# D’Alembert’s Principle

By BYJU'S Exam Prep

Updated on: September 25th, 2023

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.

## 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 he 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.

## 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:

• 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.