Dalamber's principle is one of the main principles of dynamics. According to it if to forces operating on points of a mechanical system to add inertia forces, then the received system will become balanced.
Dalamber's principle for a material point
If to consider a system which consists of several material points, allocating one certain point with the known weight, then under the influence of the external and internal forces applied to it she receives some acceleration in relation to an inertial reference system. Among such forces there can be both active forces, and communication reactions.
Force of inertia of a point is a vector size which is equal on the module to the work of mass of a point on its acceleration. This size is mentioned sometimes as the dalamberovsky force of inertia, it is directed opposite to acceleration. In this case the following property of a moving point is found: if in each timepoint to add inertia force to forces which are actually operating on a point, then the received system of forces will be counterbalanced. It is so possible to formulate Dalamber's principle for one material point. This statement completely corresponds to the second law of Newton.
Dalamber's principles for a system
If to repeat all reasonings for each point in a system, they lead to the following conclusion which expresses Dalamber's principle formulated for a system: if at any moment to apply inertia forces to each of points in a system, besides actually operating external and internal forces, then this system will be in balance therefore it is possible to apply all equations which are used in a statics to it. If to apply Dalamber's principle to the solution of problems of dynamics, then the equations of the movement of a system can be worked out in the form of the balance equations known to us. This principle considerably simplifies calculations and does approach to the solution of tasks uniform.
Use of the principle of Dalamber
It is necessary to consider that the moving point in a mechanical system is affected by only external and internal forces which arise as result of interaction of points among themselves and also with the bodies which are not entering this system. Points move with certain accelerations under the influence of all these forces. Forces of inertia do not affect moving points, otherwise they would move without acceleration or were at rest. Forces of inertia are entered only to work out dynamics equations by means of simpler and convenient methods of a statics. It is considered also that the geometrical sum of internal forces and the sum of their moments is equal to zero. Use of the equations which follow from Dalamber's principle makes process of the solution of tasks easier as these equations do not contain internal forces any more.