

The derivation of D’Alembert’s Principle is based on the concept of virtual work and results in an equation that demonstrates the principle. Here’s the step-by-step process:

Total Force on Each Particle:
The total force (𝐹ᡒ⁽ᡀ⁾) acting on the 𝑖-th particle is equal to its mass (π‘šα΅’) times its acceleration (π‘Žα΅’β€‹): 𝐹ᡒ⁽ᡀ⁾=π‘šα΅’π‘Žα΅’

Inertial Force:
The inertial force is introduced by subtracting it from the total force: πΉα΅’β½α΅€βΎβˆ’π‘šα΅’π‘Žα΅’=0 This represents a state of quasi-static equilibrium.

Virtual Work Expression:
The virtual work (π›Ώπ‘Š) equation sums the forces and inertial terms multiplied by their virtual displacements

(π›Ώπ‘Ÿα΅’β€‹): π›Ώπ‘Š=βˆ‘α΅’πΉα΅’β½α΅€βΎβ‹…π›Ώπ‘Ÿα΅’βˆ’βˆ‘α΅’π‘šα΅’π‘Žα΅’β‹…π›Ώπ‘Ÿα΅’=0

Separation of Applied and Constraint Forces:
The total force can be separated into applied forces (𝐹ᡒ) and constraint forces (𝐢ᡒ​): π›Ώπ‘Š=βˆ‘α΅’πΉα΅’β‹…π›Ώπ‘Ÿα΅’+βˆ‘α΅’πΆα΅’β‹…π›Ώπ‘Ÿα΅’βˆ’βˆ‘α΅’π‘šα΅’π‘Žα΅’β‹…π›Ώπ‘Ÿα΅’=0

Final Equation:
After simplifying, the final equation for virtual work becomes:


This final equation states that the sum of the external forces and the inertial forces, each multiply by their respect virtual displacements, equals zero. This result forms the basis of D’Alembert’s Principle, convert dynamic problems into static-like equilibrium problems.

Uses of DAlembert’s Principle

Uses of DAlembert's Principle

D’Alembert’s Principle provides a useful framework in classical mechanics, enabling several practical applications:

  1. Simplifying Dynamic Problems: First, you can use D’Alembert’s Principle to convert dynamic problems into static-like equilibrium problems by introduce fictitious inertial forces. This transformation allows you to analyze systems with the same techniques used for static systems.
  2. Analyzing Rigid Body Motion: In addition, you can apply this principle to study the motion of rigid bodies, making it easier to handle problems involving rotating or accelerating frames of reference.
  3. Understanding Vibrating Systems: Moreover, D’Alembert’s Principle helps you analyze vibrating systems, such as springs and dampers, by simplifying the calculation of forces involved.
  4. Investigating Mechanical Systems: Consequently, you can investigate various mechanical systems, from simple machines to complex engineering structures, using the principle’s ability to represent the balance between applied and inertial forces.
  5. Formulating Equations of Motion: Finally, D’Alembert’s Principle allows you to formulate the equations of motion for a system, providing a basis for applying Lagrange’s equations or Hamiltonian mechanics.
  6. Analyzing Particle Systems: Additionally, the principle enables you to analyze systems with multiple interacting particles by considering how the forces of one particle affect another.

Examples for DAlembert’s Principle

Here are a few examples of how you can apply D’Alembert’s Principle:

  • Rotating Disc: When analyzing a rotating disc, you can use D’Alembert’s Principle to consider inertial forces. By introducing these forces, you can treat the disc as if it were in static equilibrium. Simplifying the analysis of internal stresses and reactions.
  • Moving Car: In the case of a car accelerate forward, D’Alembert’s Principle allows you to analyze it by introducing a fictitious force in the opposite direction of acceleration. This way, you can assess the car’s equilibrium under the influence of both real and inertial forces, helping you calculate the forces on each axle.
  • Pendulum Motion: For a swinging pendulum, applying D’Alembert’s Principle lets you simple the analysis of the bob’s motion. You consider the gravitational and tension forces alongside an inertial force that represents the bob’s acceleration, which makes it easier to understand the pendulum’s equilibrium at any point in its swing.
  • Crane Movement: When a crane moves its load horizontally, you can analyze the problem using D’Alembert’s Principle. By including inertial forces due to acceleration. You can determine how much force the crane must exert to keep the load in balance.
  • Elevator Acceleration: In an accelerating elevator, you can apply D’Alembert’s Principle to introduce a fictitious force opposite the direction of acceleration. This adjustment lets you treat the system as if it were in equilibrium. And calculate the tension required in the support cables.


What is D’Alembert’s Principle?

D’Alembert’s Principle states that the virtual work of inertia forces in a system in dynamic equilibrium is zero.

How does it differ from Newton’s Laws?

Newton’s Laws focus on acceleration and external forces, while D’Alembert’s Principle considers virtual work and internal forces.

When is D’Alembert’s Principle applied?

It’s used to solve problems in dynamics, particularly for systems in dynamic equilibrium or under the influence of constraint forces.

What’s the significance of virtual work?

Virtual work helps analyze the effect of inertial forces on dynamic systems, simplifying the calculation of equilibrium conditions.

Can it be applied to systems with constraints?

Yes, it’s particularly useful for systems with constraints, where it simplifies calculations by treating constraints as forces.

How does it simplify dynamic analysis?

By balancing applied and inertial forces, it simplifies the analysis of systems undergoing dynamic motion or in equilibrium.

How is D’Alembert’s Principle mathematically expressed?

It’s expressed as the sum of external forces and virtual forces, which account for inertial effects in dynamic equilibrium equations.

Can it be applied to non-conservative systems?

Yes, D’Alembert’s Principle can be extended to non-conservative systems, incorporate external forces and damping effects.

What’s the relation to Hamilton’s Principle?

Both principles describe the dynamics of mechanical systems, with D’Alembert’s focusing on equilibrium and Hamilton’s on variational principles.

How does it contribute to engineering analysis?

D’Alembert’s Principle provides a powerful tool for analyzing and solving problems in dynamics, aiding in the design and optimization of mechanical systems.

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